The Mineral Gypsum
Composition: Hydrated Calcium Sulfate
Crystal System: Monoclinic - prismatic
Fracture: conchoidal and fibrous
Specific Gravity: 2.3
Luster: glassy, pearly
Color: Colorless, white, and light tints
Cleavage: two, one perfect
Transparency: Transparent to opaque
Associated Minerals: borax, calcite, halite, pyrite, sulfur and others
Hydrous calcium sulfate (32.6% CaO, 46.5% SO3, 20.9% H20).
Gypsum is usually white, colorless or gray, but can also be shades of red, brown and yellow. Gypsum has several variety names that are widely used in the mineral trade. The three we see most are Selenite, Satin Spar and Alabaster. Gypsum is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. These beds are softer than those of anhydrite or marble, and gypsum will not bubble in acid. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of water and air. Gypsum crystals can be scratched with a finger nail which is about the only test needed. The clear plates bend but lack the elasticity of mica, and are softer than brucite. Thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Crystals are often fluorescent yellow, showing hourglass pattern within crystal. They may also be phosphorescent.
Gypsum is one of the more common minerals in sedimentary environments. It occurs in massive beds, as free crystals in clay beds and crystallized in limestone cavities.
"Selenite" is the colorless and transparent variety that shows a pearl like luster. Crystals are common and most often assume a tabular habit. Fish tail twins are characteristic and spear head twins or swallow tail twins are also formed. The commonest crystals are found loose and free-growing in clay beds, coming out whole. "Satin Spar" comes from compact fibrous aggregate veins. This variety has a very satin like look that gives these crystals a play of light. "Alabaster" is A fine grained massive material used for centuries in ornamental stone carving. Other Crystal Habits include bladed or blocky crystals with a slanted parallelogram outline. Long thin crystals show bends and some specimens bend into spirals called "Ram's Horn Selenite". Also massive, crusty, granular, earthy and fibrous.
Tests: Soluble in hot dilute hydrochloric acid; the addition of barium chloride solution makes a white precipitate. After firing, fluorescent and phosphorescent in long-wave ultraviolet light.
Gypsum is a widespread, commercially important mineral. The massive beds are quarried, or mined, for the manufacture of plaster of Paris and various plaster products such as sheet rock. Abundant deposits which have formed from the alteration of the water-free variety, anhydrite. Are mined for their economic applications, in New York State, Michigan, Texas, Iowa, California, Nova Scotia, Mexico, Australia and many more localities. Good crystals are found in clay beds in Ohio and Maryland, and interesting cave rosettes of twisting fibers (gypsum flowers), were found in Kentucky. The most beautiful gypsum (selenite) crystals are foreign in origin. Large water-clear crystals came from the Sicilian sulfur mines, often with inclusions of sulfur. These are classics. In Naica, Chihuahua, Mexico, a cavern in the mine contained long, slender, slightly milky needles with tubular water-filled cavities and movable bubbles.
Sheet Rock wall board, paint fillers, some cements, plaster of Paris, fertilizer, ornamental Alabaster and as rare mineral specimens.
FACTS & HISTORY:
The name plaster of Paris comes from its early production from quarries in Montmartre, Paris.
The name gypsum comes from the Greek word for the calcined or "burned" mineral.
The word selenite comes from the Greek word meaning moon rock. A Greek comparison of the pearly luster of the cleavage to moonlight.
Gypsum is used in drywall because it has very low thermal conductivity Which makes it a good insulator.