Selenite crystals. Selenite is a transparent and colourless form of gypsum (hydrated calcium sulphate), a common mineral deposit formed from precipita

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Selenite crystals. Selenite is a transparent and colourless form of gypsum (hydrated calcium sulphate), a common mineral deposit formed from precipitated salt (sedimentary evaporite). Gypsum has natural insulating properties, and all varieties feel warm to the touch. It is used in the making of plaster of Paris for construction, as a retarder in Portland cement, as a flux for pottery and as a fertiliser. Here one of the selenite crystals has developed in a classic 'ram's horn' shape (left).

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Photo credit: © DIRK WIERSMA/SCIENCE PHOTO LIBRARY / Alamy / Afripics
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