| 词条 | Anhydrite |
| 释义 |
| name = Anhydrite | category = Sulfate mineral | boxwidth = | boxbgcolor = | image = Anhydrite_HMNH1.jpg | imagesize = 260px | alt = | caption = Anhydrite, Chihuahua, Mexico | formula = Anhydrous calcium sulfate:CaSO4 | strunz = 7.AD.30 | dana = 28.3.2.1 | system = Orthorhombic | class = Dipyramidal (mmm) H–M symbol: ({{sfrac|2|m}} {{sfrac|2|m}} {{sfrac|2|m}}) | symmetry = Amma | unit cell = a = 6.245(1) Å, b = 6.995(2) Å c = 6.993(2) Å; Z = 4 | color = Colorless to pale blue or violet if transparent; white, mauve, rose, pale brown or gray from included impurities | 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 | 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 = 3.5 | luster = Pearly on {010} vitreous to greasy on {001} vitreous on {100} | refractive = nα = 1.567–1.574 nβ = 1.574–1.579 nγ = 1.609–1.618 | opticalprop = Biaxial (+) | 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 = 56–84° | streak = White | gravity = 2.97 | melt = | fusibility = 2 | diagnostic = | solubility = | diaphaneity = Transparent to translucent | other = Some specimens fluoresce; many more fluoresce after heating | references = [1][2][3][4] }} 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 {{convert|200|°C|-2}} under normal atmospheric conditions.[5] Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits. OccurrenceAnhydrite 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 {{convert|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 nodulesAnhydrite 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 rocksMassive amounts of anhydrite occur when salt domes form a caprock. Anhydrite is 1–3% of the minerals 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] Igneous rocksAnhydrite has been found in some igneous rocks, for example in the intrusive dioritic pluton of El Teniente, Chile and in trachyandesite pumice erupted by El Chichón volcano, Mexico.[9] Naming historyThe 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.[10] Other usesThe 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. {{clear}}References1. ^{{cite book|last1=Klein |first1=Cornelis |first2=Cornelius S. |last2=Hurlbut |date=1985 |title=Manual of Mineralogy |edition=20th |publisher=John Wiley and Sons |location=New York |isbn=978-0-471-80580-9}} 2. ^{{cite web|url=http://webmineral.com/data/Anhydrite.shtml |title=Anhydrite |website=Webmineral}} 3. ^{{cite web|url=http://www.mindat.org/min-234.html |title=Anhydrite |website=Mindat.org}} 4. ^{{cite web|url=http://rruff.geo.arizona.edu/doclib/hom/anhydrite.pdf |title=Anhydrite |website=Handbook of Mineralogy}} 5. ^{{cite book|last1=Deer |last2=Howie |last3=Zussman |title=An Introduction to the Rock=Forming Minerals |publisher=Pearson Education |location=England |edition=2nd |date=1992 |page=614 |isbn=978-0-582-30094-1}} 6. ^{{cite book|last=Michael A.|first=Church |title=Encyclopedia of Sediments & Sedimentary Rocks |publisher=Springer |date=2003 |page=17–18 |isbn=978-1-4020-0872-6}} 7. ^{{cite journal |first=C. W. |last=Walker |title=Origin of Gulf Coast salt-dome cap rock |journal=AAPG Bulletin |date=Dec 1976 |volume=60 |issue=12 |pages=2162–2166 |doi=10.1306/c1ea3aa0-16c9-11d7-8645000102c1865d}} 8. ^{{cite journal |title=Origin of 'exotic' minerals in Mississippi salt dome cap rocks: results of reaction-path modeling |journal=Applied Geochemistry |volume=11 |issue=5 |date=September 1996 |pages=667–676 |doi=10.1016/S0883-2927(96)00032-7 |first1=James A. |last1=Saunders |first2=Robert C. |last2=Thomas}} 9. ^{{cite journal | url=http://www.sciencedirect.com/science/article/pii/S0377027308001613 | title=Primary igneous anhydrite: Progress since its recognition in the 1982 El Chichón trachyandesite | last=Luhr | first=James F. | journal=Journal of Volcanology and Geothermal Research | year=2008 | volume=175 | issue=4 | pages=394–407 | doi=10.1016/j.jvolgeores.2008.02.016}} 10. ^{{cite web|url=http://www.mindat.org/min-232.html |title=Angelite |website=Mindat.org}} Further reading
6 : Calcium minerals|Sulfate minerals|Evaporite|Salt domes|Luminescent minerals|Orthorhombic minerals |
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