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Almandine



Almandine

General
CategoryMineral
Identification
Colorreddish orange to red, slightly purplish red to reddish purple and usually dark in tone
Cleavagenone
Fractureconchoidal [1]
Mohs Scale hardness7 - 7.5
Lustergreasy to vitreous
Polish lustervitreous to subadamantine [1]
Refractive index1.790 (+/- .030) [1]
Optical PropertiesSingle refractive, and often anomalous double refractive [1]
Birefringencenone
Dispersion.024 [1]
Pleochroismnone
Ultraviolet fluorescenceinert
Absorption spectrausually at 504, 520, and 573nm, may also have faint lines at 423, 460, 610 and 680-690nm [1]
Specific gravity4.05 (+.25, -.12) [1]

Almandine, also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet, of deep red color, inclining to purple. It is frequently cut with a convex face, or en cabochon, and is then known as carbuncle. Viewed through the spectroscope in a strong light, it generally shows three characteristic absorption bands. Almandine is one end-member of a mineral solid solution series, with the other end member being the garnet pyrope. The almandine crystal formula is: Fe3Al2(SiO4)3. Magnesium substitutes for the iron with increasingly pyrope-rich composition.

Additional recommended knowledge

Almandine occurs rather abundantly in the gem-gravels of Sri Lanka, whence it has sometimes been called Ceylon-ruby. When the color inclines to a violet tint, the stone is often called Syrian garnet, a name said to be taken from Syriam, an ancient town of Pegu. Large deposits of fine almandine-garnets were found, some years ago, in the Northern Territory of South Australia, and were at first taken for rubies and thus they were known in trade for some time afterwards as Australian rubies.

  Almandine is widely distributed. Fine rhombic dodecahedra occur in the schistose rocks of the Zillertal, in Tyrol, and are sometimes cut and polished. An almandine in which the ferrous oxide is replaced partly by magnesia is found at Luisenfeld in German East Africa. In the United States there are many localities which yield almandine. Fine crystals of almandine embedded in mica-schist occur near Fort Wrangell in Alaska. The coarse varieties of almandine are often crushed for use as an abrasive agent.

References

  1. ^ a b c d e f g Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN:0-87311-019-6
  • This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Almandine". A list of authors is available in Wikipedia.
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