- © The Mineralogical Society Of America
Evaporite deposits preserve a wealth of information on Earth’s past surface conditions. The deposits are sensitive indicators of depositional environment and climate, recorded in a wide variety of mechanically and chemically produced sedimentary structures and fabrics. Unique to evaporite deposits is the record they carry of the hydrochemistry of surface waters, including seawater. Sulfate minerals are important components in understanding the hydrochemistry of ancient surface waters.
Sedimentological aspects of evaporite deposits were compiled by Melvin (1991). In the compilation, sedimentology specific to Ca-bearing sulfates was described by Warren (1991); Lowenstein and Hardie (1985), Handford (1991), and Smoot and Lowenstein (1991) discussed the sedimentology of evaporites in general. These general principles can be applied specifically to the sulfate minerals in evaporite deposits. Sedimentological aspects of sulfate minerals in evaporite deposits are not discussed here. For this information, the reader is referred to the above references.
Hardie et al. (1985) and Spencer and Lowenstein (1988) discussed the petrography of evaporites and the interpretation of fabrics of evaporite minerals. Examples of petrographic aspects of sulfate minerals in evaporites are presented here along with references specific to the sulfate minerals. For a more complete understanding of these aspects the reader is referred to the above references.
The major focus of this chapter is on the chemical aspects of the common sulfate minerals that occur in evaporite deposits. The discussion begins with an explanation of the concept of chemical divides and the determination of evaporation paths, and the evolution of brines in accordance with mineral solubility and the composition of dilute inflow waters. Determination of the precipitation pathways of the more soluble salts is more complex. Examples of sulfate-mineral assemblages in evaporation systems, calculated via thermochemical modelling, are given for both marine and non-marine systems.
SOLUBILITY CONTROLS ON MINERAL PRECIPITATION AND PATHS OF EVAPORATION
The mineralogy of evaporite deposits is …