The Naica crystals grew for hundreds of thousands of years in water held at 58 °C, just below the temperature where anhydrite dissolves and gypsum forms, and were only found in 2000 when the mine pumped the chamber dry. The structure is sheets of calcium and sulfate held together by layers of water molecules, bonded only through their hydrogens; the sheets peel apart along the water, which is why selenite splits into thin clear plates.
Research Elements Compound · CaSO4(H2O)2 One unit cell, the smallest block that repeats in every direction to build the whole crystal. Atoms on its faces, edges and corners are shared with the neighboring cells, and atoms just outside it are drawn so every sulfate and every water molecule is whole. Farther atoms are drawn fainter. Atom positions from the Crystallography Open Database entry 2300258 (Henry, Paul F.; Weller, Mark T.; Wilson, Chick C., 2009). Evaporite beds left where seas and salt lakes dried up, usually with halite and anhydrite, its waterless twin: across the Permian of Texas, New Mexico and Oklahoma, in Nova Scotia, and under Paris, whose Montmartre quarries gave plaster of Paris its name. White Sands in New Mexico is 700 km² of gypsum dunes. The clear crystal is selenite, the fine-grained rock is alabaster, and the Cave of Crystals at Naica in Mexico holds selenite beams eleven meters long. Quarried in open pits; it is so soft (2 on the Mohs scale, scratched by a fingernail) that it is ripped loose by bulldozer rather than blasted. Crushed and heated to about 150 °C it gives up three quarters of its water and becomes plaster of Paris, which sets back into gypsum when mixed with water: that is drywall, plaster and the retarder ground into every bag of cement. Power stations that scrub sulfur dioxide from their flue gas with limestone make gypsum too, and much drywall is made from it.Gypsum
Elements
Crystal structure
Cell: a = 6.28689 Å, b = 15.2131 Å, c = 6.52996 Å; angles 90°, 127.43° and 90°
Space group: C 1 2/c 1Where it's found
How it's mined
Notes