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Chemical Formula | CaSO4 |
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Composition | Calcium sulfate |
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Color | Colorless, white, light yellow, gray, blue, orange-red, red, pink, lilac |
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Streak | White to light gray |
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Hardness | 3 - 3½ |
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Crystal Forms and Aggregates |
(Orthorhombic) Crystals, which are tabular and prismatic, are very rare. Usually occurs as fibrous, parallel veins that break off into cleavage fragments. Also occurs grainy, massive, nodular, as rectangular cleavage fragments, and as easily cleavable crystal groupings. |
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Transparency | Transparent to translucent |
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Specific Gravity | 2.9 - 3.0 |
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Luster | Vitreous to pearly |
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Cleavage | 2,3 - forming a cube |
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Fracture | Uneven to splintery |
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Tenacity | Brittle |
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Other ID Marks | 1) Some specimens fluoresce; many more fluoresce after heating |
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Varieties |  Angelite - nodular, polished form of Anhydrite |
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In Group | Sulfates ; Anhydrous sulfates |
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All About | Anhydrite is a rare mineral, since most
existing specimens alter
to the much more common mineral Gypsum (CaSO4 · 2H2O)
from the addition of water. Some specimens only partly alter
to Gypsum, leaving one part Anhydrite and the other part
Gypsum. Many deposits that once contained Anhydrite in
abundance now contain an abundance of Gypsum. Anhydrite is also rare as other minerals form pseudomorphs over it. Some localities, such as the famous Paterson, New Jersey zeolite occurrence, contain a large amount of pseudomorphs of Quartz and zeolites over Anhydrite, as well as empty, hollow casts of Anhydrite. However, intact Anhydrite specimens are rarely found there. Anhydrite usually occurs in arid regions, forming from the dehydration of Gypsum. The best crystals come from the rock area above salt domes, where the domes absorb any underground water and prevent it from entering the structure of the Anhydrite, which would cause it to alter to Gypsum. Large Anhydrite deposits may also alter to Gypsum if exposed to the surface of the earth and not collected right away. Specimens in a collection may alter if kept in moist conditions. It is recommended that this mineral be stored in a dry area or with silica gel. Some specimens sold to collectors from old collections may actually be Gypsum that has altered due to improper care. |
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Uses | Anhydrite is used for the production of sulfuric acid and as a filler in paper. Specimens of this mineral are rare and are in demand by collectors. |
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Striking Features | Cleavage properties, crystal habits, and low specific gravity |
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Complex Tests | May fluoresce after heating |
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Popularity (1-4) | 2 |
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Prevalence (1-3) | 3 |
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Demand (1-3) | 1 |
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Distinguishing Similar Minerals |
Calcite - effervesces in hydrochloric acid Barite - much heavier (4.3 - 4.6) |
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Commonly Occurs With |
Gypsum, Halite, Calcite, Brucite |
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Noteworthy Localities |
Anhydrite was first found in Innsbruk,
Austria. It was recently found as lilac crystals during
the drilling of the Simpleton Tunnel in Switzerland. It
also occurs in some of the salt mines of central Europe. Rounded light blue crystals, as well as radiating crystal bundles (some quite large) are found in Niaca, Chihuahua, Mexico. In Canada, Anhydrite is found in McLaren's Mine in North Burgess, Ontario, and in the Faraday Mine in Bancroft, Ontario. In the U.S., many Quartz and zeolite specimens pseudomorphed over existing Anhydrite, as well as empty Anhydrite casts were found in Paterson, Passaic Co., New Jersey. Intact specimens have come from Ajo, Pima Co., Arizona; Bisbee, Cochise Co., Arizona; the Carlsbad District of Eddy Co., New Mexico; Death Valley, Inyo Co., California; Isle Royale, Houghton Co., Michigan; and Balmat, Essex Co., New York. |
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