Anhydrite

Not to be confused with anhydride.

Anhydrite Anhydrite, Chihuahua, Mexico General Category Sulfate mineral Chemical formula Anhydrous calcium sulfate:CaSO4 Strunz classification 07.AD.30 Dana classification 28.3.2.1 Crystal symmetry Orthorhombic 2/m 2/m 2/m Unit cell a = 6.245(1) Å, b = 6.995(2) Å, c = 6.993(2) Å; Z = 4 Identification Color Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities Crystal habit Rare tabular and prismatic crystals. Usually occurs as fibrous, parallel veins that break off into cleavage fragments. Also occurs as grainy, massive, or nodular masses Crystal system Orthorhombic – dipyramidal Twinning Simple or repeatedly on {011} common; contact twins rare on {120} Cleavage [010] perfect [100] perfect [001] good, resulting in pseudocubic fragments Fracture Conchoidal Tenacity Brittle Mohs scale hardness 3.5 Luster Pearly on {010} vitreous to greasy on {001} vitreous on {100} Streak White Diaphaneity Transparent to translucent Specific gravity 2.97 Optical properties Biaxial (+) Refractive index nα = 1.567–1.574 nβ = 1.574–1.579 nγ = 1.609–1.618 Birefringence δ = 0.042–0.044 Pleochroism For violet varieties X = colorless to pale yellow or rose Y = pale violet or rose Z = violet. 2V angle 56–84° Fusibility 2 Other characteristics Some specimens fluoresce; many more fluoresce after heating References [1][2][3][4] Crystal structure of anhydrite

Anhydrite is a mineral – anhydrous calcium sulfate, 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 hardness is 3.5 and the specific gravity 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 vitreous. 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 ~200°C under normal atmospheric conditions.^[5] Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite and pyrite in vein deposits.

Occurrence

Anhydrite is most frequently found in evaporite deposits with gypsum; it was, for instance, first discovered, in 1794, in a salt mine near Hall in Tirol. In this occurrence depth is critical since nearer the surface anhydrite has been altered to gypsum by absorption of circulating ground water.

From an aqueous solution calcium sulfate is deposited as crystals of gypsum, but when the solution contains an excess of sodium or potassium chloride anhydrite is deposited if temperature is above 40 °C. This is one of the several methods by which the mineral has been prepared artificially, and is identical with its mode of origin in nature, the mineral is common in salt basins.

Tidal flat nodules

Anhydrite occurs in a tidal flat environment in the Persian Gulf sabkhas as massive diagenetic replacement nodules. Cross sections of these nodular masses have a netted appearance and have been referred to as chicken wire anhydrite. Nodular anhydrite occurs as replacement of gypsum in a variety of sedimentary depositional environments.^[6]

Salt dome cap rocks

Massive amounts of anhydrite occur when salt domes form a caprock. Anhydrite is 1–3% of the salt in salt domes and is generally left as a cap at the top of the salt when the halite is removed by pore waters. The typical cap rock is a salt, topped by a layer of anhydrite, topped by patches of gypsum, topped by a layer of calcite.^[7] Interaction with oil can reduce SO₄ creating calcite, water, and hydrogen sulfide (H₂S).^[8]

Naming history

The name anhydrite was given by A. G. Werner in 1804, because of the absence of water of crystallization, as contrasted with the presence of water in gypsum. Some obsolete names for the species are muriacite and karstenite; the former, an earlier name, being given under the impression that the substance was a chloride (muriate). A peculiar variety occurring as contorted concretionary masses is known as tripe-stone, and a scaly granular variety, from Volpino, near Bergamo, in Lombardy, as vulpinite; the latter is cut and polished for ornamental purposes.

References

1. ^ Klein, Cornelis and Cornelius S. Hurlbut, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, ISBN 0-471-80580-7
2. ^ Anhydrite. Webmineral
3. ^ Anhydrite. Mindat.org
4. ^ Handbook of Mineralogy
5. ^ Deer, Howie, & Zussman. "An Introduction to the Rock Forming Minerals." Pearson Education Limited, England, 2nd Edition, 1992, Page 614. ISBN 0-582-30094-0
6. ^ Michael A. Church, Encyclopedia of Sediments & Sedimentary Rocks, Springer, 2003, pp. 17-18 ISBN 978-1402008726
7. ^ Walker, C. W. (Dec 1976). "Origin of Gulf Coast salt-dome cap rock". AAPG Bulletin 60 (12): 2162–2166. 
8. ^ Saunders, James A.; Thomas, Robert C. (September 1996). "Origin of ‘exotic’ minerals in Mississippi salt dome cap rocks: results of reaction-path modeling". Applied Geochemistry 11 (5): 667–676. doi:10.1016/S0883-2927(96)00032-7. 

Further reading

• Spencer, Leonard James. Anhydrite. 1911 Encyclopædia Britannica
• Mineralgalleries.com
• Minerals.net