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Le gypse se retrouve à l'état naturel sous la forme d'une roche blanche, d'aspect mat, finement cristallin, tendre (rayable à l'ongle) et de densité 2,3. Quelques traces jaunes de soufre peuvent apparaître. On le retrouve un peu partout dans le monde (Europe, Asie, Amérique du Sud, Amérique centrale, Amérique du Nord, Moyen-Orient ...).

 

Chimie du gypse
Cliquez sur l'image Chimie du gypse pour la voir en grand - alphaplatre - Chimie du gypse

CaSO4-2(H2O),
Hydrated Calcium Sulfate
Class: Sulfates

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.

 

Propriétés physiques
  • Color is usually white, colorless or gray, but can also be shades of red, brown and yellow.
  • Luster is vitreous to pearly especially on cleavage surfaces.
  • Transparency crystals are transparent to translucent.
  • Crystal System is monoclinic; 2/m
  • 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.
  • Cleavage is good in one direction and distinct in two others..
  • Fracture is uneven but rarely seen.
  • Hardness is 2 and can be scratched by a fingernail.
  • Specific Gravity is approximately 2.3+ (light)
  • Streak is white.
  • Associated Minerals are halite, calcite, sulfur, pyrite, borax and many others.
  • Other Characteristics: 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 its use in drywall as an insulating filler). A crystal of Gypsum will feel noticeably warmer than a like crystal of quartz.