Hardness: 3.5 to 4
A common Kansas mineral, gypsum is calcium sulfate containing water (CaSO42H2O). Calcium sulfate without water is anhydrite (CaSO4), a very different mineral. Gypsum is colorless or white to light gray (or, rarely, bright red), and is so soft that it can be scratched by a fingernail.
Gypsum is divided into three varieties. The first, selenite, consists of flat, clear, diamond-shaped crystals. Another variety of gypsum is called satin spar. It is white or pink, fibrous, and has a silky luster. It is found as thin layers in beds of rock gypsum and in certain shales. The third variety is called massive or rock gypsum. Rock gypsum is coarsely to finely granular, white to gray, and contains varying amounts of impurities.
Read more about gypsum in Rocks and Minerals of the Red Hills and Arkansas River Lowlands.
The sample pictured above is from Barber County, Kansas
Buchanan, Rex C., Tolsted, Laura L., and Swineford, Ada, 1986, Kansas Rocks and Minerals: Kansas Geological Survey, Educational Series 2, 60 p.
Klein, Cornelis, 1993, Manual of Mineralogy (after James D. Dana), 21st Edition: New York, Wiley, 681 p.
Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey; text by Liz Brosius, Kansas Geological Survey.
Rocks and Minerals | Next Sample | Previous Sample
Mineral Identification | Mineral Hardness Scale