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PLASTER, mineral, water sulfate of calcium, CaSO4Ch2N2O. The name comes from a Greek "gipsos" that in the ancient time designated and actually plaster, and swept. The dense snow-white, cream or pink fine-grained kind of plaster is known as alabaster (in construction alabaster call also a product of roasting of natural plaster). Fibrous pinkish, white or yellowish and cream plaster with silky gloss is called silk spar, or the Ural selenit, and crystal with well expressed spaynost - plaster spar, maiden or maryiny glass (and abroad - selenit). Usually composes white and gray dense units, the colourless lamellar crystals having the form of the extended rhombus often meet. Singoniya is monoklinny. The doubles reminding a larkspur are very characteristic. Hardness 2 (it is scratched by a nail) that allows to distinguish this mineral from many others, including waterless sulfate of calcium, anhydrite and calcite. Lamellar crystals of plaster possess three directions of a spaynost of different degree of perfection. One spaynost very perfect also causes formation of Skolkovo in the form of slyudopodobny plates; two other less perfect, their cracks dissect plates.
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