anhydrite - teaching hand specimen of rock gypsum - minus the water
Teaching hand specimen of anhydrite. Weiser Ridge, Clark Co., Nevada
Chemical formula: CaSO4
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. The 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 its greater Mohs hardness, 3.5 versus 2.0, so anhydrite cannot be scratched by a fingernail (2.5) while gypsum can be. It superficially resembles much harder quartzite, so a student who neglects to examine hardness might fall into this trap.
These specimens ship in a small flat rate box unless combined with other purchases.
Select a specimen: You can select a specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.
Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.