gypsum - hand/display specimen of white rock gypsum - Weiser Ridge, Clark Co., Nevada
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.
This somewhat resembles crystallized dolomite or marble, but its softness readily identifies it as gypsum. It is further distinguished from marble by its lack of reaction to hydrochloric acid - marble effervesces and gives off carbon dioxide. Gypsum does not.
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