Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

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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