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Gypsum    (Selenite)
Current inventory:  0 gems
 

Chemistry:  CaSO₄ •2H₂O   [Hydrated Calcium Sulfate]

Discovered in Prehistory;   IMA status: (Gypsum) Valid (pre-IMA; Grandfathered).
Gypsum was named in antiquity from the Greek word "gypsos," meaning plaster. Selenite is named from the Greek word σελήυη, for "moon light," in allusion to the moon-like white reflections of the mineral.

Classification

Mineral Classification:

Sulfates

Strunz 8th Ed. ID:

6/C.22-20

Nickel-Strunz 10th Ed. ID:

7.CD.40

7 : SULFATES (selenates, tellurates, chromates, molybdates, wolframates)
C : Sulfates (selenates, etc.) without additional anions, with H₂O
D : With only large cations

Related to:

Isostructural with Brushite. Hydrated form of Anhydrite.

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Crystals as acicular to stubby prismatic crystals, thin to thick tabular, with more than 20 other forms noted, coarsely striated, to 17 m; lenticular in rosettes, may be curved, bent; fibrous, earthy, concretionary, granular, massive.

Twinning:

Very common forming cruciform and V-shaped twins; as butterfly or heart-shaped twins.

Physical Properties

Cleavage:

[010] Perfect, [100] Distinct

Fracture:

Splintery parallel to [001], conchoidal on [100]

Tenacity:

Flexible, Inelastic

Hardness (Mohs):

1.5 - 2.0 (varies with direction)

Density:

2.317 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioacitve

Other:

Slightly soluble in water

Optical Properties

Color:

Colorless, White; if colored by impurities, Yellow, Tan, Blue, Pink, Brown, Reddish Brown, Gray, Black

Transparency:

Transparent to Translucent

Luster:

Sub-vitreous, pearly on cleavages, silky if fibrous

Refractive Index:

1.521 - 1.530  Biaxial ( + )

Birefringence:

0.166 (very high)

Dispersion:

Strong; r > v

Pleochroism:

n/a

Occurances

Geological Setting:

Commonest of the sulphate minerals, gypsum is found in marine evaporites, in caves where the air is dry enough to allow it to be deposited and remain, at fumaroles, and in the oxidized zones of sulfide deposits on occasion. A common constituent of sedimentary rocks, particularly marine salt deposits, and soils formed directly by evaporation or later by hydration of anhydrite. Formed by the reaction between sulfuric acid and carbonate rock in oxidizing sulfide deposits, and by the action of sulfurous volcanic gases on surrounding Ca-bearing rock. As efflorescences in mines and speleothems in caves.

Common Associations:

Anhydrite, Aragonite, Calcite, Celestine, Dolomite, Halite, Sulfur

Type Locality:

n/a

Year Discovered:

Prehistory

View mineral photos:

Gypsum Mineral Photos and Locations
Selenite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Gypsum is one of the most abundant minerals but not often faceted. It is very soft and rarely transparent enough for faceting. Selenite is the name given to colorless, transparent Gypsum crystals and faceted gems. Alabaster is the fine grained massive variety that has been used for carving vases, bowls and other decorative items for thousands of years. Satin Spar is the name given to the compact fibrous aggregate type of Gypsum. Plaster of Paris is made by heating Gypsum to about 300 degrees Fahrenheit, removing 75% of the water. Heating Gypsum to about 350 degrees removes the remaining water and converts Gypsum to the mineral Anhydrite.

There are many locations for Gypsum but only a few locations for gem quality Selenite crystals including Rio Grande do Sul, South Region, Brazil; Willow Creek, Nanton, Alberta, Canada; Liupanshui Prefecture, Guizhou Province, China.
 

  
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