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Gypsum (Quaternary; Great Salt Plains, Oklahoma, USA) | by James St. John
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Gypsum (Quaternary; Great Salt Plains, Oklahoma, USA)

Gypsum from the Quaternary of Oklahoma, USA.


A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5600 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.


The sulfate minerals all have one or more sulfate anions (SO4 -2).


Gypsum is a moderately common hydrous calcium sulfate mineral (CaSO4·2H2O). Gypsum has a nonmetallic luster, is usually clearish to whitish, is soft (H≡2), and rather fragile/brittle. Gypsum has 3 cleavage planes, one of which is well developed (“one good cleavage), and the other two are not well developed (“two poor cleavages”). Broken gypsum specimens are frequently thin plates - the consequence of the one good cleavage plane. Unbroken crystals show monoclinic symmetry.


Gypsum has economic value as a mineral. It is the starting ingredient for making plaster and wallboard.


The specimen shown above is an polycrystalline mass of intergrown gypsum crystals from Oklahoma. Many refer to such things as "selenite", which is an unnecessary junior synonym of gypsum. Selenite differs in no way from gypsum. Selenite is gypsum. Use of the term "selenite" should be discouraged.


This Oklahoma gypsum formed by evaporation of rising groundwater that is rich in dissolved minerals ultimately derived from dissolution of ancient evaporite rocks. Sediments through which the groundwater percolated have become incorporated in the gypsum (click on the photo to zoom in and look around).


Locality: unrecorded/undisclosed site at Great Salt Plains, northwestern Oklahoma, USA


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Taken on April 8, 2017