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www.mineralgallery.co.za Minerals F-J |
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Gypsum |
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Name
Gypsum
Chemistry
CaSO4-2(H2O), Hydrated Calcium Sulfate
Uses
Plaster of paris, wall board, some cements, fertilizer, paint filler, ornamental stone, etc
Color
Color is usually white, colorless or gray, but can also be shades of red, brown and yellow.
Hardness
2 (Can be scratched with a fingernail)
Specific gravity
2.3-2.4
Crystals
The crystals of gypsum is tabular, often with curved faces. The colorless, transparent variety is called selenite, the fine grained variety is called alabaster.
Accompanied by:
Halite and anhydrite
Luster
Vitreous and shows a pearly color parallel to the cleavage
Cleavage
The first cleavage is perfect the others are good
Streak
White
Similar to:
Mica
Gypsum is the most common sulfate mineral. A hydrated calcium sulfate, it serves as a raw material in plaster of Paris and is also used as fertilizer.
Alabaster, a massive, fine-grained form, and satin spar, a fibrous form with a silky luster, are employed as ornamental stone. A transparent, colorless variety called selenite is used as optical material.
Gypsum occurs with halite and other evaporite minerals in thick, extensive beds that were deposited from seas or playa lakes. Such beds often alternate with limestone and shale. It also forms by the alteration of anhydrite and in other ways.
Gypsum is found extensively in many areas of the world. The mineral is soft (hardness 2), clear, white, or tinted. It has a relative density of 2.3 and perfect cleavage in one direction.
Thin crystals are flexible but not elastic, meaning they can be bent but will not bend back on their own. Also some samples are fluorescent. Gypsum has a very low thermal conductivity (hence it's use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz.
Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters. Since it forms easily from saline water, gypsum can have many inclusions of other minerals and even trapped bubbles of air and water.
Gypsum has several variety names that are widely used in the mineral trade.
"Selenite" is the colorless and transparent variety that shows a pearl like luster and has been described as having a moon like glow. The word selenite comes from the Greek for Moon and means moon rock.
Another variety is a compact fibrous aggregate called "satin spar" . This variety has a very satin like look that gives a play of light up and down the fibrous crystals.
A fine grained massive material is called "alabaster" and is an ornamental stone used in fine carvings for centuries, even eons.
Luster is vitreous to pearly especially on cleavage surfaces. Crystal habits include the tabular, bladed or blocky crystals with a slanted parallelogram outline. The pinacoid faces dominate with jutting prism faces on the edges of the tabular crystals. Long thin crystals show bends and some specimens bend into spirals called "Ram's Horn Selenite"
Two types of twinning are common and one produces a "spear head twin" or "swallowtail twin" while the other type produces a "fishtail twin". Also massive, crusty, granular, earthy and fibrous.
