Evaporites

Evaporites are sedimentary rocks that form as water evaporates from aqueous environments enriched by concentrated salts and minerals. Minerals in these environments may be derived from the surrounding sediment or from the dissolution of organic and inorganic matter within the environment. Evaporite minerals are classified according to the type of salts found within the minerals.

Types of Evaporites

Evaporites are a type of mineral solid that forms from the accumulation of minerals and chemicals dissolved in aqueous solutions. Evaporites are generally classified as sedimentary rocks, one of the three major types of rock. Sedimentary rocks form from the accumulation of small particles and dissolved minerals that are concentrated and compacted to become solid rock.

Evaporites form in aqueous solutions only, and specifically in environments with high salt content, including salt marshes, lakes, and marine environments. As their name suggests, evaporites form as water evaporates from an aqueous solution, leaving behind elements that did not transition to gas during evaporation. Different types of evaporites may form from the same source, depending on the types of minerals dissolved in the initial solution.

Chloride evaporites are those that contain chloride ions, a negatively charged ionic form of atomic chlorine. Chloride and the acidic version of chloride, hydrochloric acid (HCl), are highly water soluble, leaving free chloride ions in aqueous solution. Chloride evaporites are also known as halides, which are the salts that derive from chemical reactions involving halide ions, such as chloride, bromide, and fluoride. Table salt, which is also called sodium chloride (NaCl) or halite, is one of the best-known types of chloride evaporites. Another example of the chloride-based evaporite is sylvite, which combines chloride with potassium, in the formula KCl, and forms into crystals similar to rock salt.

Sulfate evaporites contain an ionic sulfate group (SO₄^-2) consisting of a sulfur atom bonded with four oxygen atoms. Common sulfate evaporites include anhydrite (CaSO₄) and its hydrated form gypsum (CaSO₄•2H₂O). Gypsum is one of the most economically important of the sulfate evaporites and is used in a variety of industrial and chemical processes.

A third major variety of evaporite rocks are the carbonates, which contain a carbonate ion (CO₃^2-) in their chemical structure. Common carbonate evaporites include calcite (CaCO₃) and dolomite (CaMg(CO₃)₂). Many carbonate evaporite minerals are economically important and are used for deriving minerals for industrial processes and chemical reactions. The evaporite mineral trona (Na₃(CO₃)(HCO₃)•2H₂O) is another common carbonate evaporite that is used in a number of industrial applications.

Formation of Evaporite Rocks

Evaporite minerals form in environments where aqueous solutions become enriched with dissolved minerals generally derived from rocks surrounding the body of water. Minerals become more concentrated in areas where the body of water is partially enclosed, such as in lakes and marine estuary environments.

Temporary bodies of water that form in warm or arid environments are among the most common sources for evaporite minerals. The dissolved minerals present in the aqueous solution are derived from the atmosphere, as mineral inclusions in rainwater, and from the lithosphere, as rocks within bodies of water gradually dissolve, releasing minerals into the water.

Solar radiation causes water to evaporate by exciting water molecules and thereby breaking the bonds between them. When this occurs, molecules at the top of the body of water transition to a gaseous state, called water vapor, which rises into the atmosphere. Molecules that remain solid, such as carbonate, sulfur, calcium, potassium, and phosphorus, remain in the solution as a concentrated mineral residue. Evaporite minerals form when the rate of evaporation exceeds the total rate of water inflow into the environment, including the amount of water deposited by precipitation.

At this point, the relative temperature and humidity greatly affect the types of minerals that will form as the remaining water evaporates from the solution. For instance, halite will form in environments where the ambient humidity is less than 65 percent, while environments with higher humidity will not support the growth of halite crystals. The types of evaporites found in a certain environment are therefore governed by the overall temperature and humidity of the region.

Evaporite minerals also form in permanent marine environments, such as the bottom of seas and oceans. In these environments, evaporite formations develop slowly in time as minor quantities of mineral are deposited in layers. Areas around hydrothermal vents, which are jets of superheated water at the bottom of deep marine environments, appear to be prime locations for the development of evaporite rocks because the heated water ejected from the vents contains high concentrations of dissolved minerals, which settle in the areas immediately surrounding the vents.

While deep marine environments may give rise to large deposits of evaporite materials through millions of years, evaporites form more rapidly in semi-marine environments where saltwater and freshwater mix and in environments where the presence of water is temporary on more immediate geological scale. In marshes and salt lakes, for example, evaporite minerals may grow by more than 10 meters (33 feet) in thickness in one thousand years.

Formational Environments of Evaporite Deposits

A variety of evaporite minerals have been derived from salt flats also known as playas or sabkhas, which develop in areas where the sea level is slowly receding from the shore, generally in arid environments. Commonly, salt flats form in an area where ocean waters drain into an arid environment or where there was once a salt lake that gradually dried out.

Salt marshes, which are wetland areas surrounding marine environments, produce similar types of evaporites to those found in playas. In both salt flats and salt marshes, saltwater from the nearby ocean infiltrates the soil with the incoming tide, saturating the soil directly beneath the surface. Solar radiation causes the water to evaporate, leaving mineral deposits in the soil.

In playas and saline lakes, algae may assist in the deposition of evaporite minerals by providing a fibrous mass that helps to bind mineral crystals as they form within the soil or at the edges and bottom of the lake. As the ocean recedes and water evaporates from the system, the algal mats gradually shrink, leaving concentrated mineral deposits. The Great Salt Lake in Utah is an example of an environment where evaporite deposition is occurring. Utah’s saline lake will eventually evaporate completely, leaving behind a variety of evaporite salts.

Sabkhas and salt marshes generate both gypsum and anhydrite, which form in small ponds on the surface of the marsh. Gypsum forms first, as the water recedes, but further evaporation will dehydrate the gypsum deposits, transforming them into anhydrite. An influx of rain can rehydrate anhydrite deposits, giving rise to gypsum. In this way, gypsum and anhydrite exist in a transitional flux alternating between either form depending on the relative moisture remaining in the environment. In thousands of years, thick deposits of anhydrite and gypsum can develop within the upper layers of the soil. Sabkha environments also give rise to considerable quantities of halite, which can be harvested to manufacture table salt for consumption.

Evaporite minerals also can form in shallow saltwater environments, such as salt ponds and temporary salt lakes. These environments, like the salt flats or salt marshes, give rise to halite in large quantities. The creation of artificial salt ponds, called salterns or salt evaporation ponds, allows for the distillation of salt from marine water and provides a way to harvest halite to be used in making table salt. In addition, environments of this type produce a variety of evaporite carbonates, some derived from carbonate forming from the decomposition of organisms living within the environment.

In deeper aqueous environments, such as shallow seas, marine estuaries, and other marine environments, evaporites form within the water column and settle at the bottom of the environment, where they condense into larger bodies. Gypsum forms in deeper environments as thin, needle-like rods that form primarily at the upper surface as solar radiation evaporates the layer of molecules at the surface of the water. The gypsum then gathers at the floor of the ecosystem, where it forms into fibrous mats. Halite forms in a similar manner, as do several types of carbonate evaporite minerals, developing into thin mats at the floor of the system.

Within millions of years, oceans and seas may dry out completely because of tectonic shifts changing the position of the continental crust relative to the oceans. Shallow seas that once covered portions of the intercontinental area often contain thick evaporite deposits that accumulated through millions of years of gradual evaporation and condensation as the seas shifted. The formation of ancient marine evaporite deposits is more difficult to interpret because the exact combination of climatic changes that occurred during the extended period of deposition is unknown. Geologists believe that the same basic mechanisms involved in the formation of evaporites in semi-aqueous environments also function in deeper oceanic environments, but that the process is more gradual. Ancient marine environments often contain the largest and most complex evaporite rocks, some of which may contain thousands of pounds of evaporite salts.

Uses of Evaporites

Halite is one of the most economically important evaporite minerals. Salt mines around the world are generally the remnants of extinct marine ecosystems or salt flats. In these environments, halite may be found in thick deposits at or beneath the earth’s crust.

Halite is a fragile mineral and will dissolve in areas where humidity is high or where there is sufficient influx of water from drainage or precipitation. Halite is the only mineral that is consumed directly as a food source. Salt mining is a multibillion-dollar international industry.

Gypsum is another important evaporite mineral often used in the manufacture of construction materials. Gypsum is one of the primary ingredients in plaster and also is a component in drywall sheets, also known as gypsum boards. Gypsum forms in mixed beds containing halite, gypsum, and anhydrite.

Gypsum is generally white in color and has been used in the manufacture of gesso, a base paint used by artists to prepare canvases. Gypsum also is widely used in agriculture as an ingredient in many soil fertilizers and as a treatment for water. In some cases, gypsum may be used as a coagulant in the production of food, and it is most notably used to add stability in soy bean curd, or tofu. In this regard, gypsum provides calcium and therefore serves as a dietary supplement.

Trona is a relatively common carbonate evaporite that is derived from playas and salt pans in Africa and North America. Trona can be processed to produce sodium carbonate, which is mixed with limestone to create fertilizer, and it also can be used as a water treatment chemical. Sodium carbonate also is used in the manufacture of soaps and detergents.

Another carbonate evaporite, dolomite, is used as a decorative stone and in building construction. Crushed dolomite is used as a component in the manufacture of concrete. One of the most important uses of dolomite is as a substrate for deriving magnesium metal by subjecting the dolomite to a reductive chemical environment that results in the concentration of magnesium solids held within the rock.

Principal Terms

algae: diverse group of autotrophic organisms that live in aqueous and subaqueous environments

aqueous: solution that contains water acting as a solvent or medium for reactions

carbonate: salt that forms when carbonic acid reacts with a basic material

evaporation: process by which molecules of water at the surface of a body of water transition to a gaseous state in response to energy derived from heat and from solar radiation

halide: salt formed from the reaction between a halide ion, including chloride, bromide, and fluoride, with a basic material in the environment

halite: mineral form of rock salt or common salt formed from a reaction between a chloride ion and sulfur atoms in an aqueous environment

playa: dried salt lake environment known for concentrations of evaporite minerals

salt: ionic compound resulting from chemical reactions forming solids through the reaction of an acidic material and a basic material

sedimentary rock: one of the three basic rock types composed of solids derived from dust, debris, and fragments of material that condense and solidify into larger solid structures

sulfate: salt that forms from a chemical reaction between sulfuric acid and a basic material in the environment

Bibliography

Bao, Xiujuam, et al. "Quantifying Climate Conditions for the Formation of Coals and Evaporites." National Science Review, vole. 10. no. 6, June 2023, doi.org/10.1093/nsr/nwad051. Accessed 28 July 2024.

Blatt, Harvey, Robert J. Tracy, and Brent E. Owens. Petrology: Igneous, Sedimentary, and Metamorphic. 3d ed. New York: W. H. Freeman, 2006.

"Evaporites." Geology Science, 20 Nov. 2023, geologyscience.com/rocks/sedimentary-rocks/non-clastic-sedimentary-rock/evaporites/?amp. Accessed 25 July 2024.

Leeder, Mike. Sedimentology and Sedimentary Basins: From Turbulence to Tectonics. 2d ed. Malden, Mass.: Wiley-Blackwell, 2011.

Schreiber, B. C., S. Lugli, and M. Babel, eds. Evaporites Through Space and Time. Geological Society Special Publication 285. Bath, England: Geological Society, 2007.

Tucker, Maurice E. Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks. 3d ed. Malden, Mass.: Blackwell Science, 2001.

Warren, John. Evaporites: Sediments, Resources, and Hydrocarbons. New York: Springer, 2006.

Wicander, Reed, and James S. Monroe, eds. Historical Geology: Evolution of Earth and Life Through Time. 6th ed. Belmont, Calif.: Brooks/Cole, 2010.