EVAPORITE DEPOSITS

http://www.soton.ac.uk/~imw/sabkha.htm

(A supratidal part of a large coastal sabkha at Umm Said in Quatar.)

The term evaporites is used for all deposits, such as salt deposits, mainly chemical sediments that are composed of minerals that precipitated from saline solutions concentrated by evaporation. Evaporite deposits are composed dominantly of varying proportions of halite (rock salt) (NaCl), anhydrite (CaSo₄) and gypsum (CaSo₄.2H₂O). Evaporites may be classified as chlorides, sulfates or carbonates on the basis of their chemical composition (Tucker, 1991).Chemist's Usiglio and Vant Hoff undertook experiments to evaporate sea water and to synthesize evaporite minerals with a view to determining stability fields and precipitational controls. Evaporates precipitated in a lab do so in sequence:

1) Minor quantities of carbonate minerals begin to form when the original volume of sea water is reduced by evaporation to about one half.

2) Gypsum appears when the original volume has been reduced to about 20%.

3) Halite forms when the water volume reaches approximately 10% of the original volume. Precipitation of gypsum causes an increase in the Mg/Ca ratio in remaining water favoring dolomitization. Magnesium and potassium salts are deposited when less than about 5% of the original volume of sea water remains (Boggs, 1995).

click here for mineral compositions and pictures

Rocks of gypsum-anhydrite possess distinctive structures and textures and are susceptible to replacement crystallization and dissolution. Both may be precipitated at the earth's surface, subaqueousley in shallow and deep water and subaerially in coastal and inland sabkhas. Gypsum has a low relief and weak birefringence and belongs to the monoclinic crystal system; anhydrite has moderate birefringence and higher relief and is orthorhombic. Both may show a prominent cleavage (Tucker, 1991).

Halite is the main evaporite mineral of modern salt lakes and saline pans and is isotopic, usually showing strong cleavage and fracture plains and posses fluid inclusions. The NaCl for most halite deposits is derived from the evaporation of sea water.

Potassium and magnesium salts occur in some marine evapotite deposits and include sulfates, kieserite, kainite, and the chlorides sylvite and carnalite.

Evaporite deposits are generally restricted to areas of low latitude, very high temperature, relative humidity is low and evaporation far exceeds precipitation (Tucker, 1991).

Evaporite depositional settings can be classified as follows:

Diagram adapted from Boggs, (1996).

To enter the virtual field trip use this clickable image map. The fieldtrip consists of four routes, when each route page is accessed an introduction explaining the type of evaporite environment is displayed, with access to the three localities within that environment. At the last locality in each environment you are able to move forwards to the next route page. To start the fieldtrip click on Route 1.

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This page is created by Pamela Williams

Last Modified: 18/06/02