A colorless, white, gray, blue, or lilac mineral of anhydrous calcium sulfate, CaSO4, occurring as layers in gypsum deposits.
[ANHYDR(OUS) + -ITE1.]
anhydrite
Dictionary:
an·hy·drite (ăn-hī'drīt)  |
n.
A colorless, white, gray, blue, or lilac mineral of anhydrous calcium sulfate, CaSO4, occurring as layers in gypsum deposits.
A colorless, white, gray, blue, or lilac mineral of anhydrous calcium sulfate, CaSO4, occurring as layers in gypsum deposits.
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A mineral with the chemical composition CaSO4. Anhydrite occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals. Fracture is uneven and luster is pearly to vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is 2.98. Anhydrite is an important rock-forming mineral and occurs in association with gypsum, limestone, dolomite, and salt beds. Under natural conditions, anhydrite hydrates slowly, but readily, to gypsum. It is not used as widely as gypsum. Anhydrite is of worldwide distribution. Large deposits occur in the Carlsbad district, Eddy County, New Mexico, and in salt-dome areas in Texas and Louisiana. See also Gypsum.
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A natural mineral calcium sulfate, used in the manufacture of portland cement to control its set.
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CaSO
Orthorhombic -- Rhombic bipyramidal
Environment
Sedimentary beds, gangue in ore veins, in traprock zeolite occurrences, and as insoluble sand from halite brines.
Crystal descriptionLarger crystals relatively rare, rectangular pinacoidal, or elongated parallel to domes. Also coarse to fine-grained, granular formations capping salt domes.
Physical propertiesColorless, white, gray, blue, lilac. Luster glassy to pearly; hardness 3-3Ɖ; specific gravity 3.0; fracture uneven to splintery, cleavage 3 good, pinacoidal. Transparent to translucent; sometimes fluorescent.
CompositionCalcium sulfate (41.2% CaO, 58.8% SO 3 ).
TestsCracks, decrepitates mildly, and fuses with difficulty on charcoal. Gives good sulfur test with silver. Fluoresces various colors after roasting: pink (Balmat, New York), yellow-green (Paterson, New Jersey), or blue-white (Switzerland).
Distinguishing characteristicsThe cubic aspect of its cleavage and the sulfur test, coupled with the low specific gravity, make its identification easy. Often associated with gypsum in same specimen.
OccurrenceAnhydrite is surprisingly rare in good mineral specimens, probably because it is easily altered to gypsum through hydration. It can be deposited directly from seawater. It is found most abundantly in sedimentary rocks, especially associated with salt beds. The metamorphic rock occurrences were probably derived from recrystallized gypsum. For a long time, the best occurrences were European. Highly prized, attractive lilac crystals and cleavages were encountered in drilling the Simplon Tunnel in Switzerland. It is abundant in Poland in the salt mines near Kraków, and in Germany at the Stassfurt mine. In these occurrences it is usually massive, as well as in small crystals all through the water-soluble salt.
A most interesting U.S. occurrence has little anhydrite today. It is only a silica skeleton preserved in the form of hollow 3-faced (sides and top) pseudomorphs of quartz and zeolites, after what must have been long blades of anhydrite, preserved in zeolite pockets in the West Paterson (New Jersey) traprock area. Some residual anhydrite, altered in part to white gypsum, has been found; the discovery solved a controversy about the origin of the rectangular voids. Countless numbers of tiny clear crystals have been found in salt-well insoluble residues in Louisiana and Texas. Equally tiny colorless crystals of anhydrite were recovered from halite that was caught up in a pegmatitic intrusion encountered in the zinc mine at Balmat, New York, and a violet anhydrite is associated with it in the bounding dike. Anhydrite is an important gangue mineral in some ore veins. It is found in lilac masses in Peru and Chile and is abundant in giant 12-in. (30 cm) crystals in the gangue of the Faraday Mine, Bancroft, Ontario. Freestanding, jagged-ended, Mayan-arch-tipped, 6-in. (15-25 cm) blue crystals are found with the Naica, Mexico lead ores in great abundance, making it now probably the world's richest specimen source.
Anhydrite beds once occurred in Nova Scotia, but they are now largely altered to gypsum; hydration swelling caused a beautiful crumpling of the weak layers, providing very graphic small-scale illustrations of the geologic process of the folding of stratified beds under compression. Pressure or elevated temperatures, or both, seem essential to anhydrite formation.
| Wikipedia: Anhydrite |
Not to be confused with anhydride.
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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. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite and pyrite in vein deposits.
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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.[4]
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.[5] Interaction with oil can reduce SO4 creating calcite, water, and hydrogen sulfide (H2S).[6]
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 Vulpino, near Bergamo, in Lombardy, as vulpinite; the latter is cut and polished for ornamental purposes.
References
- ^ Klein, Cornelis and Cornelius S. Hurlbut, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, ISBN 0-471-80580-7
- ^ http://webmineral.com/data/Anhydrite.shtml Webmineral
- ^ http://www.mindat.org/min-234.html Mindat.org
- ^ Michael A. Church, Encyclopedia of Sediments & Sedimentary Rocks, Springer, 2003, pp. 17-18 ISBN 978-1402008726
- ^ Walker, C. W. (Dec 1976). "Origin of Gulf Coast salt-dome cap rock". AAPG Bulletin 60 (12): 2162-2166.
- ^ 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:.
- Spencer, Leonard James. Anhydrite. 1911 Encyclopædia Britannica
- Mineralgalleries.com
- Minerals.net
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Architecture. McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Rock & Mineral Guide. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Anhydrite". Read more |
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Mentioned in
- karstenite
- allomorphite (mineralogy)
- anhydrock (petrology)
- calcium sulfate
- rim gypsum (geochemistry)
- tripestone
- vulpinite
- solution porosity (petroleum engineering)
- annual layer (geology)
- penesaline (ecology)
- cap rock (geology)
- halite
- cube
- Gypsum (mineralogy and petrology)
