Roger Weller, geology instructor
Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur silky and fibrous, in which case it is commonly called satinspar. Finally it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque.
Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. It is often associated with the minerals halite and sulfur. Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 kmē (275 sq mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.
Commercial quantities of gypsum are found in Thailand, Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada, and in New York, Michigan, Iowa, Kansas, Oklahoma, Arizona, New Mexico, Colorado, Utah and Nevada in the United States. .[i]
This example of calcite / gypsum is from the Hilltop Mine in the Chihuahua Mountains of Cochise County, AZ. Specimens are from the private collection of Roy Parsons.
and Gypsum, normal light
Calcite is mottled brown, Gypsum is light
Gypsum, under ultraviolet light
Calcite is orange-red, Gypsum is cream white
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