gypsum - sand selenite rose from a dry lake near Merzouga, Morocco
Selenite gypsum CaSO4・2H2O
Selenite gypsum sand roses form in seasonal desert lakes as they dry. These selenite “desert roses” were newly discovered in 2018 in Dyet Sriji, a seasonal salt lake west of the village of Merzouga, Morocco, in the Sahara Desert near the Algerian border. Dyet Sriji is dry in summer.
Selenite is typically transparent, but when it crystallizes in sandy conditions it forms rosettes of flattened crystals, the rose “petals,” which are either encrusted with sand or with sand embedded throughout.
Unlike the white-edged desert roses you see for sale in rock shops, these are unaltered and completely natural. Since gypsum has two attached water molecules, mineral dealers cook selenite roses with a weed burner to drive off the water, turning the edges of the crystals white. They are not found like this naturally, and we prefer that students see this mineral in unaltered form.
Here’s a photo of selenite roses from Mexico being burned. This is also how plaster is made. Gypsum is heated to above 130˚ Celsius to drive off roughly 75% of the water. When powdered, it becomes plaster. When water is added, the plaster rehydrates to become gypsum. Between two thick paper sheets, it becomes wallboard.
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