Salty Matters

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Brine evolution and origins of potash ore salts: Primary or secondary? Part 1 of 3

John Warren - Wednesday, October 31, 2018

Introduction

There is a dichotomy in mineralogical associations and precipitation series in both modern and ancient potash ore deposits. Interpretations of ancient potash ore mineralogies across time are generally tied to the evolution of the hydrochemical proportions in modern and ancient oceans. We have already discussed this in previous Salty Matters articles and will not repeat the details here (see August 10, 2015; July 31, 2018).

At times in the past, such as in the Devonian and the Cretaceous, the world ocean was depleted in the Mg and SO4 relative to the present-day ocean (Figure 1a). In the relevant literature, this has led to the application of the term MgSO4-depleted versus MgSO4-enriched oceans. In terms of brine evolution, this is related to the gypsum divide, with the term MgSO4-enriched used to describe the ocean chemistry of today and other times in the past, such as in the Permian, when MgSO4 bittern salts typify co-precipitates with sylvite/carnallite (Figure 1b).


The validity of the ocean chemistry argument is primarily based on determinations of inclusion chemistries as measured in chevron halites (Figure 1a; Lowenstein et al., 2014). Inclusions in growth-aligned primary halite chevrons are assumed to preserve the chemical proportions of the ambient oceanic brine precipitating the halite. That is, the working assumption is that pristine aligned-halite chevrons have not been subject to significant diagenetic alteration once the salt was deposited and permeability was lost due to ongoing halite cementation in the shallow (eogenetic) subsurface realm.

The same assumption as to the pristine nature of chevron halite is applied to outcomes of biological experiments where Permian archaeal/halobacterial life has been re-animated using ancient salt samples (Vreeland et al., 2000).

Primary potash ore?

But does the same assumption of pristine texturing across time also apply to the halite layers associated with the world’s potash ores? In my experience of subsurface potash ores and their textures, I have rarely seen primary-chevron halite interlayered with potash ore layers of either sylvite or carnallite. An obvious exception is the pristine interlayering of chevron halite and sylvite in the now-depleted Eocene potash ores of the Mulhouse Basin, France (Lowenstein and Spencer, 1990). There, the sylvite layers intercalate at the cm-scale with chevron halite, and the alternating layering is thought to be related to precipitation driven by temperature fluctuations in a series of shallow density-stratified meromictic brine lakes (documented in the third Salty Matters article).

More typically, ancient potash ore textures are diagenetic and indicate responses to varying degrees of dissolution, brine infiltration and alteration. The simpler styles of brine infiltration consist of a background matrix dominated by cm-dm scale chevron halite layers that have been subject to dissolution and karstification during shallow burial. Resultant cm-dm scale voids typically retain a mm-thick selvedge of CaSO4 lathes and needles, followed by fill of the remaining void by varying amounts of sparry halite, carnallite and sylvite. This type of texture dominates in Quaternary stratoid potash layers in the southern Qaidam Basin in China and the Cretaceous carnallite-rich layers of the Maha Sarakham Fm in NE Thailand and southern China (Warren, 2016). Then there are the even more altered and recrystallised, but still bedded, textures in the potash ore zones of Devonian Prairie Evaporite of western Canada (Wardlaw, 1968) and potash layers in the Permian Basin in west Texas and New Mexico (Lowenstein, 1988; Holt and Powers, 2011). Beyond this level of diagenetic texturing are the flow-orientated and foliated structural textures of the Permian potash ores in potash mines in the diapiric Zechstein evaporites of Germany and Poland, the Kungurian diapirs of the Cis-Urals of Russia and the Devonian diapirs of the Pripyat Basin.

And so, herein lies the main point of discussion for this and the next two Salty Matters articles, namely, what, where and when is(are) the mechanism(s) or association(s) of hydrochemical mechanisms that sufficiently concentrate or alter a brine’s chemistry to where it precipitates economic levels of a variety of potash salts, as either muriate of potash or sulphate of potash. Notably, there are no Quaternary-age solid-state ore systems that are mined for potash.

In this article, we look at the main modern brine systems where muriate of potash (MOP) is produced economically by solar evaporation (Salar de Atacama, Chile; Qarhan sump, China; and the southern Basin of the Dead Sea). In the second article we will focus on sulphate of potash (SOP) production in Quaternary saline sumps (Great Salt Lake, USA and Lop Nur, China). In the third article we shall discuss depositional and diagenetic characteristics of solid-state potash ores some of the world’s more substantial deposits (e.g. Devonian of western Canada) and relate the observations of ancient potash texture to time-based evolution of potash precipitating brines, and subsequent alteration or the ore textures, which are typically driven by later cross-flushing by one or more pulses of diagenetically-evolved brines.


Potash from brine in Salar de Atacama (MOP in a simple near-uniserial set of brine concentration pans)

Potash production in Salar de Atacama is a byproduct of the output of lithium carbonate from shallow lake brines pumped into a series of solar concentration pans (Figure 2). The inflow feed to the concentrator pans comes from fields of brine wells extracting pore waters from the salt nucleus facies in the central and southern part of the Atacama saltflats (Figure 3a,b). However, Atacama pore brines are not chemically homogeneous across the salar (Alonso and Risacher, 1996; Risacher and Alonso, 1996; Carmona et al., 2000; Pueyo et al. 2017). The most common primary inflow brines to the Atacama sump are sulphate-rich (SO4/Ca > 1), but there are areas in the salt flat at the southern end of the playa, such as those near the Península de Chépica, where pore brines are richer in calcium (SO4/Ca < 1- Figure 4). These brines also contain elevated levels of lithium (Figure 3c; Risacher et al., 2003).


Ion proportions in the natural salar inflow and pore waters are dominated by sodium and chloride, followed by potassium, then magnesium then sulphate in the more saline regions of the salar sump (Figure 4; Lowenstein and Risacher, 2009). In addition, owing to the progressive reduction of porosity in depth, driven mainly by diagenetic halite cementation, the pore brine in the upper 40 meters of the salar sediment column accumulates by advection in the area of greatest porosity, i.e., in this top 40 m of sediments of the salt flat at the southern end of Atacama (Pueyo et al., 2017). When pumped from the hosting salar sediments into the concentrator pans the final brines contain elevated levels of lithium chloride (≈ 6000 ppm). These lithium-enriched acidic waters are then pumped to a nearby industrial plant and processed to obtain lithium carbonate as the main commercial product.

In a benchmark paper, Pueyo et al. (2017) document the brine evolution and products recovered in the solar pans of Rockwood Lithium GmbH (Figure 2b; formerly Sociedad Chilena del Litio) in the Península de Chépica. There, a bittern paragenesis of salts precipitates that is mostly devoid of magnesium sulphate salt due to the low levels of sulphate attained in the various concentator pans via widespread precipitation of gypsum in the early concentrator pans (Figures 4, 5).


The depletion of sulphate levels in the early concentrators is done via artificial manipulation of ionic proportions in the feeders. Without alteration of the ionic proportion in halite-stage brines, the evaporation of the saltflat brine feeds, which are rich in sulfate, would result in assemblages that, in addition to potassic chlorides, would contain contain problematic magnesium sulfates (such as schoenite, kainite, glaserite as in Great Salt Lake). The presence of such sulphate salts and ions in the liquor feeding the lithium carbonate plant would complicate the lithium carbonate extraction process. So the aim in the Atacama pans is to remove most of the sulphate via constructing a suitably balanced chemistry in the early concentrator brine stage (to illustrate this compare ionic proportions in sulphate between Figure 4 and Figure 5).


Such a sylvite/carnallite brine paragenesis, sans sulphate (as seen in Figure 5), is similar to that envisaged as the feed chemistry for ancient Mg-sulphate-free marine potash deposits (Braitsch, 1971). That is, as the brines pass through the concentrators, with successive pans transitioning to higher salinities, the potash salts carnallite and sylvite precipitate, without the complication of the widespread magnesium sulphate salts that complicate the processing of marine-derived bitterns. Such MgSO4 double-salts typify SOP production in the Ogden Salt flats, with their primary feed of sulphate-rich Great Salt Lake waters. Relative proportions of sulphate are much higher in the Great Salt Lake brine feed (see article 2 in this series).


Once the balance is accomplished by mixing a Ca-rich brine from further up the concentration series, with the natural SO4-brine in an appropriate ratio, the modified brines are then pumped and discharged into the halite ponds of the saltwork circuit (ponds number 17 and 16, as seen in Figures 5 and 6). In these ponds, halite precipitates from the very beginning with small amounts of accessory gypsum as brines are saturated with both minerals. Subsequently, the brines are transferred to increasingly smaller ponds where halite (ponds 15 and 14), halite and sylvite (pond 13), sylvite (ponds 12, 11 and 10), sylvite and carnallite (pond 9), carnallite (pond 8), carnallite and bischofite (pond 7), bischofite (ponds 6, 5 and 4), bischofite with some lithium-carnallite [LiClMgCl26H2O] (pond 3), and lithium-carnallite (ponds 2 and 1) precipitate. The brines of the last ponds (R-1 to R-3), whose volumes undergo a reduction to 1/50th of the starting volume, are treated at the processing factory to obtain lithium carbonate as the main commercial product.

As documented in Pueyo et al. (2017), the average daily temperature in the Salar de Atacama ranges between 22 °C in February and 8 °C in July, with a maximum oscillation of approximately 14 °C. Wind speed ranges daily from< 2 ms−1 in the morning to 15 ms−1 in the afternoon. Rainfall in the area of the salt flat corresponds to that of a hyperarid desert climate with an annual average, for the period 1988–2011, of 28 mm at San Pedro de Atacama, 15.1 mm at Peine and 11.6 mm at the lithium saltworks, in the last case ranging between 0 and 86 mm for individual years. The adjoining Altiplano to the east has an arid climate with an average annual rainfall of approximately 100 mm. The average relative humidity in the saltpan area, for the period 2006–2011, is 19.8% with a maximum around February (27%) and a minimum in October (15%) and with a peak in the morning when it may reach 50%. The low relative humidity and the high insolation (direct radiation of 3000 kWh m−2 yr−1) in the salt flat increase the efficiency of solar evaporation, giving rise to the precipitation and stability of very deliquescent minerals such as carnallite and bischofite. The average annual evaporation value measured in the period 1998–2011, using the salt flat interstitial brine, is approximately 2250 mm with a peak in December–January and a minimum in June–July. This cool high-altitude hyperarid climatic setting, where widespread sylvite and carnallite accumulates on the pan floor, is tectonically and climatically distinct from the hot-arid subsealevel basinwide desert seep settings envisaged for ancient marine-fed potash basins (as discussed in the upcoming third article in this series).

MOP from brine Dabuxum/Qarhan region, Qaidam Basin, China

The Qarhan saltflat/playa is now the largest hypersaline sump within the disaggregated lacustrine system that makes up the hydrology of Qaidam Basin, China (Figure 7a). The Qaidam basin sump has an area of some 6,000 km2, is mostly underlain by bedded Late Quaternary halite. Regionally, the depression is endorheic, fed by the Golmud, Qarhan and Urtom (Wutumeiren) rivers in the south and the Sugan River in the north, and today is mostly covered by a layered halite pan crust. Below, some 0 to 1.3m beneath the playa surface, is the watertable atop a permanent hypersaline groundwater brine lens (Figure 7b).


The southern Qaidam sump entrains nine perennial salt lakes: Seni, Dabiele, Xiaobiele, Daxi, Dabuxum (Dabsan Hu), Tuanjie, Xiezuo and Fubuxum north and south lakeshore (Figure 7). Dabuxum Lake, which occupies the central part of the Qarhan sump region, is the largest of the perennial lakes (184 km2; Figures 7b, 8a). Lake water depths vary seasonally from 20cm to 1m and never deeper than a metre, even when flooded. Salt contents in the various lakes range from 165 to 360 g/l, with pH ranging between 5.4 and 7.85. Today the salt plain and pans of the Qarhan playa are fed mostly by runoff from the Kunlun Mountains (Kunlun Shan), along with input from a number of saline groundwater springs concentrated along a fault trend defining an area of salt karst along the northern edge of the Dabuxum sump, especially north of Xiezuo Lake (Figure 8a).


The present climate across the Qaidam Basin is cool, arid to hyperarid (BWk), with an average yearly rainfall of 26 mm, mean annual evaporation is 3000–3200 mm, and a yearly mean temperature 2-4° C in the central basin (An et al., 2012). The various salt lakes and playas spread across the basin and contain alternating climate-dependent evaporitic sedimentary sequences. Across the basin the playa sumps are surrounded by aeolian deposits and wind-eroded landforms (yardangs). In terms of potash occurrence, the most significant region in the Qaidam Basin is the Qarhan sump or playa (aka Chaerhan Salt Lake), which occupies a landscape low in front of the outlets of the Golmud and Qarhan rivers (Figure 7a, b). Overall the Qaidam Basin displays a typical exposed lacustrine geomorphology and desert landscape, related to increasing aridification in a cool desert setting. In contrast, the surrounding elevated highlands are mostly typified by a high-alpine tundra (ET) Köppen climate.


Bedded and displacive salts began to accumulate in the Qarhan depression some 50,000 years ago (Figure 9). Today, outcropping areas of surface salt crust consist of a chaotic mixture of fine-grained halite crystals and mud, with a rugged, pitted upper surface (Schubel and Lowenstein, 1997; Duan and Hu, 2001). Vadose diagenetic features, such as dissolution pits, cavities and pendant cements, form wherever the salt crust lies above the watertable. Interbedded salts and siliciclastic sediments underlying the crust reach thicknesses of upwards of 70m (Kezao and Bowler, 1986).

Bedded potash, as carnallite, precipitates naturally in transient volumetrically-minor lake strandzone (stratoid) beds about the northeastern margin of Lake Dabuxum (Figure 8a) and as cements in Late Pleistocene bedded deposits exposed in and below nearby Lake Tuanje in what is known as the sediments of the Dadong ancient lake (Figure 8b). Ongoing freshened sheetflow from the up-dip bajada fans means the proportion of carnallite versus halite in the evaporite unit increases with distance from the Golmud Fan across, both the layered (bedded) and stratoid (cement) modes of occurrence.

At times in the past, when the watertable was lower, occasional meteoric inflow was also the driver for the brine cycling that created the karst cavities hosting the halite and carnallite cements that formed as prograde cements during cooling of the sinking brine (Figure 9). Solid bedded potash salts are not present in sufficient amounts to be quarried, and most of the exploited potash resource resides in interstitial brines that are pumped and processed using solar ponds.

Modern halite crusts in Qarhan playa contain the most concentrated brine inclusions of the sampled Quaternary halites, suggesting that today may be the most desiccated period in the Qarhan-Tuanje sump recorded over the last 50,000 years (values in the inset in figure 9 were measured on clear halite-spar void-fill crystals between chevrons). Inclusion measurements from these very early diagenetic halite show they formed syndepositionally from shallow groundwater brines and confirm the climatic record derived from adjacent primary (chevron) halite. The occurrence of carnallite-saturated brines in fluid inclusions in the diagenetic halite in the top 13 m of Qarhan playa sediments also imply a prograde diagenetic, not depositional, origin of carnallite, which locally accumulated in the same voids as the more widespread microkarst halite-spar cements.

Today, transient surficial primary carnallite rafts can accumulate along the northern strandline of Lake Dabuxum (Figure 9; Casas, 1992; Casas et al., 1992). Compositions of fluid inclusions in the older primary (chevron) halite beds hosting carnallite cements in the various Qarhan salt crusts represent preserved lake brines and indicate relatively wetter conditions throughout most of the Late Pleistocene (Yang et al., 1995). Oxygen isotope signatures of the inclusions record episodic freshening and concentration during the formation of the various salt units interlayered with lacustrine muds. Desiccation events, sufficient to allow halite beds to accumulate, occurred a number of times in the Late Quaternary: 1) in a short-lived event ≈ 50,000 ka, 2) from about 17 - 8,000 ka, and 3) from about 2,000 ka till now (Figure 9).

The greatest volume of water entering Dabuxum Lake comes from the Golmud River (Figure 7b). Cold springs, emerging from a narrow karst zone some 10 km to the north of the Dabuxum strandline and extending hundreds of km across the basin, also supply solutes to the lake. The spring water discharging along this fault-defined karst zone is chemically similar to hydrothermal CaCl2 basin-sourced waters as defined by Hardie (1990), and are interpreted as subsurface brines that have risen to the surface along deep faults to the north of the Dabuxum sump (Figure 9, 10; Spencer et al., 1990; Lowenstein and Risacher, 2009). Depths from where the Ca–Cl spring waters rise is not known. Subsurface lithologies of the Qaidam Basin in this region contains Jurassic and younger sediments and sedimentary rock columns, up to 15 km thick, which overlie Proterozoic metamorphic rocks (Wang and Coward, 1990).


Several lakes located near the northern karst zone (Donglin, North Huobusun, Xiezhuo, and Huobusun) receive sufficient Ca–Cl inflow, more than 1 part spring inflow to 40 parts river inflow, to form mixtures with chemistries of Ca equivalents > equivalents HCO3 + SO4 to create a simple potash evaporation series (this is indicated by the Ca-Cl trend line in Figure 10a). With evaporation such waters, after precipitation of calcite and gypsum, evolve into Ca–Cl-rich, HCO3–SO4-poor brines (brines numbered 5, 7-12 in figure 11a).


Dabuxum is the largest lake in the Qarhan region, with brines that are Na–Mg–K–Cl dominant, with minor Ca and SO4 (Figure 10d, 11a). These brines are interpreted by Lowenstein and Risacher (2009) to have formed from a mix of ≈40 parts river water to 1 part spring inflow, so that the equivalents of Ca ≈ equivalents HCO3 + SO4 (Figure 10b). Brines with this ratio of river to spring inflow lose most of their Ca, SO4, and HCO3 after precipitation of CaCO3 and CaSO4, and so form Na–K–Mg–Cl brines capable of precipitating carnallite and sylvite (Figure 11a). This chemistry is similar to that of ancient MgSO4-depleted marine bitterns (Figure 1)

The chemical composition of surface brines in the various lakes on the Qarhan Salt plain vary and appear to be controlled by the particular blend of river and spring inflows into the local lake/playa sump. In turn, this mix is controlled geographically by proximity to river mouths and the northern karst zone. Formation of marine-like ionic proportions in some lakes, such as Tuanje, Dabuxum and ancient Dadong Lake, engender bitterns suitable for the primary and secondary precipitation of sylvite/carnallite (Figures 10b-d; 11a). The variation in the relative proportion of sulphate to chloride in the feeder brines is a fundamental control on the suitability of the brine as a potash producer.

Figure 11b clearly illustrates sulphate to chloride variation in pore waters in the region to the immediate north and east of Dabuxum Lake. Brine wells in the low-sulphate area are drawn upon to supply feeder brines to the carnallite precipitating ponds. The hydrochemistry of this region is a clear indication of the regional variation in the sump hydrochemistry (Duan and Hu, 2001), but also underlines why it is so important to understand pore chemistry, and variations in aquifer porosity and permeability, when designing a potash plant in a Quaternary saline setting.

Compared to the MOP plant in Atacama, there as yet no lithium carbonate extraction stream to help ameliorate costs associated with carnallite processing. Lithium levels in the Qaidam brines, whilee levated, are much lower than in the Atacama brine feeds. Regionally, away from the Tuanje-Dadong area, most salt-lake and pore brines in the Qaidam flats are of the magnesium sulphate subtype and the ratio of Mg/Li can be as high as 500. With such brine compositions, the chemical precipitation approach, which is successfully applied to lithium extraction using low calcium and magnesium brines (such as those from Zabuye and Jezecaka Lake on the Tibetan Plateau and in the Andean Altiplano), would consume a large quantity of chemicals and generate a huge amount of solid waste. Accordingly, brine operations in the Qarhan region are focused on MOP production from a carnallitite slurry using extraction techniques similar to those utilised in the Southern Dead Sea. But owing to its cooler climate compared to the Dead Sea sump, the pond chemistry is subject to lower evaporation rates, higher moisture levels in the product, and a longer curing time.

Potash in the Qarhan region is produced by the Qinghai Salt Lake Potash Company, which owns the 120-square-kilometer salt lake area near Golmud (Figure 7). The company was established and listed on the Shenzhen Stock Exchange in 1997. Currently, it specialises in the manufacture of MOP from pore brines pumped from appropriate low-sulphate regions in the lake sediments (Figure 12). The MOP factory processes a carnallite slurry pumped from pans using a slurry processing stream very similar to the dual process stream utilized in the pans of the Southern Basin in the Dead Sea and discussed in the next section.

The final potash product in the Qaidam sump runs 60-62% K2O with >2% moisture and is distributed under the brand name of “Yanqiao.” With annual production ≈3.5 million tonnes and a projected reserve ≈ 540 million tonnes, the company currently generates 97% of Chinese domestic MOP production. However, China’s annual agricultural need for potash far outpaces this level of production. The company is jointly owned by Qinghai Salt Lake Industry Group and Sinochem Corporation and is the only domestic producer of a natural MOP product.

Dead Sea Potash (MOP operation in the Southern Basin)

The Dead Sea water surface defines what is the deepest continental position (-417 m asl) on the earth’s current terrestrial surface. In the Northen Basin is our only modern example of bedded evaporitic sediments (halite and gypsum) accumulating on the subaqueous floor of a deep brine body, where water depths are hundreds of metres (Warren, 2016). This salt-encrusted depression is 80 km long and 20 km wide, has an area of 810 km2, is covered by a brine volume of 147 km3 and occupies the lowest part of a drainage basin with a catchment area of 40,650 km2 (Figure 13a). However, falling water levels in the past few decades mean the permanent water mass now only occupies the northern part of the lake, while saline anthropogenic potash pans occupy the Southern Basin, so that the current perennial “Sea” resides in the Northern Basin is now only some 50 km long (Figure 13b).


Rainfall in the region is 45 to 90 mm, evaporation around 1500 mm, and air temperatures between 11 and 21°C in winter and 18 to 40°C in summer, with a recorded maximum of 51°C. The subsiding basin is surrounded by mountain ranges to the east and west, producing an orographic rain shadow that further emphasises the aridity of the adjacent desert sump. The primary source of solutes in the perennial lake is ongoing dissolution of the halokinetic salts of the Miocene Sedom Fm (aka Usdum Fm) a marine evaporite unit that underlies the Dead Sea and approaches the surface in diapiric structures beneath the Lisan Straits and at Mt. Sedom (Garfunkel and Ben-Avraham, 1996).

A series of linked fractionation ponds have been built in the Southern Basin of the Dead Sea to further concentrate pumped Dead Sea brine to the carnallite stage (Figure 13). On the Israeli side this is done by the Dead Sea Works Ltd. (owned by ICL Fertilisers), near Mt. Sedom, and by the Arab Potash Company (APC) at Ghor al Safi on the Jordanian side. ICL is 52.3% owned by Israel Corporation Ltd.(considered as under Government control), 13.6% shares held by Potash Corporation of Saskatchewan and 33.6% shares held by various institutional investors and the general public (33.64%). In contrast, PotashCorp owns 28% of APC shares, the Government of Jordan 27%, Arab Mining Company 20%, with the remainder held by several small Middle Eastern governments and a public float that trades on the Amman Stock Exchange. This gives PotashCorp control on how APC product is marketed, but it does not control how DSW product is sold.

In both the DSW and APC brine fields, muriate of potash is extracted by processing carnallitite slurries, created by sequential evaporation in a series of linked, gravity-fed fractionation ponds. The inflow brine currently pumped from the Dead Sea has a density of ≈1.24 gm/cc, while after slurry extraction the residual brine, with a density of ≈1.34 gm/cc, is pumped back into the northern Dead Sea basin water mass. The total area of the concentration pans is more than 250 km2, within the total area of 1,000 km2, which is the southern Dead Sea floor. The first stage in the evaporation process is pumping of Dead Sea water into header ponds and into the gravity-fed series of artificial fractionation pans that now cover the Southern Basin floor. With the ongoing fall of the Dead Sea water level over the past 60 years, brines from the Northern Basin must be pumped higher and over further lateral distances. This results in an ongoing need for more powerful brine pumps and an increasing problem with karst dolines related to lowered Dead Sea water levels. Saturation stages of the evolving pan brines are monitored and waters are moved from pan to pan as they are subject to the ongoing and intense levels of natural solar evaporation (Figure 13b, c; Karcz and Zak, 1987).

The artificial salt ponds of the Dead Sea are unusual in that they are designed to trap and discard most of the halite precipitate rather than harvest it. Most other artificial salt ponds around the world are shallow pans purpose-designed as ephemeral water-holding depressions that periodically dry out so that salts can be scrapped and harvested. In contrast, the Dead Sea halite ponds are purpose-designed to be permanently subaqueous and relatively deep (≈4m). Brine levels in the ponds vary by a few decimetres during the year, and lowstand levels generally increase each winter when waste brine is pumped back into the northern basin.

As the Dead Sea brine thickens, minor gypsum, then voluminous halite precipitates on the pan floor in the upstream section of the concentration series, where the halite-precipitating-brines have densities > 1.2 gm/cc (Figure 13c). As the concentrating brines approach carnallite-precipitating densities (around 1.3 gm/cc), they are allowed to flow into the carnallite precipitating ponds (Figure 13c). Individual pans have areas around 6-8 km2 and brine depths up to 2 metres. During the early halite concentration stages, a series of problematic halite reefs or mushroom polygons can build to the brine surface and so compartmentalise and entrap brines within isolated pockets enclosed by the reefs. This hinders the orderly downstream progression of increasingly saline brines into the carnallite ponds, with the associated loss of potash product.


When the plant was first designed, the expectation was that halite would accumulate on the floor of the early fractionation ponds as flat beds and crusts, beneath permanent holomictic brine layers. The expected volume of salt was deposited in the pans each year (Talbot et al., 1996), but instead of accumulating on a flat floor aggrading 15-20 cm each year, halite in some areas aggraded into a series of polygonally-linked at-surface salt reefs (aka salt mushrooms). Then, instead of each brine lake/pan being homogenized by wind shear across a single large subaqueous ponds, the salt reefs separated the larger early ponds into thousands of smaller polygonally-defined inaccessible compartments, where the isolated brines developed different compositions (Figure 14). Carnallitite slurries crystallised in inter-reef compartments from where it could not be easily harvested, so large volumes of potential potash product were locked up in the early fractionation ponds (Figure 14a, b). Attempts to drown the reefs by maintaining freshened waters in the ponds during the winters of 1984 and 1985 were only partly successful. The current approach to the salt reef problem in the early fractionation ponds is to periodically breakup and remove the halite reefs and mushrooms by a combination of dredging and occasional blasting (Figure 14c).


Unlike seawater feeds to conventional marine coastal saltworks producing halite with marine inflow salinities ≈35‰, the inflow brine pumped into the header ponds from the Dead Sea already has a salinity of more than 300‰ (Figure 15). Massive halite precipitation occurs quickly, once the brine attains a density of 1.235 (≈340‰) and reaches a maximum at a density of 1.24 (Figure 13c). Evaporation is allowed to continue in the initial halite concentrator ponds until the original water volume pumped into the pond has been halved. Concentrated halite-depleted brine is then pumped through a conveyance canal into a series of smaller evaporation ponds where carnallite, along with minor halite and gypsum precipitates (Figure 13c). Around 300–400 mm of carnallite salt slurry is allowed to accumulate in the carnallite ponds, with 84% pure carnallite and 16% sodium chloride as the average chemical composition (Figure 6a; Abu-Hamatteh and Al-Amr, 2008). The carnallite bed is harvested (pumped) from beneath the brine in slurry form and is delivered through corrosion-resistant steel pipes to the process refineries via a series of powerful pumps.

This carnallitite slurry is harvested using purpose-specific dredges floating across the crystalliser ponds. These dredges not only pump the slurry to the processing plant but also undertake the early part of the processing stream. On the dredge, the harvested slurry is crushed and size sorted, with the coarser purer crystals separated for cold crystallisation. The remainder is slurried with the residual pan brine and then further filtered aboard the floating dredges. At this stage in the processing stream the dredges pipe the treated slurries from the pans to the refining plant.


On arrival at the processing plant, raw product is then used to manufacture muriate of potash, salt, magnesium chloride, magnesium oxide, hydrochloric acid, bath salts, chlorine, caustic soda and magnesium metal (Figure 16a). Residual brine after carnallitite precipitation contain about 11-12 g/l bromide and is used for the production of bromine, before the waste brine (with a density around 1.34 gm/cc) is returned to the northern Dead Sea water mass. The entire cycle from the slurry harvesting to MOP production takes as little as five hours.

In the initial years of both DSW and APC operations, MOP was refined from the carnallite slurry via hot leaching and flotation. In the coarser-crystalline carnallitite feed, significant volumes of sylvite are now produced more economically in a cold crystallisation plant (Figure 16b). The cold crystallisation process takes place at ambient temperature and is less energy-intensive than the hot crystallisation unit. The method also consumes less water but requires a higher and more consistent grade of carnallite feed (Mansour and Takrouri, 2007; Abu-Hamatteh and Al-Amr, 2008). Both hot (thermal) and cold production methods can be utilized in either plant, depending on the quality of the slurry feed.

Sylvite is produced via cold crystallisation using the addition of water to incongruently dissolve the magnesium chloride from the crystal structure. If the carnallite slurry contains only a small amount of halite, the solid residue that remains after water flushing is mostly sylvite. As is shown in Figure 16b, if the MgCl2 concentration is at or near the triple-saturation point (the point at which the solution is saturated with carnallite, NaCl, and KCl), the KCl solubility is suppressed to the point where most of it will precipitate as sylvite. For maximum recovery, the crystallising mixture must be saturated with carnallite at its triple-saturation point. If the mixture is not saturated, for example, it contains higher levels of NaCl, then more KCl will dissolve during the water flushing of the slurry. Industrially, the cold crystallizers are usually fed with both coarse and fine carnallite streams, such that 10% carnallite remains in the slurry, this can be achieved by adjusting addition of process water (Mansour and Takrouri, 2007).

Successful cold crystallisation depends largely on a consistent high-quality carnallite feed. If a large amount of halite is present in the feed slurry, the resulting solid residue from cold crystallisation is sylvinite, not sylvite. This needs to be further refined by hot crystallisation, a more expensive extraction method based on the fact that the solubility of sylvite varies significantly with increasing temperature, while that of salt remains relatively constant (Figure 16c). As potash brine is hot leached from the sylvinite, the remaining halite is filtered off, and the brine is cooled under controlled conditions to yield sylvite.

Residual brine from the crystallisation processes then undergoes electrolysis to yield chlorine, caustic soda (sodium hydroxide) and hydrogen. Chlorine is then reacted with brine filtered from the pans to produce bromine. The caustic soda is sold, and the hydrogen is used to make bromine compounds, with the excess being burnt as fuel. Bromine distilled from the brine is sold partly as elemental bromine, and partly in the form of bromine compounds produced in the bromine plant at Ramat Hovav (near Beer Sheva). This is the largest bromine plant in the world, and Israel is the main exporter of bromine to Europe. About 200,000 tons of bromine are produced each year.

Residual magnesium chloride-rich solutions created by cold crystallisation are concentrated and sold as flakes for use in the chemical industry and for de-icing (about 100,000 tons per year) and dirt road de-dusting. Part of the MgCl2 solution produced is sold to the nearby Dead Sea Periclase plant (a subsidiary of Israel Chemicals Ltd.). At this plant, the brine is decomposed thermally to give an extremely pure magnesium oxide (periclase) and hydrochloric acid. In Israel, Dead Sea Salt Work’s (DSW) production has risen to more than 2.9 Mt KCl since 2005, continuing a series of increments and reflecting an investment in expanded capacity, the streamlining of product throughput in the mill facilities, and the amelioration of the effects salt mushrooms, and increased salinity of the Dead Sea due to extended drought conditions (Figure 17).

On the other side of the truce line in Jordan, the Arab Potash Co. Ltd. (APC) output rose to 1.94 Mt KCl in 2010 The APC plant now has the capacity to produce 2.35 Mt KCl and like the DSW produces bromine from bittern end brines. Early in the pond concentration stream, APC also has to remove salt mushrooms from its ponds, a process which when completed can increase carnallite output by over 50,000 t/yr. Currently, APC is continuing with an expansion program aimed at increasing potash capacity to 2.5 Mt/yr.


MOP brines and Quaternary climate

As mentioned in the introduction, exploited Quaternary potash deposits encompass both MOP and SOP mineral associations across a range of climatic and elevation settings. This article focuses on the three main MOP producing examples, the next deals with SOP Quaternary producers (Great Salt Lake, USA and Lop Nur, China). Interestingly, both sets of Quaternary examples are nonmarine brine-fed depositional hydrologies. All currently-active economic potash plants hosted in Quaternary systems do not mine a solid product but derive their potash solar evaporation of pumped hypersaline lake brines. For MOP processing to be economic the sulphate levels in the brines held in bittern-stage concentrator pans must be low and Mg levels are typically high, so favoring the precipitation of carnallite over sylvite in all three systems.

In Salar de Atacama the low sulphate levels in the bittern stage is accomplished by artificially mixing a CaCl2 brine from further up the evaporation stream with a less saline more sulphate-enriched brine. The mixing proportions of the two brine streams aims to maximise the level of extraction/removal of CaSO4 in the halite pans prior to the precipitation of sylvite and carnallite. In the case of the pans in the Qarhan sump there is a similar but largely natural mixing of river waters with fault-fed salt-karst spring waters in a ratio of 40:1 that creates a hybrid pore brine with a low sulphate chemistry suitable for the precipitation of both natural and pan carnallite. In the case of the Dead Sea brine feed, the inflowing Dead Sea waters are naturally low in sulphate and high in magnesium. The large size of this natural brine feed systems and its homogeneous nature allows for a moderate cost of MOP manufacture estimated in Warren 2016, chapter 11 to be US$ 170/tonne. The Qarhan production cost is less ≈ US$ 110/tonne but the total reserve is less than in the brine system of the Dead Sea. In Salar de Atacama region the MOP cost is likely around US$ 250-270/tonne, but this is offset by the production of a bischofite stage brine suitable for lithium carbonate extraction.

Outside of these three main Quaternary-feed MOP producers there are a number of potash mineral occurrences in intermontane depressions in the high Andes in what is a high altitude polar tundra setting (Koeppen ET), none of which are commercial (Figure 18a). Similarly, there a number of non-commercial potash (SOP) mineral and brine occurrences in various hot arid desert regions in Australia, northern Africa and the Middle East (Koeppen BWh) that we shall look at in the next article. In the Danakhil depression there is the possibility of a future combined MOP/SOP plant (see Salty Matters April 19, 2015; April 29, 2015; May 1, 2015; May12, 2015 and Bastow et al., 2018). In the Danakhil it is important to distinguish between the current non-potash climate (BWh - Koeppen climate) over the Dallol saltflat in Ethiopia, with its nonmarine brine feed and the former now-buried marine fed potash (SOP)/halite evaporite system. The latter is the target of current exploration efforts in the basin, focused on sediments now buried 60-120m below the Dallol saltflat surface. Nowhere in the Quaternary are such dry arid desert climates (BWh) associated with commercial accumulations of potash minerals.


Climatically most commercial potash brine systems in Quaternary-age sediments are located in cooler endorheic intermontane depressions (BWk, BSk) or in the case of the Dead Sea an intermontane position in the sump of the Dead Sea, the deepest position of any continental landscape on the earth’s surface (-417 msl). The association with somewhat cooler and or less arid steppe climates implies a need for greater volumes of brine to reside in a landscape in order to facilitate the precipitation of significant volumes of potash bitterns (Figure 18a,b).

In summary, all three currently economic Quaternary MOP operations are producing by pumping nonmarine pore or saline lake brines into a series of concentrator pans. The final bittern chemistry in all three is a low-sulphate liquour, but with inherently high levels of magnesium that favors the solar pan production of carnallite over sylvite that is then processed to produce the final KCl product. The brine chemistry in all three examples imitates the ionic proportions obtained when evaporating a ancient sulphate-depleted seawater (Figure 1). The next article will discuss the complexities (the double salt problem at the potash bittern stage when concentrating a more sulphate-enriched mother brine.

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Calcium Chloride (CaCl2), Article 2 of 2: CaCl2 minerals in evaporites

John Warren - Wednesday, May 31, 2017

 

Introduction

Found in association with some highly-saline calcium chloride brines are four naturally occurring CaCl2 minerals with evaporite associations, namely; 1) tachyhydrite CaMg2Cl6.12H2O), 2) antarcticite (CaCl2.6H2O), 3) sinjarite (CaCl2.2H2O) and 4) chlorocalcite (KCaCl3). For any CaCl2 salt to precipitate in a saline setting requires a Ca-enriched mother brine completely depleted in sulphate (Table 1). Otherwise, gypsum precipitates and in so doing removes all calcium from the concentrating brine, well before the bittern stage.

 

Of the four, tachyhydrite is perhaps the most common solid phase, sinjarite or chlorocalcite the least. Bischofite is sometimes a co-precipitate with tachyhydrite and indicative of a mother brine with an elevated MgCl2 content (Figure 1). Even so, tachyhydrite is still a rare bittern mineral phase across the Phanerozoic of Earth, although as we shall see later, the situation may be a little different on the martian surface. Along with carnallite and bischofite, tachyhydrite typifies highly saline marine bittern assemblages in only a few ancient potash-rich evaporite systems. These same systems are also MgSO4 poor, and tend to define times in Earth history of MgSO4-depleted seas. Occurrences include: Silurian potash in Michigan Basin; Carboniferous potash in the Canadian Maritimes and the Paradox Basin; Permian Zechstein-2 Stassfurt potash seams in Stassfurt, Germany; Triassic Saharan potash of North Africa; Cretaceous potash evaporites in Brazil, Gabon and Thailand and Oligocene potash in the Rhine Graben. Tachyhydrite-bearing successions are much less common within a broader MgSO4-poor group of potash deposits (see Warren 2016, chapters 2 and 11; Warren, 2017).


To form antarcticite (CaCl2).6H2O) at the bittern stage, a CaCl2 brine must be depleted in Mg, otherwise tachyhydrite forms, along with carnallite and bischofite. Carnallite, bischofite and tachyhydrite are not part of the bittern suite precipitated from today’s MgSO4-rich oceans, where sulphate and magnesium levels are too high relative to calcium. At times in the past, these three salts were more common precipitates in the bittern stages of ancient CaCl2 or MgSO4-depleted oceans. These tended to be at times when rates of seafloor spreading, and hence volumes of through-circulated seawater, were greater than today (Lowenstein et al., 2014). It seems that most the highly saline calcium chloride brines form in sedimentary basins with a thick evaporite near the base of the sediment fill (see article August 11, 2015). But, not all are basins with highly saline CaCl2 brines are co-associated with bedded tachyhydrite occurrences in the buried and dissolving evaporite units.

Antarcticite

Antarcticite (CaCl2.6H2O) is extremely hygroscopic, colourless with perfect basal cleavage and good to perfect prismatic cleavage, is brittle with a hardness between 2 and 3, and has a specific gravity of 1.715 ± 0.010 (14 °C), and a density of 1.700 g/cm3 (Dunning and Cooper, 1969). The lowest temperature for antarcticite to precipitate is -50 °C and it’s melting point is 30 °C.

There are only two known modern hypersaline saline deposits where the pure calcium chloride mineral antarcticite (CaCl2.6H2O) is present as thin crystal beds and efflorescences; they are, Bristol Dry Lake (BWh) in California and Don Juan Pond (EF) in the Wright Valley, near Lake Vanda in Antarctica (Dunning and Cooper, 1969; Horita, 2009). Both saline lakes possess similar CaCl2-enriched, MgCl2-depleted chemistries and are fed in part by hydrothermal waters. In Bristol Dry Lake the concentration process is driven by solar evaporation, while cryogenesis is the primary driver in Don Juan Pond. Only in Bristol Dry Lake is a CaCl2 brine is commercially extracted from Quaternary sediments. There is also a minor occurrence as isolated small crystals in the Kunteyi Lake of the Tibetan Plateau (Kezao and Bowler, 1986; Zheng and Liu, 2009)


Don Juan Pond, Antarctica

Don Juan Pond is a perennial free-standing water body, approximately 300 m long and 100 m wide, surrounded by glacial moraine deposits in the Wright Valley, Antarctica (Figure 2). It defines the outcrop portion of a liquid water drainage sump that retains a 10-cm-deep CaCl2 brine at the surface (Torri and Ossaka, 1965; Marion, 1997; Burt et al., 2003) (Figure 2). This pool contains some of the saltiest free-standing perennial waters on the earth’s surface, with concentrations ranging up to 40% (by mass; Dickson et al., 2013). Mean annual precipitation the Don Juan Pond region is 5 to 10 cm and air temperatures range from –55°C to +10°C (Marion, 1997). The pond does not freeze because it is a eutectic brine of H2O, CaCl2, and NaCl, with a freezing point of -52°C and a density of approximately 1.4 (Marion, 1997; Burt et al., 2002). Crystals of antarcticite were first found sticking out of the brine at the bottom of the pond, with halite and gypsum fringes at the edges of the pond (Torri and Ossaka, 1965).

Precipitation of antarcticite in this cryogenic environment is the result of the arid climate, ongoing cold temperatures and a CaCl2 brine chemistry (Torri and Ossaka, 1965). The CaCl2 brine is in part derived from springs fed by a deep, confined, basaltic aquifer, with a groundwater similar in composition to the surface brine (Harris et al., 1979; Marion, 1997; Burt et al., 2002). The origin of the CaCl2 is still contentious, but is most likely be related to mineral weathering and periodic deliquescence of calcium chloride held in the sediments(Dickson et al., 2013).


Brine in the Don Juan Pond is predominantly from CaCl2 solutions (Figure 3; Marion, 1997). Only Na of the remaining cations contributes significantly to the overall charge balance. Both Mg and K are present at very low concentrations, and SO4 is usually not detectable. In contrast to low ionic variability in seawater composition, levels of the dominant ions (Ca, Na, and Cl) in the Pond are highly variable (Figure 3). The concentration of the brine varies seasonal, controlled mainly by a complex hydrologic system including groundwater discharge, melting of nearby permafrost, rare precipitation, and evaporation or sublimation (Harris et al., 1979; Dickson et al., 2013). Despite the changes in the absolute concentration of Ca and Cl across an annual cycle, the Ca/Cl molal ratio remained relatively constant over the 23-year sampling period; it varies between 0.432 and 0.506 with an average of 0.462 ± 0.018 (Figure 2; Marion, 1997).

Bristol Dry Lake, California

Bristol Dry Lake is a 155 km2 continental playa located within a closed Basin and Range valley in the Mojave Desert of southern California (Figure 4a). It is the second known location of natural antarcticite and is thought to precipitate from lake brines via a lowering of brine temperatures (Dunning and Cooper, 1969). No antarcticite was identified in the nearby Cadiz or Danby dry lakes. Lake brines first concentrate by solar evaporation beyond halite precipitation Mountains surrounding Bristol Dry Lake are composed mostly of pre-Cambrian and Mesozoic granite, some Palaeozoic carbonates, and Tertiary and Pleistocene volcanic rocks including Quaternary olivine basalts that erupted from nearby Amboy Crater. Alluvial fans are present between the mountains and the lowest parts of the valley (Dunning and Cooper, 1969; Handford, 1982).


Bristol Dry Lake playa contains more than 300 m of interbedded halite and salt-bearing sands, silts and clay. Halite and gypsum are currently deposited from ephemeral bodies of water under evaporative conditions, with a gypsum pavement surrounding more saline halite-saturated sumps in the central lake depression (Figure 4a; Dunning and Cooper, 1969; Handford, 1982; Rosen and Warren, 1990). The primary evaporite minerals at Bristol Dry Lake are gypsum, anhydrite, and halite with rare antarcticite, celestite (SrSO4), calcite, and possibly chlorocalcite (KCaCl3). Bristol Dry Lake is mined for its chloride minerals and has been since the 1900s, and so is covered with pits and trenches (Figure 4b).

Antarcticite was identified in one of these previously excavated trenches approximately 9 kilometres south of Amboy, near the road from Amboy to Twenty-nine Palms. Dunning and Cooper (1969) examined this and other trenches during the winters of 1961, 1962, and 1968. Only one trench had antarcticite. The trench was 3 m deep, 20 m long, and contained a 10 to 25 cm deep pool of CaCl2 brine extending along the length of the trench. At the base of the pit was a massive halite deposit, with acicular groups of antarcticite protruding outward from the halite and within the brine. Antarcticite is not present on the playa surface as no surface brines are present. The only CaCl2 brines exposed to solar radiation are subsurface brines that have seeped into, and are now exposed within, the excavated trenches.


Dunning and Cooper (1969) suggested that CaCl2 brines at Bristol Dry Lake originated from chlorine of volcanic origin interacting with calcium carbonate in surrounding alluvial deposits (Figure 5). Lowenstein et al. (2009) argue that the nearby active magma chamber drives the formation of Ca-Cl brines at elevated temperatures, as well as the thermally-driven transportation of these brines to the surface. Other evidence for the magma chamber is the Amboy crater and its associated recent lava flows, which occur directly North of Bristol Lake. Evaporative concentration of this calcium chloride brine, driven by an arid climate and typical surface temperatures below 30°C, encourage antarcticite precipitation (Dunning and Cooper, 1969).

Sinjarite (CaCl2.2H2O)

Sinjarite is a modern ephemeral precipitate in surficial alluvial fan sediments near Sinjar in Iraq (Figure 6). No natural occurrences of either antarcticite or sinjarite have been documented in ancient salts, except as a volumetrically insignificant mineral phase in brine inclusions in both sediments and igneous rocks (Polozov et al., 2016), even in sedimentary basins containing highly saline CaCl2-rich pore fluids.


Sinjarite is an extremely rare pink-tetragonal-hygroscopic mineral that is highly soluble with 117 g dissolving in 100 g of water at 20 °C (Table 1; Aljubouri and Aldabbagh, 1980). The lowest temperature for sinjarite to precipitate is 42.3 °C, while its melting point is 175.5 °C (Wardlaw, 1972). Theoretical composition of sinjarite is 27.26% Ca, 48.24% Cl, and 24.50% H2O but the actual chemistry for sinjarite present at its type locality in Iraq is 25.84% Ca, 46.64% Cl, 26.55% H2O, and 0.85% Na with 226 ppm K and trace amounts of Mg, Sr, and Fe (Aljubouri and Aldabbagh, 1980). Properties include; good cleavage, average density of 1.81 g/cm3, a very deliquescent habit, hardness of 1.5, vitreous to resinous lustre, white streak, and occurs naturally in massive to granular forms.

Little research has been performed on sinjarite and knowledge of the environment of formation comes from the only place it was discovered. Sinjarite occurs in detritus in a wadi cutting through the Sinjar anticline near the town of Sinjar, Iraq (west of Mosul). Sinjarite precipitates via slow evaporation of groundwater saturated with Ca and Cl ions. CaCl2 in the groundwater solution must be 3.5 times greater than NaCl for sinjarite to precipitate instead of halite. The extremely deliquescent nature of sinjarite means that the mineral is ephemeral and quickly dissolves in wet seasons or changes to the hexahydrite antarcticite when the temperature is less than 30°C (Aljubouri and Aldabbagh, 1980).


CaCl2 salts and liquid water flows on current-day Mars

On the current Martian surface it has been recently proposed that aqueous solutions form seasonally, via the deliquescence of hygroscopic salts in contact with atmospheric water vapor. Regions of these hygroscopic salts are thought to be indicated by occurrences of surface features known as recurring slope linae - RSL (Figure 7a, b; Chevrier and Rivera-Valentin, 2012). Older studies largely focused on perchlorate species as the most likely set of hydroscopic salts driving the periodic formation of RSLs, but another Mars-relevant set of salts, with similar low eutectic temperatures, relevant to ambient conditions with a significant deliquescence potential, are the calcium chloride hydrates, antarcticite and sinjarite. Gough et al. (2016) propose hydrated calcium chloride salts are linked to RSL formation on Mars. They also note deliquescence of CaCl2, not perchlorate, is also known to be responsible for the only terrestrial RSL analog known thus far: these are the seasonal water tracks in the McMurdo Dry Valleys, especially in the vicinity of the Don Juan Pond, as documented by Dickson et al., 2013 (Figure 8).

Soluble regolith salts like perchlorate and calcium chloride salts with low eutectic temperatures are likely to deliquesce at low relative humidity (RH) values at a wide range of temperatures (Gough et al., 2016). Deliquescence is the process by which a solid crystalline phase absorbs water vapor to form a saturated aqueous (liquid) solution. This phase transition from solid salt to liquid brine occurs at a deliquescence relative humidity (DRH), the value of which is specific for each hydration state of each salt and often varies with temperature. A salt is expected to be aqueous whenever the temperature is above the eutectic temperature of the salt and the relative humidity is above the DRH, although at much higher RH values ice may form. The low temperature deliquescence of perchlorates has been extensively studied (Gough et al., 2011, 2014; Nuding et al., 2014; Zorzano et al., 2009). Many perchlorate species have deliquescence relative humidity (DRH) values below 40% RH, and so should deliquesce and be stable or metastable liquids under Martian surface conditions (Chevrier et al., 2009), although slow reaction kinetics may limit their formation (Fischer et al., 2014). Not as much attention has been paid to other soluble, deliquescent salts, especially the calcium chloride salts, that may be present in the martian regolith.

Chlorine has been detected by rovers and landers in every Martian soil sample analyzed to date and is found at similar concentrations (≈0.2 to 1%) in all locations (Glavin et al., 2013). The form of the chlorine (i.e. chloride vs. perchlorate) is unknown at some sites, although measurements of the regolith at the Phoenix landing site confirmed chloride, perchlorate (Hecht et al., 2009) and probably chlorate (Hanley et al., 2012). The identity of the associated cation(s) is also generally unknown, although calcium, sodium, magnesium and potassium are the most likely candidates (Hecht et al., 2009). Regionally and globally widespread chloride deposits have also been detected by orbiting spectrometers (Figure 7c; Osterloo et al., 2010; Keller et al., 2006).

The global distribution of chloride deposits across the Martian surface is similar to that of recurring slope lineae (RSL), prompting hypotheses of a relationship between the two (Figure 7c; McEwen et al., 2011; Stillman et al., 2017). Chevrier and Rivera-Valentin (2012) suggest that CaCl2 is one of the best candidates for the formation of RSL, the narrow, dark features that appear and grow seasonally on Mars and appear to be caused by flowing liquid.


Two hydrated chloride salts, MgCl2 and CaCl2, may be the most appropriate salts because the seasonality of observed RSL formation best matches the seasonality of the melting of ice associated with these salts (Chevrier and Rivera-Valentin, 2012). Specifically, these chloride salts have eutectic temperatures less than or equal to the threshold temperature of 250 K that seems to mark the start of RSL activity (McEwen et al., 2011), but the eutectic temperatures of these salts are not so low that brines on Mars would be permanently liquid. Additionally, calculations show that CaCl2 in particular will cause seasonal melting of water ice throughout the top 20 cm of the martian regolith, therefore providing greater potential fluid flow than other salts (Chevrier and Rivera-Valentin, 2012). More recently, hydrated chloride salts (although not specifically calcium chloride) were observed to be present in RSL but not in surrounding regions (Ojha et al., 2015). It is not generally believed that deliquescence could be fully responsible for RSL formation because the small amount of water vapor in the martian atmosphere would limit the condensed phase water that could form. It is certainly possible, however, that salt deliquescence may be involved in the formation of RSL or in their appearance or behavior (McEwen et al., 2011; Ojha et al., 2015; Dickson et al., 2013; Stillman et al., 2017).

In summary, determining whether liquid water exists on the Martian surface is central to understanding the hydrologic cycle and potential for extant life on Mars (Ohja etal., 2015; Stillman et al., 2017). Recurring slope lineae, narrow streaks of low reflectance compared to the surrounding terrain, appear and grow incrementally in the downslope direction during warm seasons when temperatures reach about 250–300 K (-23°C to +26°C), a pattern consistent with the transient flow of a volatile material. Brine flows (or seeps) associated with seasonal deliquesence of hydrated salts (possibly hydrated calcium chloride salts or perchlorates) are proposed to explain the formation of recurring slope linea. As yet, no direct evidence for either liquid water or actual hydrated salt mineralogies has been found.


Tachyhydrite

Tachyhydrite (CaMg2Cl6.12H2O) (occasionally spelled tachydrite or tachhydrite) is a yellow, transparent to translucent, trigonal-rhombohedral mineral with very high solubility. Pure tachyhydrite is composed of 7.74% Ca, 9.39% Mg, 41.10% Cl, 37.09% O, and 4.67% H (Wardlaw, 1972). Its chemical formula is most accurately expressed as [Mg(H2O)6]2[CaCl6] because the structure consists of Mg(H2O)6 octahedra and CaCl6 octahedra loosely linked together by hydrogen bonds. Physical properties include; good cleavage, vitreous to greasy lustre, white streak, massive habit, bitter taste, deliquescent habit (dissolves or liquefies upon exposure to air), a density of 1.66 g/cm3, a hardness of 2, and it is hygroscopic (Braitsch, 1971).

Tachyhydrite is extremely soluble with 160 g dissolving in 100 g of water at 20 °C, and its solubility increases with increasing temperature (D’Ans, 1961; Wardlaw, 1972). Its hygroscopic and extremely deliquescent nature means at earth surface temperatures a crystal becomes liquid by absorbing moisture in the air. On exposure, tachyhydrite quickly alters to a residue of bischofite (MgCl2.6H2O) and a CaCl2.nH2O phase (Wardlaw, 1972). This is why tachyhydrite is typically documented in natural occurences where a crystal is still encased in halite. Tachyhydrite has a wide thermal stability field from 21.95°C to at least 167°C (Braitsch, 1971; Clark et al., 1980). The lowest temperature for tachyhydrite precipitation is 21.95°C at a concentration of 92.7 mol CaCl2/1000 mol H2O (450 g CaCl2/liter (D’Ans, 1961; Braitsch, 1971; Wardlaw, 1972). Its lowest temperature of formation increases by 0.0162ºC for every atmosphere of pressure (D’Ans, 1961).

Modern tachyhydrite occurrences

Natural tachyhydrite is documented only in a few modern hypersaline settings and it never forms a bed with primary precipitation textures (Figure 9). It is found in greater quantities in some ancient potash deposits. Tachyhydrite occurs seasonally as a minor interstitial cements and efflorescences, along with antarcticite (the hexahydrate form of calcium chloride), in the modern ephemeral halite crusts, atop sabkhas, of the Gavkhoni Playa (BWk), southeast of Isfahan, Iran (Pakzad and Ajalloeian, 2004), in mine wates in the Salar de Pedenales in the andean Altiplano, and perhaps as minor salt effloresecnes in the uppermost parts of the Abu Dhabi sabkha (Wood et al., 2005).

Much of the elevated ionic content of various Iranian playas in the Great Kavir is due to salt dissolution of the crests of nearby at- or near-surface diapirs and namakiers, where the mother salt sourced in halokinetic Miocene marine salts (Warren, 2008). These outcropping diapirs have carried carnallite and sylvite remnants into the namakiers that now reside at the surface (Rahimpour-Bonab and Kalantarzadeh, 2005). Tachyhydrite is also found as white feathery efflorescences within waste piles at an abandoned borate working along the southwestern margin of Salar de Pedernales, Chile (a BWk Koeppen climate location very near the ET climate boundary; Ericksen et al., 1989).

It seems that tachyhydrite’s high solubility means it has little or no preservation potential as a solid salt in any modern at-surface depositional setting; in the Peruvian case, it is a winter precipitate that disappears with the end of winter. In Abu Dhabi it, along with other highly soluble salts in the salty surface efflorescences, can disappear in morning dew (Warren, pers. obs.). In both the Iranian and the Peruvian settings the climate is cool and elevated. It seems the arid desert environments where bedded subaqueous-textured tachyhydrite accumulated in the drawndown Aptian seepage basins of the opening Atlantic were very different to its occasional rare occurrence in the evaporite settings of today.

Cretaceous of Brazil & W. Africa

Lower Cretaceous (Aptian) evaporite deposits in Sergipe, Brazil, and Gabon and Congo in western Africa contain significant amounts of tachyhydrite in halite-carnallite beds, along with other SO4-poor bitterns (Figure 10; Wardlaw, 1972; Borchert, 1977; de Ruiter, 1979; Hardie, 1990; Garrett, 1995; Zhang et al., 2017). These evaporite basins formed during the early Cretaceous rifting of Africa and South America. In each basin, potash-rich halite-dominant evaporites are transitional between older continental pre-rift and rift siliciclastics and younger post-rift marine shales and carbonates (Wardlaw, 1972, Borchert, 1977; de Ruiter, 1979; Szatmari et al., 1979; Hardie, 1990; Garrett, 1996).


The basic salt cycle of the Gabon and Congo basins includes from bottom to top: (1) thin black shale, (2) halite, (3) combination of halite and carnallite (carnallitite, and (4) bischofite and/or tachyhydrite (Figure 10;de Ruiter, 1979; Zhang et al., 2017). Although variations exist, a similar sequence is present in Sergipe, Brazil comprising the Ibura Member of the Muribeca Formation. The Ibura Member includes from bottom to top: (1) halite, (2) carnallite, (3) tachyhydrite, (4) sylvite (KCl) and halite, and (5) anhydrite (CaSO4) (Wardlaw, 1972; Borchert, 1977; Garrett, 1996). Tachyhydrite is located within the central and deepest portions of the Sergipe basins (Wardlaw, 1972; Borchert, 1977; Szatmari et al., 1979).

The origin of the evaporite sequences in Sergipe, Gabon and Congo is controversial with some authors suggesting a marine origin (Wardlaw, 1972; de Ruiter, 1979; Szatmari et al., 1979) or diagenetic origin (Borchert, 1977). More recent literature discussing Sergipe, Gabon, Congo, and other similar tachyhydrite deposits cite a non-marine/hydrothermal origin based on geochemical and textural studies (Lowenstein et al., 1989; Hardie, 1990; Garrett, 1996; El Tabakh et al., 1999). Yet other recent authors indicate a marine source based on its dominance in beds with a CaCl2 oceanic timing (Warren, 2016; Zhang et al., 2017).

The brine that tachyhydrite precipitated from must have been high in Ca because in waters with more SO4 than Ca present, the Ca is used-up by gypsum or anhydrite. Only if the Ca level is greater than SO4 in the early stages of precipitation will Ca stay in the brine allowing for potential precipitation of tachyhydrite (Wardlaw, 1972; Hardie 1990). The CaCl2-rich brine that produced the tachyhydrite and other potash salts is thought to have formed either by the interaction of hot meteoric groundwater with rift-related sediment and bedrock, or an influx of seawater at a time of a MgSO4-depleted ocean. Based on figure 1, Hardie (1990) suggests heated seawater could also have interacted with sediment and/or bedrock to produce a CaCl2 brine. This hot CaCl2 brine was perhaps driven to the surface by thermal circulation (Hardie, 1990). Alternatively, without arguing that the proportions of major ions in seawater change over time, the fact that modern marine water cannot precipitate tachyhydrite precludes it being the dominant mother brine in the Sergipe, Congo, and Gabon Basin evaporite basins,  (Lowenstein et al., 2014; Warren  2016).

Textures recovered in core, and visible in mine walls in Brazil, indicate Sergipe tachyhydrite was a primary bedded precipitate in those circum-Atlantic Cretaceous evaporite basins with stratiform tachyhydrite units in potash beds (Figure 10; Wardlaw, 1972). The Cretaceous world ocean at the time this tachyhydrite precipitated was a MgSO4-free marine bittern. Figure 9 shows tachyhydrite does not occur as a bittern phases in many other potash sequences deposited in basinwide settings at other times of CaCl2 oceans (e.g. the Devonian of Canada, the Silurian of the Michigan Basin). This has led to the argument that stratabound tachyhydrite in potash beds indicates an additional hydrothermal/basinal source for the mother brine. If so, the presence of tachyhydrite is not solely indicative of a MgSO4-depleted marine feed to the basin (Hardie 1990).

Cretaceous occurrences in Thailand

Another significant tachyhydrite occurrence is within slightly older mid Cretaceous (Cenomanian) evaporites of the Maha Sarakham Formation on the Khorat Plateau of northeastern Thailand (Figure 11). The majority of the Maha Sarakham Formation consists of alternating anhydrite and halite, separated by mudstone and siltstone “redbeds”. A 10- to 75-m-thick salt unit overlies a lower halite unit and underlies mudstone red beds of the lower clastic unit. Carnallite is most abundant in this salt unit, but sylvite and halite are also locally abundant. Tachyhydrite is present with halite and carnallite, but not with sylvite and is perhaps concentrated more in former basin centre or sump positions (El Tabakh et al., 1999; Warren 2016). Hite and Japakasetr (1979) estimated that tachyhydrite comprises less than 30% of the total carnallitite deposit, but tachyhydrite can form pure layers up to 16 m thick. Tachyhydrite crystals are euhedral and average 1 mm in size (El Tabakh et al., 1999). Sylvite and halite are also associated with accessory amounts of hilgardite [Ca2BCl(OH)2] and boracite (Mg3ClB7O13).


The origin of the Maha Sarakham Formation evaporites is still contentious; Hite and Japakasetr (1979) advocate a purely marine origin (highly restricted and concentrated sea), Hardie (1990) advocating an exclusively non-marine origin (hydrothermal CaCl2 brines), and El Tabakh et al. (1999) advocate an alternating marine and non-marine origin. They envisage an inland basin that was periodically inundated by the rising sea and at the time of tachyhydrite deposition received an additional influx of hydrothermal CaCl2 brines. Hardie (1990) states that formation of Maha Sarakham evaporites may have been similar to that of tachyhydrite formation in the Cretaceous basins of Brazil and western Africa, although he offers no evidence for the origin of the hydrothermal CaCl2 brines. El Tabakh et al. (1999) suggested that hydrothermal CaCl2 waters entered the restricted marine basin and created the right conditions for tachyhydrite precipitation and they cite contemporaneous granitic intrusions as possible evidence of thermal activity during the time of tachyhydrite formation.

Hydrothermal oceanic CaCl2 brine

Today, large volumes of relatively dilute, oceanic CaCl2 brines form via hydrothermal circulation and thermally-driven alteration of mid-ocean ridge (MOR) lithosphere (see part 1). These CaCl2 waters occur in and near active fracture zones, wherever seawater interacts with labile basalt (oceanic crust) at elevated temperatures and converts the circulating fluid from a Na-Mg-Cl water into a low-salinity Na-Ca-Cl brine, depleted in Mg and SO4. Similar hydrothermally-driven alteration of continental basalts via deeply circulated seawater interactions forms modern CaCl2-rich brine seeps, for example, within the thermally active continental Danakil rift valley (Hardie, 1990).

Tachyhydrite is a 'Swift Water" mineral

There may be an additional factor at work in terms of our recognition of tachyhydrite’s basin-by-basin distribution across the Phanerozoic. That factor is tachyhydrite’s high solubility in most subsurface waters and drilling muds. Even the name, tachyhydrite, was chosen from the Greek tachy hydros for “swift water” to indicate its rapid deliquescence on exposure to atmospheric conditions. This property means that any ground-up remains (rock chips) of tachyhydrite in a return mudstream in a conventional drilling operation typically do not make it to the surface. So, as most oil companies prefer not to core evaporites when drilling for hydrocarbons trapped beneath salt, there may be more tachyhydrite occurrences in deeply buried basin evaporites than the few currently documented.

Ancient tachyhydrite has only been found encased in halite in recovered cores and mine walls, never in drill cuttings, and its presence or absence can be easily misinterpreted in wireline logs, especially when drilling through thick salt masses in the search for oil and gas (companies tend to run a more limited log suite in thick salt units).

 

Tachyhydrite’s lack of shear strength and extremely high solubility makes mining any associated potash beds difficult; it presence leads to head beam and floor stability problems in conventional underground mines in Brazil and West Africa. Holle, a West African potash mine in the 1970s, was never economical and was lost to flooding after a few years of conventional mining and the Brazilian mine has ongoing floor and wall stability problems ties to tachyhydrite-induced heave. Today, solution mining is proposed as a more efficient means to exploit areas of Cretaceous potash associated with tachyhydrite beds, as in Aptian halite-potash associations on both sides of the Atlantic. High levels of tropical humidity mean at-surface potash evaporation pans are not viable in either area, so brine concentration and processing will probably require a more expensive option involving motorised dryers.

 

Not only is tachyhydrite highly soluble, but it is also highly reactive whenever in comes into contact with subsurface bitterns. Based on its occurrence in the Permian Stassfurt series in Germany, D’Ans (1961) suggested that much of the widespread polyhalite found in the CaCl2 brine-rich parts of the Zechstein Basin was a reaction product indicating former tachyhydrite. He noted that, at temperatures higher than room, there is a reaction between gypsum/anhydrite, sylvite and tachyhydrite-bischofite, or their dissolution-related brines, which removes tachyhydrite to form diagenetic polyhalite and calcium chloride brines;

CaMg2Cl6.12H2O+4KCl+8CaSO4--> 2CaSO4Mg(SO4)4(H2O)2 + 5CaCl2 + 4H2O

This was the mechanism suggested by Manheim (1974) to explain the lack of tachyhydrite, and the formation CaCl2-rich brines that characterise hydrothermal pore waters circulating in sediments associated with buried Miocene evaporites along the deep axial trough of the Red Sea.

Terrestrial CaCl2 minerals across time

The high solubility of CaCl2 minerals means that, with the exception of some parts of the Aptian Atlantic Potash association, even in past times of likely CaCl2-rich oceans, calcium chloride minerals are relatively rare as primary-textured occurrences in solid salt beds. On the other hand, as we saw in part 1, CaCl2 brines are commonplace in basinal or formation waters in many Phanerozoic sedimentary basins entraining thick sequences of dissolving ancient salt. For example, since 1914, CaCl2 brines have been extracted from Silurian strata adjacent to Salina Salt of the Michigan Basin USA, yet there are no significant volumes of tachyhydrite documented in the Michigan Basin(Garrett, 2004).

Historically, before micro-inclusion studies of chevron halite showed that the ionic proportions of seawater likely varied across the Phanerozoic, the various CaCl2 basinal brines occurrences in Silurian and Cretaceous strata were explained as an indicator of widespread dolomitisation and other diagenetic reactions, which preferentially extracted magnesium from pore waters. Since then, some authors have argued that CaCl2 enrichments in many ancient basinal brines, including brines in the Detroit group are partial leftovers of primary seawater chemistries (Lowenstein and Timofeeff, 2008). That is, Ca-enriched (MgSO4-depleted) pore brines are indicators of ancient CaCl2 oceans, with the pore brines being remnants from time the enclosing evaporitic and marine sediments were deposited (relict or connate brines).

Others, such as Houston et al. (2011), conclude this is not necessarily so, they agree that there are two end-members typifying highly saline subsurface brines. But they conclude that end-member chemistries relate to either substantial subsurface halite dissolution, or to preservation of early reflux-related seawater. Houston et al. (2011) go on to argue that CaCl2-enriched formation water chemistries from many basins worldwide, including the Michigan Basin, do not support an interpretation of variation in ionic proportions in seawater across the Phanerozoic. They find that CaCl2-rich brines formed either by dissolving bittern salts in the subsurface, or simply lost water in the subsurface after significant rock-fluid interaction had taken place. Water loss might be achieved by interaction with a gas phase at the elevated temperatures of deep burial or, alternatively, water may have been lost to clays. Both these mechanisms would have the effect of dehydrating (concentrating) the brine.

However, whatever the mechanism, it does seem that there is a Quaternary continental/pre-Quaternary marine dichotomy in the nature and distribution of CaCl2 brines and the associated antarcticite/tachyhydrite minerals, much as there is for the world’s potash deposits (MgSO4-enriched versus MgSO4-depleted). On Earth, there are no documented examples of pre-Quaternary bedded antarcticite. The situation may be different on Mars.

Today terrestrial antarcticite either precipitates in the hot-arid Bristol Dry Lake or the icecap-polar setting of Don Juan Pond, neither is associated with brine sourced in seawater or derived by the recycling of older (“connate”) marine evaporite successions. Rather the occurrenceof the calcium chloride minerals is in part a reponse to low temperatures and regolith cycling. Other present-day examples of regions with somewhat enriched levels of CaCl2 surface waters, lack both tachyhydrite and antarcticite precipitates, and contain higher levels of Mg that are tied to deeply circulated marine/hydrothermal waters and variable interactions with MgSO4-enriched marine salts (e.g., Lake Asal and the Dallol Salt Pan). CaCl2 brines of the Dead Sea and the Red Sea show even more elevated levels of MgCl2 as they are derived, at least in part by the fractionated dissolution of bedded and halokinetic Miocene marine evaporites. The Mg-rich clay palygorskite is co-associated with these CaCl2 brines in the subsurface of the Red Sea and the Dallol.

In contrast, tachyhydrite in some ancient marine-fed basinwide evaporite associations is found as somewhat rare, but at times bedded, units in the bittern-rich portions of the halite succession, as in the Cretaceous basinwide evaporites of Brazil, the Congo and Thailand, and as a cementing phase in the Permian Stassfurt 2 in the Zechstein (where it is commercially extracted in association with MgCl2 brines).

All the ancient tachyhydrite examples mentioned above, are associated with the presence of widespread potash salts within adjacent salt beds. However, there are many other even larger and richer ancient potash deposits, such as the intracratonic Alberta basin (Devonian) and the Kama basins (Permian) where no tachyhydrite is documented. It seems that the terrestrial precipitation of bedded tachyhydrite is not just favoured by times of CaCl2 oceans, it also requires additional input from saline hydrothermal/basinal waters. Such settings are most likely in the transition phases in an actively-opening hydrothermally-influenced continental rift as passes into the marine seepage realm at a time when the adjacent ocean was a MgSO4-depleted system.

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Rahimpour-Bonab, H., and Z. Kalantarzadeh, 2005, Origin of secondary potash deposits; a case from Miocene evaporites of NW Central Iran: Journal of Asian Earth Sciences, v. 25, p. 157-166.

Renner, T., 1912, Uber Baeumlerit, ein neues Kalisalzmineral. Centralbl. Mineralogie, p. 106-107.

Rosen, M. R., 1991, Sedimentologic and geochemical constraints on the evolution of Bristol Dry Lake Basin, California, U.S.A.: Paleogeography Paleoclimatology Paleoecology, v. 84, p. 229-257.

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Calcium Chloride (CaCl2), Article 1 of 2: Usage and brine chemistry

John Warren - Sunday, April 30, 2017

 

Introduction

Calcium chloride minerals in the natural state are rare and only found in a few specific evaporite associations. On the other hand, calcium chloride-rich brines are commonplace in the burial diagenetic realm, especially in deep high-salinity basinal brines and in a number of hypersaline lake waters, especially in rift settings. In the subsurface, these brines also play a significant role in the formation of a number of metal ores. Occurrences of both the brine and the minerals have significance in modelling rock-fluid interactions and seawater chemistry across geological time.

At earth-surface temperatures, calcium chloride can exist in the solid state as the anhydrous form (CaCl2) as well as in four levels of hydration – CaCl2.H2O; CaCl2.2H2O; CaCl2.4H2O; CaCl2.6H2O (Table 1). Of this group, CaCl2 occurs naturally as two rare minerals; antarcticite and sinjarite. All of the early studies on calcium chloride and its hydrates were done with laboratory-prepared samples of brines and hydrates, since CaCl2 was not produced on a commercial scale until after the ammonia–soda process for the manufacture of soda ash (Solvay Process) was in operation. Before its industrial uses were discovered, calcium chloride was considered a waste product of brine production. Today, its primary industrial use is predicated on the very high enthalpy change of solution, indicated by considerable temperature rise accompanying dissolution of the anhydrous salt in water (Table 1 – Heat of solution in water). This property is the basis for its largest scale application, namely road de-icing.

 

In the natural state, most CaCl2 occurs in solution in basinal waters in sedimentary basins and modified pore waters in specific hydrothermal associations. Calcium chloride, in a mineral state in the natural world, occurs as the rare evaporite minerals; sinjarite (CaCl2.2H2O) and antarcticite(CaCl2.6H2O). The related potassic and magnesian calcium chloride minerals, chlorocalcite (KCaCl3) and tachyhydrite (calcium magnesium chloride, CaMg2Cl6•12H2O) are also rare in the sedimentary realm, and have particular evaporite associations and implications (see Part 2).

Outside of an industrial byproduct of the Solvay Process, most CaCl2 is derived from the processing of hypersaline basinal brines. The only current commercially, exploited natural CaCl2 surficial brine source is in Bristol Dry Lake, California (Figure 1). In the USA, for example, basinal brines are the primary commercial source of calcium chloride. Some of these brines in Michigan, Ohio, West Virginia, Utah, and California contain >4% calcium, with the Michigan Basin as the dominant producer. In the USA, a former commercially important source of calcium chloride was as a by-product of the Solvay Process used to produce soda ash. Because of environmental concerns and high energy costs, the Solvay Process has been discontinued as a source of CaCl2 in the USA.

This article will focus on the utility and geological significance of CaCl2 brines, while the next will focus on the geochemical significance of various calcium chloride minerals in particular evaporitic settings.


Usage

Calcium chloride depresses the freezing point of water, and its principal use is to prevent ice formation, especially on winter roads. Calcium chloride released to the environment is relatively harmless to plants and soil in diluted form. As a de-icing agent, it is more efficient at lower temperatures than sodium chloride. Solutions of calcium chloride can prevent freezing at a temperature as low as -52 °C (Figure 2). Hence, more than 50% of world CaCl2 usage is for road de-icing during winter. The second largest application of calcium chloride brine exploits its hygroscopic properties and the tackiness of its hydrates. In summer, it is used for roadbed stabilisation in unsealed roads and as a dust palliative. When sprayed onto the road surface, a concentrated CaCl2 solution maintains a cushioning layer on the surface of dirt roads and so suppresses formation of road dust. Without brine treatment dust particles blow away, eventually larger aggregate in the road also begins to shift around, and the road surface breaks down. Using calcium chloride reduces the need for grading by as much as 50% and the need for fill-in materials as much as 80%.


Calcium chloride’s low-temperature properties also make it ideal for filling agricultural implement tyres as a liquid ballast, aiding traction in cold climates. It is also used as an accelerator in the ready-mix concrete industry, although there is concern about its usage because of possible long-term chloride-induced corrosion of steel in highways and buildings. Calcium chloride is also widely used to increase mud fluid densities in oil- and gas-well drilling. It is also used in salt/chemical-based dehumidifiers in domestic and other environments to absorb dampness/moisture from the air.

Calcium chloride is used in the food industry to increase firmness of fruits and vegetables, such as tomatoes, cucumbers, and jalapenos, and prevent spoilage during processing. Food-grade calcium chloride is used in cheese-making to aid in rennet coagulation and to replace calcium lost in pasteurisation. It also is used in the brewing industry both to control the mineral salt characteristics of the water and as a basic component of certain beers.


Calcium Chloride brines production

Generally, CaCl2 brines are found in permeable strata either below, adjacent to, or above evaporite deposits, gradually becoming more dilute as brines approach the surface, and modified somewhat in proportion to distance from a potash or salt layer (Figure 1; Table 2). Other natural calcium chloride brines are derived from hydrothermally-modified marine waters. Dilute calcium chloride brines are also occasionally found in coastal aquifers, and some oil or gas formation waters that have been formed from seawater, possibly by a dolomitization reaction supplemented by the leaching of certain types of rocks (Garrett, 2004).

Basinal brines are chemically similar to CaCl2 brines forming hydrothermally at modern mid-ocean ridges, where seawater is being converted by interaction with basalt at elevated temperatures into low-salinity Na-Ca-Cl brines, depleted in Mg and SO4. These CaCl2 waters occur in and near active fracture zones, wherever seawater interacts with labile basalt (oceanic crust) at elevated temperatures and converts the circulating fluid from a Na-Mg-Cl water into a low-salinity Na-Ca-Cl brine, depleted in Mg and SO4. Similar hydrothermally-driven alteration of continental basalts via deeply circulated seawater interactions forms modern CaCl2-rich brine seeps, for example, within the thermally active continental Danakil rift valley (Hardie, 1990; Warren, 2016).

 

Calcium chloride is produced in commercial amounts using a variety of procedures: 1) refining of natural brines, typically with heating to increase concentration, 2) reaction of calcium hydroxide with ammonium chloride in Solvay soda ash production, and 3) reaction of hydrochloric acid with calcium carbonate. The first two processes account for over 90% of the world’s total calcium chloride production. Historically, natural brines sources are the dominant CaCl2 source. There is currently an excess of capacity in the calcium chloride industry, a situation which is only expected to become more acute as synthetic and byproduct capacity increases.

As we shall see in the next article, calcium chloride crystals are relatively rare as primary-textured occurrences in solid salt beds. On the other hand, CaCl2 brines are commonplace in basinal or formation waters in many Phanerozoic sedimentary basins entraining thick sequences of dissolving ancient salt. For example, since 1914, CaCl2 brines have been extracted from Silurian strata adjacent to Salina Salt of the Michigan Basin USA (Figure 1; Ludington; Oxy Chemicals, Formerly Dow Chemicals)). These brines are recovered from Detroit Group sediments that overlie the Silurian Salina evaporites. Based on fluid inclusions in primary halite chevrons in the Salina Salt, the Silurian was a time characterised by a CaCl2-enriched MgSO4-depleted ocean (but no tachyhydrite is known from Silurian strata, here or elsewhere). Industrial production of CaCl2

CaCl2 brines in the Michigan Basin (commercial)

This Silurian halite/potash basin has many aquifers with calcium chloride brines, both above and below the Silurian Salina Group’s halite and potash levels (Figures 3, 4; Garrett, 2004). Major aquifers are the overlying Devonian carbonate and sandstone beds, with many lesser aquifers. In the first porous bed above the potash (Sylvania Sandstone Formation) there is an extensive area of rich calcium chloride brine sitting directly above the potash deposit and extending to the south-southeast. Brine concentrations at nearly the same concentration as potash end liquor in fractures in the intrasalt carbonates. The less voluminous sandstone of the Filer Formation to the northwest contains a similar, but slightly more dilute CaCl2 brine. Several thinner and less abundant aquifers also occur under the potash beds with equally strong, or stronger calcium chloride brines (Figure 3).


The porous 0-90 m thick Sylvania Sandstone lies at the base of the Detroit River Group and is the main source of CaCl brine production. It is in direct erosional contact with the salt succession (Figure 4). The remainder of the Group consists of 0-350 m of variably porous carbonates (Garrett, 1995, 2004). Both sandstones and carbonates contain CaCl2-rich brines and extend across some 40% of the Michigan Basin at depths of from 300-1,400m. Brine concentration and the relative amount of CaCl2 increases with depth. Typically, the brines are only considered to be of economic importance below about 880 m depth. In carbonate hosts, the CaCl2 content varies from 3-23% and KCl from 0.2-2.9%, usually increasing in concert with concentration, as the NaCl content decreases. CaCl2 content in the Sylvania Sandstone varies from 14-22%, KCl from 0.6-2.1%, and both are more uniformly concentrated compared to the carbonate-hosted brines (Garrett, 2004).


Each aquifer entrains roughly the same ratio of ions, but pore waters become progressively more dilute as beds approach the surface about the basin margin. It seems likely that in this basin, a potash liquor originally seeped through and under the potash deposit (and reacted with calcite) was much later forced from its original sediments into the overlying porous strata into the overlying porous strata as they were compressed by deep burial, possibly aided by load-induced pumping induced by the waxing and waning of thick glacial ice that formed over this basin (McIntosh et al., 2011). Variable ionic content, as seen in Table 2, results from their considerably different migration history and variable dilution by meteoric or other groundwater (as is strongly indicated by the brine’s deuterium and 18O analyses), precipitation (such as gypsum), and their different contact with rocks that they could partially leach or react with. However, in the Michigan Basin these reactions were limited, since the porous carbonate strata (average, 20%) contains fairly pure carbonates, and the sandstone strata fairly pure silica (quartz arenites) cemented by dolomite or quartz (Martini, 1997).

There is a general synclinal structure to the strata under the Michigan Basin, and examples of the specific stratigraphy to the southeast of the centre of the basin at Midland are shown in Figure 4. The Detroit River Group consists of 0–350 m of variable porosity carbonates, and at its base there is 0–90 m of porous sandstone called the Sylvania Formation. Each of these formations cover about 40% or more of the Michigan Basin, and contain strong calcium chloride brines at depths of 300–1400 m. Their brines have been commercially recovered in the past, and were generally only considered to be economic below about 880 m. The brines’ total dissolved solids (TDS) and the amount of CaCl2 increases fairly consistently with depth from 3 to 23% CaCl2, and the NaCl and MgCl2 concentrations vary inversely with the CaCl2 . In the Sylvania Formation, the CaCl2 usually ranges from 14 to 22% (Figure 3). Additional information on the brine in other aquifers and the various reactions and changes that have occurred with them are discussed in Martini a(1997), Wilson and Hewett, (1992) and Wilson and Long (1992).

The Michigan Basin brines’ very low pH (4.5 to 5.3) helps to explain an ability to leach and react with other rocks, as is indicated by their high contents of strontium, barium and metals, much of the Sr and Ba probably came from the reaction with calcite. Geothermal water also probably mixed with some of the formations, as indicated by the variable presence of iodine, boron, lithium, caesium, rubidium and other rare metals. With most of the brines, the calcium concentration is somewhat higher than its magnesium equivalent in seawater end-liquor from a potash deposit, and the potassium a little lower. Wilson and Long (1992) speculated that this occurred by the conversion of the clays kaolinite and smectite to illite: Small amounts of glauberite (CaSO4.Na2SO4) and polyhalite (2CaSO4.K2SO4.MgSO4; have also been found in the basin. Finally, some of the calcium chloride aquifers have a slightly elevated ratio of 87Sr/86Sr (range from 0.7080 to 0.7105; seawater is 0.70919), further indicating that there was some rock leaching during burial(Martini, 1997).

What does a CaCl2 basinal brine indicate?

Pore fluids in the deeper parts of many sedimentary basins, especially if they contain a significant unit of evaporite, tend to be CaCl2 brines, entraining large volumes of hypersaline brine and in places, hydrocarbons (e.g., Michigan Basin, the U.S. Gulf Coast, European North Sea Basin, Western Canada Basin and Volga Basin).

Worldwide, one of the principal geochemical characteristics of saline waters in sedimentary basins is the progressive shift in their major ion composition from Na–Cl to Na–Ca–Cl to Ca–Na–Cl dominated waters with increasing chlorinity or salinity (Hanor and McIntosh, 2006). Such basinal brines (also called oil-field brines or formation waters) with significant calcium chloride contents have salinities that typically range up to 300,000 mg/l (Hanor, 1994; Lowenstein et al., 2003). The majority of these basinal brines are chemically distinct in their high Ca concentrations, separating their hydrogeochemistries from modern seawater and other common surface and near-surface waters which tend to be Na-Cl-SO4, Ca-HCO3, or Na-CO3 types (Drever, 1997).

Calcium levels in a CaCl2 basinal brine typically exceed the combined concentrations of SO4, HCO3, CO3 ions, (specifically, mCa > ∑(mSO4 + 1/2mHCO3 + mCO3); Lowenstein et al., 2004). And yet, the evaporative concentration of modern seawater leads to brines depleted in Ca, as required by the principle of chemical divides (CaCO3 and CaSO4 divides) for any evaporating water (Hardie and Eugster, 1970). Explanations for the origin of CaCl2 basinal brines remain problematic.

There is no simple pathway by which modern seawater, and most other surface and near-surface waters trapped in sedimentary deposits, can be converted to CaCl2 basinal brines during burial, without invoking significant rock-fluid interaction. Historically, before micro-inclusion studies of chevron halite showed that the ionic proportions of seawater likely varied across the Phanerozoic, the various CaCl2 basinal brines occurrences, for example in in Silurian and Cretaceous age strata were explained as an indicator of widespread dolomitisation and other diagenetic reactions, which preferentially extracted magnesium from pore waters (Garrett, 2004).

Since the mid-1990s, others have argued that CaCl2 enrichments in many ancient basinal brines with thick evaporites in the stratigraphy, including brines in the Detroit Group, are partial leftovers from times of CaCl2-enriched seawater chemistries (Table 3; Lowenstein and Timofeeff, 2008; Lowenstein et al., 2014). That is, Ca-enriched (MgSO4-depleted) pore brines adjacent to thick evaporites are indicators of ancient CaCl2 oceans, with the pore brines being remnants from the time the enclosing evaporitic and marine sediments were deposited (relict or connate brines).


Other authors, such as Houston et al. (2011), conclude this is not necessarily so; they agree that there are two end-members typifying highly saline subsurface brines (MgSO4 depleted or enriched). But, they conclude that end member chemistries relate to either substantial subsurface halite dissolution, or to preservation of early reflux-related seawater. Houston et al. (2011) go on to argue that CaCl2-enriched formation water chemistries from many basins worldwide, including the Michigan Basin, do not support an interpretation of variation in ionic proportions in seawater across the Phanerozoic. They find that CaCl2-rich brines formed either by dissolving bittern salts in the subsurface, or simply lost water in the subsurface after significant rock-fluid interaction had taken place. Water loss might be achieved by interaction with a gas phase at the elevated temperatures of deep burial or, alternatively, water may have been lost to clays. Both these mechanisms would have the effect of dehydrating (concentrating) the brine.

Calling upon a CaCl2 seawater source as an explanation for the origin of basinal brines was criticized also by Hanor & McIntosh (2006) who pointed out that no matter what the starting composition of a paleoseawater, significant diagenetic alteration must have occurred to produce the present major ion chemistry of Illinois and Michigan basin brines specifically, and basinal brines in general. In their view, the diagenetic mineral–brine interactions that occur during burial mask any original compositional variations in the starting seawater.

Hanor & McIntosh (2006) also argued that due to ongoing fluid escape and crossflow, it is difficult, if not impossible, to assign specific ages to basinal brines in a sedimentary basin. If the age of a basinal brine is not known, then the possible parent seawater, whether CaCl2 or MgSO4 type, cannot be determined. Hanor and McIntosh (2007) illustrated further complications in the interpretation of the timing of the origin of basinal brines. They showed that some brines in the Gulf of Mexico basin were not formed during the Middle Jurassic, contemporaneous with deposition of the Louann Salt, but formed during the Cenozoic from the dissolution of the Louann salt.

Interestingly, many marine potash deposit end-liquor brines have a high to medium–high lithium content, such as the Angara-Lena basin, Russia’s 1600–1900 ppm, the Paradox Basin’s 66–173 ppm Li, the Michigan Basin’s Sylvania Formation’s 36–72 ppm and the English Zechstein Formation’s 7– 65 ppm, etc. (Garrett, 2004)). However, some end-liquors have only a nominal lithium contents, such as from the Saskatchewan, Canada potash deposits. A few calcium chloride lakes also have medium–high values, such as the Don Juan Pond’s 235 ppm, Bristol Lake’s 30–108 ppm, Cadiz Lake’s 20–67 ppm, and Lake Vanda’s 27 ppm (Figure 1). We shall come back to this topic in a future article that will focus on lithium-rich brines.

Origin of CaCl2 brines

So, currently, there are two schools of thought used to explain the origin of CaCl2 basinal brines in evaporitic basins. One school assumes that the chemistry of the world’s ocean and its ionic proportions have remained near constant across the Phanerozoic. To form a CaCl2 enriched basinal brine then requires substantial subsurface rock-fluid interactions, utilising mechanisms and processes that include nonmarine parent waters, diagenetic alteration, pervasive dolomitization of carbonates, or bacterial sulphate reduction. All these mechanisms can reduce the proportion of Mg, HCO3 and SO4 relative to Ca in subsurface pore waters (Hanor and McIntosh, 2006). Proponents of this school tend to base their argument on basin-scale variations in the hydrogeochemistry of pore fluids.

The other school (mostly based on the micro-inclusion chemistry of chevron halite) argues for long-term changes in the major ion chemistry of seawater (Table 2). For example, Upper Jurassic, Cretaceous, and Cenozoic seawater records a systematic, long-term (>150 My) shift from the Ca2+ - rich, Mg2+ - and SO42- - poor seawater of the Mesozoic (“CaCl2 seas”) to the “MgSO4 seas” (with higher Mg2+ and SO42- > Ca2+) of the Cenozoic (Lowenstein and Timofeeff, 2008; Lowenstein et al., 2003, 2014). Over that period, the Mg/Ca ratio of seawater rose from 1 in the Early Cretaceous, to 2.3 in the Eocene, and 5.2 in present-day seawater.

Suggested drivers of long-term variation in the major ion chemistry of seawater include; fluctuations in the volume of discharge of hydrothermal waters from the global mid-ocean ridge system (Hardie, 1996), changes in the rates of volcanic activity and weathering processes, and variations in the amount of dolomite formed in the oceans (Holland and Zimmermann, 2000).

In my mind, much of the conflict between to two schools of thought as to the origin of CaCl2 basinal brines stems from the source of evidence. One approach utilises micro-inclusion chemistry in halite chevrons to define the evolution of Phanerozoic seawater. This data is extracted from an intra-salt textural association that, due to its long lack of permeability (‘locked up in halite’), likely preserves the chemical composition of the original depositional setting (e.g. Zambito et al.). The other school focuses on pore fluid hydrochemistry in subsurface waters, generally using water samples in boreholes, collected from pores and fractures in a carbonate, sandstone or shale host. The nature of fluids in these non-salt sediments, some which have been permeable since deposition, mean fluids experienced ongoing re-supply via crossflow and rock-fluid interaction as the ambient temperature, pressure and salinities evolved in the burial environment. This process shutdown once matrix permeability was lost (Warren et al., 2014).

In the next article, we shall expand our discussion of the significance of CaCl2 brines with a close look at where and how particular calcium chloride minerals can precipitate and be preserved and why some types of calcium chloride salts are more common in particular evaporitic settings.

References

Drever, J. I., 1997, The geochemistry of natural waters: Surface and groundwater environments: New Jersey, Prentice-Hall Inc., p. 327-351.

Garrett, D. E., 1995, Potash: Deposits, processing, properties and uses: Berlin, Springer, 752 p.

Garrett, D. E., 2004, Handbook of Lithium and atural Calcium Chloride; Their deposits, processing, uses and properties Amsterdam, Elsevier Academic Press, 476 p.

Hanor, J. S., 1994, Origin of saline fluids in sedimentary basins: Geological Society, London, Special Publications, v. 78, p. 151-174.

Hanor, J. S., and J. C. McIntosh, 2006, Are secular variations in seawater chemistry reflected in the compositions of basinal brines?: Journal of Geochemical Exploration, v. 89, p. 153-156.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: a hypothesis: American Journal of Science, v. 290, p. 43-106.

Hardie, L. A., 1996, Secular variation in seawater chemistry: an explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.: Geology, v. 24, p. 279 - 283.

Holland, H. D., and H. Zimmermann, 2000, The Dolomite Problem Revisited: Int. Geol. Rev., v. 42, p. 481-490.

Houston, S., C. Smalley, A. Laycock, and B. W. D. Yardley, 2011, The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters: Marine and Petroleum Geology, v. 28, p. 1242-1251.

Lowenstein, T., B. Kendall, and A. D. Anbar, 2014, Chapter 8.21. The Geologic History of Seawater, Treatise on Geochemistry (2nd Edition), Elsevier, p. 569-621.

Lowenstein, T. K., L. A. Hardie, M. N. Timofeeff, and R. V. Demicco, 2003, Secular variation in seawater chemistry and the origin of calcium chloride basinal brines: Geology, v. 31, p. 857-860.

Lowenstein, T. K., and M. N. Timofeeff, 2008, Secular variations in seawater chemistry as a control on the chemistry of basinal brines: test of the hypothesis: Geofluids, v. 8, p. 77-92.

Martini, A. M., 1979, Hydrogeochemistry of Saline Fluids and Associated Water and Gas, Michigan Basin: Doctoral thesis, University of Michigan, 236 p.

McIntosh, J. C., G. Garven, and J. S. Hanor, 2011, Impacts of Pleistocene glaciation on large-scale groundwater flow and salinity in the Michigan Basin: Geofluids, v. 11, p. 18-33.

Warren, J., C. Morley, T. Charoentitirat, I. Cartwright, P. Ampaiwan, P. Khositchaisri, M. Mirzaloo, and J. Yingyuen, 2014, Structural and fluid evolution of Saraburi Group sedimentary carbonates, central Thailand: A tectonically driven fluid system: Marine and Petroleum Geology, v. 55, p. 100-121.

Warren, J. K., 2015, Seawater chemistry (1 of 2): Potash bitterns and Phanerozoic marine brine evolution, Salty Matters blog, www.saltworkconsultants.com.

Warren, J. K., 2016, Evaporites: A Compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

Wilson, T. P., and T. A. Hewett, 1992, Geochemistry and isotope chemistry of Michigan Basin brines: Devonian formations: Applied Geochemistry, v. 7, p. 81-100.

 


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