thumb| [[Halite on wood from the Wieliczka salt mine, Poland. (Gallery of Mineralogy and Geology of the French National Museum of Natural History in Paris)]]
An evaporite () is a water-soluble sedimentary mineral deposit caused by the concentration and crystallization (chemical precipitation) that results when enough of the water solvent evaporates from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.
Formation
Although all water bodies on the surface and in aquifers contain dissolved salts, the water must evaporate into the atmosphere for the minerals to precipitate. For this to happen, the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small drainage basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate after the water becomes saturated.
Depositional environments
Marine
thumb|Anhydrite
Marine evaporites tend to have thicker deposits and are usually the focus of more extensive research. When scientists evaporate ocean water in a laboratory, the minerals are deposited in a defined order that was first demonstrated by Usiglio in 1884. though only about a dozen are common enough to be considered important rock formers. Thick non-marine deposits that accumulate tend to form where evaporation rates will exceed the inflow rate, and where there is sufficient soluble supplies. The inflow also has to occur in a closed basin, or one with restricted outflow, so that the sediment has time to pool and form in a lake or other standing body of water.
For a formation to be recognised as evaporitic it may simply require recognition of halite pseudomorphs, sequences composed of some proportion of evaporite minerals, and recognition of mud crack textures or other textures.
Economic importance
Evaporites are important economically because of their mineralogy, their physical properties in-situ, and their behaviour within the subsurface.
Evaporite minerals, especially nitrate minerals, are economically important in Peru and Chile. Nitrate minerals are often mined for use in the production on fertilizer and explosives.
Thick halite deposits are expected to become an important location for the disposal of nuclear waste because of their geologic stability, predictable engineering and physical behaviour, and imperviousness to groundwater.
Halite formations are famous for their ability to form diapirs, which produce ideal locations for trapping petroleum deposits.
Halite deposits are often mined for use as salt.
Major groups of evaporite minerals
thumb|Calcite
This is a chart that shows minerals that form the marine evaporite rocks. They are usually the most common minerals that appear in this kind of deposit.
{| class="wikitable"
!Mineral class
!Mineral name
!Chemical composition
|-
| rowspan="4" |Chlorides
|Halite || NaCl
|-
|Sylvite || KCl
|-
|Carnallite ||
|-
|Kainite ||
|-
| rowspan="5" |Sulfates
|Anhydrite ||
|-
|Gypsum ||
|-
|Kieserite ||
|-
|Langbeinite ||
|-
|Polyhalite ||
|-
| rowspan="3" |Carbonates
|Dolomite ||
|-
|Calcite ||
|-
|Magnesite ||
|}
thumb|[[Hanksite, , one of the few minerals that is both a carbonate and a sulfate]]
- Halides: halite, sylvite (KCl), and fluorite
- Sulfates: such as gypsum, barite, and anhydrite
- Nitrates: nitratine (soda niter) and niter
- Borates: typically found in arid-salt-lake deposits plentiful in the southwestern US. A common borate is borax, which has been used in soaps as a surfactant.
- Carbonates: such as trona, formed in inland brine lakes.
- Some evaporite minerals, such as hanksite, are from multiple groups.
Evaporite minerals start to precipitate when their concentration in water reaches such a level that they can no longer exist as solutes.
The minerals precipitate out of solution in the reverse order of their solubilities, such that the order of precipitation from sea water is:
- Calcite (CaCO<sub>3</sub>) and dolomite ()
- Gypsum () and anhydrite (CaSO<sub>4</sub>).
- Halite (i.e. common salt, NaCl)
- Potassium and magnesium salts
The abundance of rocks formed by seawater precipitation is in the same order as the precipitation given above. Thus, limestone (dolomite) is more common than gypsum, which is more common than halite, which is more common than potassium and magnesium salts.
Evaporites can also be easily recrystallized in laboratories in order to investigate the conditions and characteristics of their formation.
Possible evaporites on Titan
Recent evidence from satellite observations and laboratory experiments suggest evaporites are likely present on the surface of Titan, Saturn's largest moon. Instead of water oceans, Titan hosts lakes and seas of liquid hydrocarbons (mainly methane) with many soluble hydrocarbons, such as acetylene, that can evaporate out of solution. Evaporite deposits cover large regions of Titan's surface, mainly along the coastlines of lakes or in isolated basins (Lacunae) that are equivalent to salt pans on Earth.
See also
- List of minerals
- List of rock types
- Salt dome
References
Other reading
- California State University evaporites page
- Hardie, Lawrence, 1984, "Evaporites: marine or nonmarine?: American Journal of Science, v. 284, pp. 193–240.
- Hardie, L.A., and Eugster, H.P., 1971, "The depositional environment of marine evaporites: a case for shallow, clastic accumulation": Sedimentology, v. 16, p. 187–220.
- Benison, K.C., and Goldstein, R.H., 2002, "Recognizing acid lakes and groundwaters in the rock record": Sedimentary Geology, v. 151, pp. 177–185.