anhydrite - teaching student specimens of white anhydrite - Unit of 5 specimens
Anhydrite is calcium sulfate like gypsum, but without the attached water molecules. Lithostatic loading, the pressure of overlying rocks, can cause gypsum to lose its water and convert to anhydrite. When Africa collided with Europe roughly 5.6 million years ago closing the Mediterranean's opening to the Atlantic, the sea completely dried up, leaving a thick gypsum bed on the what is now the sea floor. During the Permian, roughly 280 million years ago, gypsum formed in what is now Nevada in much the same way, and is part of an over 100 foot thick interbedded sequence of limestones, gypsum and anhydrite.
Anhydrite and related gypsum were mined at Weiser Ridge by Georgia Pacific for use in the manufacture of wallboard. The quarry has not been active since 1995. Anhydrite is distinguishable from softer gypsum by it's greater Mohs hardness, 3.5 versus 2.0, so anhydrite cannot be scratched by a fingernail (2.5) while gypsum can. It superficially resembles much harder quartzite, so a student who neglects to examine hardness might fall into this trap.
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