Hydrous Sulphates - Gypsum. Selenite Composition & Structure
Gypsum. Selenite
Composition. Hydrous calcium sulphate, CaSO4.2H2O = sulfur trioxide 46.6, lime 32.5, water 20.9.
Crystallization. Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edgesbeveled by prism and pyramid faces (Fig. 358). Other form rare. Sometimes twinned (Fig. 359).
Structure. Cleavage massive; foliated; granular massive; sometimes with fibrous appearance.
Physical Properties. Cleavage in three directions; perfect parallel to clinopinacoi, yielding easy thin foils; with conchoidal surface parallel to orthopinacoid; with fibrous fracture parallel to a pyramid to orthopinacoid; with fibrous fracture parallel to a pyramid. H. = 2 (can be scratched by the fingernail). G. = 2.32. usually with vitreous luster; sometimes
Silky. Colorless, white, gray; sometimes various shades of yellow, red, brown, etc., from impurities. Transparent to opaque
Test. Fusible at 3-3.5. after intense ignition, residue gives alkaline reaction on moistened test paper. Soluble in hot dilute hydrochloric acid and solution with barium chloride gives white precipitate of barium sulphate. Much water in C. T. Characterized by its softness and its perfect pinacoidal cleavage.
Varieties. Crystalline. In crystals or foliated masses. Fibrous. With coarse to fine fibrous appearance. Satin spar is fine fibrous with silky luster.
Massive. Alabaster, a fine-grained variety. Rock gypsum, massive granular or eartly; often impure.
Ocurrence. Gypsum is a common mineral which is widely distributed in sedimentary rocks, often as thick beds. It frequently occurs interstratified with limestones and shales. Usually to be found as a layer underlying beds of rocks salt and has been deposited there as one of the first minerals to crystallize because of the concentration of salt waters. Occurs also as lenticular bodies or scattered crystals in clays and shales. Commonly of primary origin but may have been derived from the alteration of anhydrite. Found at times in volcanic regions, especially where limestones have been acted upon by sulfur vapors. Also, is common as a gangue mineral in metallic veins. Associated with many different minerals, the more common ones being salt, anhydrite, dolomite, calcite, sulfur, pyrite, quartz. Deposited of gypsum of commercial importance are found in many localities in the United States, but the chief producers are located in New York, Oklahoma, Texas, Iowa, Michigan, Ohio, Kansas and Nevada gypsum is found is large deposits in Arizona and New Mexico in the form of wind-blown sand.
Name. Derived from the Greek name for the species. At times the crystalline variety is called selenite which comes from a Greek word meaning moon, probably in allusion to the moon – like white reflections from some varieties.
Use. Gypsum is chiefly used for the production of plaster of Paris. In the manufacture of this material, the dypsum is ground and then heated, until a large proportion of the water has been driven off. This plaster, when mixed with water, slowly absorbs the water and so hardens or “sets.” Plaster of Paris is used extensively for “staff,” the material from which temporary exposition buildings are built, and form molds and casts of all kinds. Gypsum is employed in making adamant plaster for interior use. Serves as land plaster, for fertilizer. Satin spar and alabaster are cut and polished for various ornamental purposes but are restricted in their uses on account of their softness.
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