Anhydrite

Anhydrite
Anhydrite, from Chihuahua, Mexico
General
CategorySulfate mineral
Formula
(repeating unit)
CaSO4
IMA symbolAnh[1]
Strunz classification7.AD.30
Dana classification28.3.2.1
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H–M symbol: (2/m 2/m 2/m)
Space groupAmma
Unit cella = 6.245(1) Å, b = 6.995(2) Å
c = 6.993(2) Å; Z = 4
Identification
ColorColorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities
Crystal habitRare tabular and prismatic crystals. Usually occurs as fibrous, parallel veins that break off into cleavage fragments. Also occurs as grainy, massive, or nodular masses
TwinningSimple or repeatedly on {011} common; contact twins rare on {120}
Cleavage[010] perfect
[100] perfect
[001] good, resulting in pseudocubic fragments
FractureConchoidal
TenacityBrittle
Mohs scale hardness3.5
LusterPearly on {010}
vitreous to greasy on {001}
vitreous on {100}
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.97
Optical propertiesBiaxial (+)
Refractive indexnα = 1.567–1.574
nβ = 1.574–1.579
nγ = 1.609–1.618
Birefringenceδ = 0.042–0.044
PleochroismFor violet varieties
X = colorless to pale yellow or rose
Y = pale violet or rose
Z = violet.
2V angle56–84°
Fusibility2
Other characteristicsSome specimens fluoresce; many more fluoresce after heating
References[2][3][4][5]

Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium (baryte) and strontium (celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5, and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions.[6] Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Klein, Cornelis; Hurlbut, Cornelius S. (1985). Manual of Mineralogy (20th ed.). New York: John Wiley and Sons. ISBN 978-0-471-80580-9.
  3. ^ "Anhydrite". Webmineral.
  4. ^ "Anhydrite". Mindat.org.
  5. ^ "Anhydrite" (PDF). Handbook of Mineralogy.
  6. ^ Deer; Howie; Zussman (1992). An Introduction to the Rock=Forming Minerals (2nd ed.). England: Pearson Education. p. 614. ISBN 978-0-582-30094-1.