Evaporite
Evaporites are water-soluble, mineral sediments that result from the evaporation of bodies of water on the Earth's surface. They are usually considered sedimentary rocks.
Formation of evaporite rocks
Evaporites are formed by evaporation of restricted bodies of water at the Earth's surface. Although all water bodies on the surface and in aquifers contain dissolved salts, in order to form minerals from these salts, the water must evaporate into the atmosphere in order to precipitate the minerals. In order 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 basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate when the water becomes oversaturated.
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 is:
- Calcite (CaCO3) and dolomite (CaMg(CO3)2)
- Gypsum (CaSO4-2H2O) and anhydrite (CaSO4).
- Halite (i.e. common salt, NaCl)
- Potassium and magnesium salts
Most evaporite formations do not contain more than a few percent of evaporite minerals, the remainder being composed of the more typical detrital clastic rocks and carbonates.
For a formation to be recognized 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.
Evaporites can also be easily recrystallized in laboratories in order to postulate the specific characteristics of their formation.
Environments where evaporite deposits are found
Evaporite deposits can be found in the following environments:
- Graben areas and half-grabens within continental rift environments fed by limited riverine drainage, usually in subtropical or tropical environments. Examples of such environments are the Denakil Depression in Ethiopia and Death Valley in California.
- 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.
- Internal drainage basins in arid to semi-arid temperate to tropical environments fed by ephemeral drainage. Example environments include the Simpson Desert in Western Australia and the Great Salt Lake in Utah.
- Non-basin areas fed exclusively by groundwater seepage from artesian waters. Examples include the seep-mounds of the Victoria Desert, fed by the Great Artesian Basin in Australia.
- Restricted coastal plains in regressive sea environments. Examples include the sabkha deposits of Iran, Saudi Arabia, and the Red Sea.
- Drainage basins feeding into extremely arid environments. Examples include the Chilean deserts, certain parts of the Sahara, and the Namib desert.
Major groups of evaporite minerals
- Halides - halite, sylvite (KCl), and fluorite
- Sulfates - such as gypsum, barite, and anhydrite
- Nitrates - nitratite (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.
Economic importance of evaporites
Evaporites are important economically because of their mineralogy, their physical properties in-situ and their behavior within the subsurface.
Evaporite minerals, especially nitrate minerals, are economically important in Peru and Chile. Nitrate minerals are often mined for the production of fertilizers 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 behavior, and imperviousness to groundwater.
Halite formations are famous for their ability to form diapirs, which produce ideal locations for trapping petroleum deposits.
See also
ReferencesISBN links support NWE through referral fees
- Guéguen, Yves, and Victor Palciauskas. 1994. Introduction to the Physics of Rocks. Princeton, NJ: Princeton University Press. ISBN 0691034524.
- Gore, Rick. December 1982. The Mediterranean: Sea of Man's Fate. National Geographic. 694-737.
- Skinner, Brian J., Stephen C. Porter, and Jeffrey Park. 2004. Dynamic Earth: An Introduction to Physical Geology. 5th ed. Hoboken, NJ: John Wiley. ISBN 0471152285
- Tucker, Maurice E. 2001. Sedimentary Petrology. 3rd ed. Oxford: Blackwell Publishing. ISBN 0632057351
- California State University evaporites page Retrieved January 4, 2008.
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