gypsum - teaching hand specimen of rock gypsum from Weiser Ridge, Clarke County, Nevada
Chemical formula: CaSO4・2H2O
Gypsum is primarily formed as a chemical precipitation in an evaporite environment. Gypsum and anhydrite are the most common sulfate minerals. This rock gypsum was deposited in a Permian age evaporite sequence over 100 feet thick interbedded with limestones of the Toroweap and Kaibab Formations. Both gypsum and anhydrite were mined at Weiser Ridge in Clark County, Nevada for use in the manufacture of gypsum wallboard. The quarry is now inactive.
Gypsum is a hydrated calcium sulfate - with two water molecules attached to the calcium sulfate. If it loses water it becomes anhydrite. If a bed of anhydrite picks up water, it swells, pushing up the sediments that lie above it to form a dome. Gypsum is distinguishable from anhydrite by its lower Mohs hardness, 2.0 versus 3.5. They often look similar. Gypsum can be scratched by a fingernail where anhydrite can not. This gypsum is fairly typical, somewhat sugary in appearance, and passes the fingernail scratch test. You have to exert some pressure, using the edge of your thumbnail.
Some of these specimens look very much like marble or dolomite and would make good specimens for student comparisons. .
We have a white dolomite that is very similar in appearance. The difference in hardness would be obvious to a student. Students should understand that rock gypsum is variable in color and that its softness is diagnostic - it can be scratched by a fingernail. Additionally, when heated in a test tube with a loose stopper, gypsum releases water, which will condense on the wall of the tube.
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