Saline brines and their precipitates

All evaporite salts are ionic salts, containing the major ions Na, Ca, Mg, K, Cl, SO4, CO3 in varying proportions along with other less common ionic constituents, such as B, Ba, Sr, Br and I and varying amounts of structural water. The mineralogy and order of precipitates in any brine pool is controlled by the ionic makeup of the parent brine. This, in turn, reflects the chemical composition of lithologies contributing dissolved ions to the parent water on its way to the site of precipitation. Because of consistent ionic proportions in seawater worldwide, the modern marine evaporite series is predictable and tied to increasing salinity. With nonmarine brines, proportions vary according to source terrain and levels of hydrothermal input. Hence, the proportions of minerals and sequence of precipitation are more variable. In addition, we now know that the ionic proportions in seawater were not constant through time and that the modern evaporite series is not necessarily a one-for-one analogue for past marine-fed precipitates.