Formed by Evaporation
Evaporation is an important mineral forming process, which supplies
valuable materials used by the housewife, farmer, builder, chemist, engineer,
manufacturer and even birds, beasts and plants.
Mineral deposits resulting from the concentration and crystallization by
evaporation from aqueous solutions are called
Mineral deposits resulting from the concentration and crystallization by evaporation from aqueous solutions are called Evaporites.
Conditions of Formation:
Evaporation proceeds most rapidly in warm humid climates.
Evaporation of bodies of saline water leads to concentration of soluble
salts, and when supersaturation is reached, the salts are precipitated.
depositional environment, and
& climatic changes
Types of Evaporites: 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.
Marine Evaporites - Deposition from Oceanic Waters:
salts of oceanic waters are mainly obtained from the weathering of terrestrial
rocks. Rain water carries soluble
salts from continental areas to the oceans, evaporates, leaving behind the
salts, to return to the continents as more rain.
formation of barrier reefs
near coasts where sills or reefs isolate sinking inland basins
formation of sand bars
If the original body of water contained 100 km3 of water
and this were to be concentrated to 50 km3, the iron oxide and
calcium carbonate present would be precipitated. The water would still contain 3500 million tons of salt of
which 2700 million tons would be common salt.
Non-Marine Evaporites - Deposition from Salt Lakes:
The deposits formed from the evaporation of salt lakes are similar
to those obtained from ocean water because salt lakes contain the same salts as
the ocean, but generally in greater proportions.
Basin and Rang Province, USA
area of the Imperial Valley, California
Salt pans throughout Central Asia and northern Africa
Evaporation of groundwater is universal and in arid regions the
evaporites may accumulate as long as the climate remains dry.
nitrate salts with iodine
calcium and sodium carbonate
Artificial brines are obtained by pumping water down wells drilled into rock salt beds, and the brines so formed are the main source of common salt production in the US. Potash is also obtained in this manner.
Deposition from Hot Springs:
Substances contained in hot-spring waters build up deposits around
their orifices. A few of theses are
of commercial importance, the others are scenically beautiful.
Important Minerals found in Evaporites:
Sea water is the prime source of minerals formed by evaporation. Sea water consists of about 3.45% of dissolved salts, of which 99.9% consists of only seven ions. These are: Na+ (30.61), Mg2+ (3.69), Ca2+ (1.16), K+ (1.10), Cl- (55.04), SO42- (7.68) and HCO3- (0.41). About 45 other elements whose concentration is known in sea water occur as trace minerals in evaporites. Evaporites often show a repeated sequence of minerals, indicating cyclic conditions with a mineralogy determined by solubility. The sequence in which minerals in marine evaporites are deposited is: calcite, gypsum, anhydrite, halite, polyhalite, and lastly potassium and magnesium salts such as sylvite, carnallite, kainite, and kieserite; anhydrite and halite dominate.
Following are the important minerals that form in marine evaporites:
Chlorides: Halite (NaCl), sylvite (KCl), carnallite (KMgCl3.6H2O), langbeinite (K2Mg2(SO4)3 ), polyhalite (K2Ca2Mg(SO4)6.H2O), kainite (KMg(SO4)Cl.3H2O)
Sulfates: Anhydrite (CaSO4), gypsum (CaSO4.2H2O), kieserite (MgSO4.H2O)
Carbonetes: Dolomite (CaMg(CO3)2 ), Calcite (CaCO3), Magnesite (MgCO3)
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)