are water-soluble mineral sediments
that result from the evaporation
of bodies of surficial water
. Evaporites are considered sedimentary rocks.
Formation of evaporite rocks
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 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 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
- 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
- Non-basin areas fed exclusively by groundwater seepage from artesian waters
- 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
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.
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), 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.
- Carbonates: such as trona, formed in inland brine lakes.
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 (CaCO3) and dolomite (CaMg(CO3)2)
- Gypsum (CaSO4-2H2O) 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 (calcite) and 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.