Gypsum. Polarised light micrograph of a thin section of gypsum. Gypsum is a chemical sedimentary rock, composed mainly of hydrated calcium sulphate. It may grow as a crystal aggregate (as here) or in giant tabular crystals up to 1 metre in length. Gypsum is used in plaster of Paris, in Portland cement and as a flux in pottery. The most compact form of gypsum is known as alabaster. Magnification: x30 at 10 centimetres wide.
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