Evaporitic minerals

From GeoClasses

Evaporite precipitation at the edge of a saline lake. Nulla Nulla in Southeastern Australia.

Evaporitic minerals are water-soluble, mineral sedimentary minearals that result from the evaporation of saline water. Most evaporites are derived from bodies of sea-water, though saline lakes may also be an important source (e.g. the Great Salt Lake in Utah).

Evaporites start to precipitate when their concentration in water reaches such a level that they can no longer exist as solutes. This supersaturation is usually the result of prolonged evaporation.

Salt crystalization on playa floor in Death Valley, USA.

Evaporite minerals are geologically important because they clearly are related to the environmental conditions that existed at the time of their deposition, namely an arid environment, such as coastal plain or restricted basinal, e.g. Death Valley, or the Dead Sea. Evaporites can also be easily recrystallized in laboratories in order to postulate the specific characteristics of their formation.

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, both often mined for use in the production on fertilizer and explosives.
• Borates - typically found in arid-salt-lake deposits plentiful in the southwestern United States|US. A common borate is borax, which has been used in soaps as a surfactant.

Order of precipitation

The minerals precipitate out of solution in the reverse order of their solubilities, such that the order of precipitation is:

1. Calcite (CaCO₃) and dolomite (CaMg(CO₃)₂)
2. Gypsum (CaSO₄-2H₂O) and anhydrite (CaSO₄).
3. Halite (i.e. common salt, NaCl)
4. Potassium and magnesium salts

The preceeding was adapted from Wikipedia:Evaporite (Feb 4th, 2006).

Calcite crystal

Gypsum

Halite

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