The Formation of Non-Clastic Chemical Evaporites
Non-clastic, Chemical Sedimentary Rocks are not composed of weathered
rock fragments. Chemical Sediments form when a solid comes out
of solution either by evaporation or precipitation.
A chemical evaporite results when ions in a supersaturated aqueous solution
bond to form minerals as the water evaporates. This occurs when a body of water
exists in a restricted environment whereby the input of additional water is
less than the rate of evaporation. In the geologic past, the evaporation of
expansive Inland Seas left behind huge evaporite deposits.
Evaporites predominantly exist as dry lake deposits. Saline lakes, found
in arid regions contain a heavy salt content. Occasional heavy rainfall with
flash flooding brings additional salts, eroded from surrounding rocks, into
the lake. When water evaporates from these lakes, saline deposits (evaporites)
are left behind. Death Valley, California, US is one area where occasional,
but heavy rains cause temporary expanses of water to form which, due to the
heat and arid conditions in the area, evaporate quickly. Therefore, evaporites
are commonly found in this area.
Perhaps the best example of an evaporite sedimentary rock is Rock Salt:
Na+ + Cl- = NaCl is the Mineral
Halite, called Rock Salt when it is found in the field
as large evaporite deposits.
However one major difference between the mineral and the rock is this: When
Halite is found in the field, in massive form, the crystal shape of the mineral
may not be discernible.
See the difference in the next 2 photos.
| Halite Mineral With an obvious cubic crystal shape. |
Rock Salt Cubic crystals are not visible. |
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Rock Salt In the Field One area where salt deposits continue to form today is Death Valley,
California, US. |
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Another common mineral found in thick evaporite beds is Gypsum (CaSO4-2H2O) and Anhydrite (CaSO4), commonly referred to as Plaster of Paris. The Mineral Gypsum is called Rock Gypsum when it is found as large evaporite deposits in the field. As with Halite, Gypsum's crystal shape, seen in the photograph to the left, may not be discernible in massive form when found in large Rock Gypsum deposits. |
