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Evaporites are water-soluble, mineral sediments that result from the evaporation of bodies of surficial water.
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.
Most evaporites are derived from bodies of sea-water, however evaporites can be created by evaporation of fresh water sources such as rivers, by aquifers or even by rainfall.
Evaporite depositional environments
Evaporite depositional environments which meet the above conditions include;
- Graben areas and half-grabens within continental rift environments fed by limited riverine drainage, usually in subtropical or tropical environments
- Example environments at the present which match this is the Denakil Depression, Ethiopia;Death Valley, California
- Graben environments in oceanic rift environments fed by limited oceanic input, leading to eventual isolation and evaporation
- 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
- Drainage basins feeding into extremely arid environments
Evaporitic formations
Evaporite formations need not be composed entirely of halite salt. In fact, 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.
In order 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 of evaporites
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.
Major groups of Evaporite Minerals
- Halides - halite, sylvite (KCl), calcite, 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.
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
Evaporites can also be easily recrystallized in laboratories in order to postulate the specific characteristics of their formation.
See also
Sedimentary rocks | Mineralogy
Evaporit | Evaporiidid | Evaporita | Évaporite | Evaporiet | Evaporiitti
"Evaporite"