PHYSICAL PROPERTIES:
Chemistry: CaS04-2(H20)
Composition: Hydrated Calcium Sulfate
Class: Sulfates
Crystal system: Monoclinic - prismatic
Fracture: conchoidal and fibrous
Hardness: 2
Specific gravity: 2.3
Refractive Index:
Luster: glassy, pearly
Streak: White
Cleavage: two, one perfect
Color: Colorless, white, and light tints
Transparency: Transparent to opaque
Associated Minerals: borax, calcite,
halite, pyrite, sulfur and others
COMPOSITION:
Hydrous calcium sulfate (32.6% CaO, 46.5% SO3,
20.9% H20).
DISTINGUISHING CHARACTERISTICS:
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.
ENVIRONMENT:
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.
CRYSTAL DESCRIPTION:
"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:
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.
LOCALITIES:
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.
USES:
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.
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