Anhydrite

Source: Wikipedia, the free encyclopedia.
"Angelite" redirects here. For the musical ensemble, see
Angelite (choir)
.
Not to be confused with
anhydride
.
Anhydrite
2V angle
56–84°
Fusibility2
Other characteristicsSome specimens fluoresce; many more fluoresce after heating
References[2][3][4][5]

Anhydrite, or anhydrous

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.

Occurrence

Crystal structure of anhydrite

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 the temperature is above 40 °C (104 °F). This is one method 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.[7]

Salt dome cap rocks

Massive amounts of anhydrite occur when

thermochemical sulfate reduction
(TSR).

Hand sample of gypsum and anhydrite drom diapir caprock showing "chicken wire" texture.

Igneous rocks

Anhydrite has been found in some

pluton of El Teniente, Chile and in trachyandesite pumice erupted by El Chichón volcano, Mexico.[10]

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.

A semi-transparent light blue-grey variety from Peru is referred to by the trade name angelite.[11]

Other uses

Relief carving of an anhydrite kiln, made from a piece of anhydrite, by Ophelia Gordon Bell

The Catalyst Science Discovery Centre in Widnes, England, has a relief carving of an anhydrite kiln, made from a piece of anhydrite, for the United Sulphuric Acid Corporation.

Extensive structural damage in the German city of Staufen im Breisgau has occurred since a 2007 geothermal drilling project allowed subsurface water to invade a layer of anhydrite below the city, causing extensive but uneven ground swelling as pockets of the anhydrite converted to gypsum.

References

  1. S2CID 235729616
    .
  2. ISBN 978-0-471-80580-9
    .
  3. ^ "Anhydrite". Webmineral.
  4. ^ "Anhydrite". Mindat.org.
  5. ^ "Anhydrite" (PDF). Handbook of Mineralogy.
  6. ISBN 978-0-582-30094-1
    .
  7. ISBN 978-1-4020-0872-6
    .
  8. doi:10.1306/c1ea3aa0-16c9-11d7-8645000102c1865d
    .
  9. doi:10.1016/S0883-2927(96)00032-7
    .
  10. doi:10.1016/j.jvolgeores.2008.02.016
    .
  11. ^ "Angelite". Mindat.org.

Further reading

Retrieved from "https://en.wikipedia.org/w/index.php?title=Anhydrite&oldid=1198964286"