Evaporite
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An evaporite (/ɪˈvæpəˌraɪt/) is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution.[1] 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
Depositional environments
Marine
Marine evaporites tend to have thicker deposits and are usually the focus of more extensive research.[2] When scientists evaporate ocean water in a laboratory, the minerals are deposited in a defined order that was first demonstrated by Usiglio in 1884.[2] The first phase of precipitation begins when about 50% of the original water depth remains. At this point, minor carbonates begin to form.[2] The next phase in the sequence comes when the experiment is left with about 20% of its original level. At this point, the mineral gypsum begins to form, which is then followed by halite at 10%,[2] excluding carbonate minerals that tend not to be evaporites. The most common marine evaporites are calcite, gypsum and anhydrite, halite, sylvite, carnallite, langbeinite, polyhalite, and kainite. Kieserite (MgSO4) may also be included, which often will make up less than four percent of the overall content.[2] However, there are approximately 80 different minerals that have been reported found in evaporite deposits,[3][4] though only about a dozen are common enough to be considered important rock formers.[2]
Non-marine
Non-marine evaporites are usually composed of minerals that are not common in marine environments because in general the water from which non-marine evaporite precipitates has proportions of chemical elements different from those found in the marine environments.
Evaporite
- riftenvironments fed by limited riverine drainage, usually in subtropical or tropical environments
- Example environments at the present that match this is the Denakil Depression, Ethiopia; Death Valley, California
- Example environments at the present that match this is the
- Graben environments in oceanic rift environments fed by limited oceanic input, leading to eventual isolation and evaporation
- Examples include the Red Sea, and the Dead Sea in Jordan and Israel
- Internal drainage basins in arid to semi-arid temperate to tropical environments fed by ephemeral drainage
- Example environments at the present include the Simpson Desert, Western Australia, the Great Salt Lake in Utah
- Non-basin areas fed exclusively by groundwater seepage from artesian waters
- Example environments include the seep-mounds of the Victoria Desert, fed by the Great Artesian Basin, Australia
- Restricted coastal plains in regressive sea environments
- Examples include the sabkha deposits of Iran, Saudi Arabia, and the Red Sea; the Garabogazköl of the Caspian Sea
- Drainage basins feeding into extremely arid environments
The most significant known evaporite depositions happened during the
Evaporitic formations
Evaporite formations need not be composed entirely of
For a formation to be recognised as evaporitic it may simply require recognition of halite
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
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
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.
Mineral Class Mineral name Chemical Composition Chlorides Halite NaCl Sylvite KCl Carnallite KMgCl3 · 6 H2O Kainite KMg(SO4)Cl · 3 H2O Sulfates Anhydrite CaSO4 Gypsum CaSO4 · 2 H2O Kieserite MgSO4 · H2O Langbeinite K2Mg2(SO4)3 Polyhalite K2Ca2Mg(SO4)6 · H2O Carbonates Dolomite CaMg(CO3)2 Calcite CaCO3 Magnesite MgCO3
- Halides: halite, sylvite (KCl), and fluorite
- barite, and anhydrite
- .
- Carbonates: such as trona, formed in inland brine lakes.
- Some evaporite minerals, such as hanksite, are from multiple groups.
Evaporite minerals start to
The minerals precipitate out of solution in the reverse order of their solubilities, such that the order of precipitation from sea water is:
- Calcite (CaCO3) and dolomite (CaMg(CO3)2)
- Gypsum (CaSO4 · 2 H2O) and anhydrite (CaSO4).
- 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 are 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
See also
References
- ^ Jackson, Julia A. (1997). Glossary of Geology (4th ed.). Alexandria, Virginia: American Geological Institute.
- ^ ISBN 0131547283.
- doi:10.3133/pp440Y.
- ISBN 978-0632053018.
- ^ ISBN 978-0444555762.
- ^ C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC Archived October 28, 2012, at the Wayback Machine
- ISSN 0019-1035.
- S2CID 202875048.
- ISSN 0016-7037.
- ISSN 1538-4357.
Other reading
- California State University evaporites page
- Gore, Rick (December 1982). "The Mediterranean: Sea of Man's Fate". National Geographic: 694–737.
- Guéguen, Yves; Palciauskas, Vector (1994). Introduction to the physics of rocks. Princeton, N.J.: Princeton University Press. ISBN 9780691034522.
- Hardie, Lawrence, 1984, Evaporites: marine or nonmarine?: American Journal of Science, v. 284, pp. 193-240. DOI: https://doi.org/10.2475/ajs.284.3.193
- 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, p. 177-185.