gypsum - teaching student specimens of white rock gypsum - Unit of 5 specimens
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 and passes the fingernail scratch test. You have to exert some pressure, using the edge of your thumbnail. Gypsum gives water in the closed tube, diagnostic of gypsum but not of anhydrite. Place a small specimen in the bottom a test tube, insert a stopper loosely and heat over a bunsen burner flame. Water droplets will condense part way up the tube.
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