Mineral name:  Gypsum variety Satin Spar Chemical formula:  CaSO4.2H2O Location:  Unknown Crystal system and class:  Monoclinic  2/m Genesis:  Selenite gypsum typically crystallizes in hydrothermal veins or through the recrystallization of massive gypsum deposits formed through the precipitation of evaporating salt water.  These long selenite crystals precipitated in solution cavities from calcium sulfate-bearing groundwater, or as a solution or hydrothermal vein deposit.  The finer-grained white material may be a recrystallized evaporite, and the gray material probably is residual clay minerals. Width x length of field of view in cm:  12 X 14 Specimen number:  E