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Satin Spar Gypsum is a varietiy of the mineral gypsum; all four varieties show obvious crystalline structure. Satin spar, along with selenite, desert rose, and gypsum flower (the four "crystalline" varieties of gypsum) are sometimes grouped together and called selenite. All varieties of gypsum, including selenite and alabaster, are composed of calcium sulfate dihydrate (meaning has two molecules of water), with the chemical formula CaSO4·2H2O. Selenite contains no significant selenium; the similarity of names comes from both substances being named from the Ancient Greek word for the Moon, "selene". Satin Spar Gypsum is most often silky, fibrous, and translucent (pearly, milky), but can exhibit some coloration. (source: Wikipedia) Photos by Michael Marcotte The thin white vein layers among the red (due to iron oxide) claystone, in the above photo at Palo Duro Canyon, Texas, are streaks of satin spar gypsum. (Photos by Michael Marcotte) Return to List Back to Michael's Hobbies page Back to Michael Marcotte's HomePage