Mineral name:  Gypsum variety Selenite Chemical formula:  CaSO4.2H2O Location:  Brewster County, Texas Crystal system and class:  Monoclinic  2/m Genesis:  Gypsum crystallizes in a number of different geologic environments, including hydrothermal veins, evaporite deposits in shallow marine seas or playa lakes, and solution cavities through precipitation from calcium sulfate-bearing groundwater.  The radiating, well-formed crystals of selenite in this "desert crystal" or "sand crystal" formed in the semi-arid climate of west Texas.  Groundwaters carrying dissolved calcium sulphate precipitated a few selenite crystals in sand that then grew outward from the central nucleation site. Width x length of specimen in cm:  13 X 14 Specimen number:  E