Dictionary:

andradite

  (ăn-drä'dīt)

A green to brown or black calcium-iron garnet, Ca₃Fe₂(SiO₄)₃.

[After José Bonifácio de ANDRADA E SILVA.]

Crystal description

Crystals are very common, especially in some varieties, depending in part on the type of their usual occurrence. Pyropes usually embedded in volcanic rocks are not in sharp and well-formed crystals. Almandine, growing in mica schists, usually shows faces of the dodecahedron or the trapezohedron, as does spessartine, which commonly forms in open cavities. Uvarovite commonly coats seams in chromite and so is free to form good dodecahedral crystals. Grossular forms on seams and shows smooth trapezohedral and dodecahedral faces, crystallizes in pockets in pegmatite (essonite or "cinnamon stone," light brown in color), or is embedded in limestone, where it is usually in good dodecahedrons. Andradite commonly coats seams and forms small lustrous crystals, unless embedded in asbestos (as in Val Malenco, Italy, and the demantoids of the Urals), when it may be rounded. Occasionally in sandy aggregates of fine grains, or in massive white, pink, or green veins (grossular, particularly: "South African jade").

Red, brown, black, green, yellow, white. Luster glassy; hardness 6-7Ɖ; specific gravity 3.5-4.3; fracture conchoidal to uneven; cleavage none; but occasional partings. Transparent to translucent; almandine weakly magnetic.

A series of aluminum silicates with magnesium, iron, and manganese; and a second series of calcium silicates with chromium, aluminum, and iron in which the SiO ₂ amounts to about 35%.

Theoretically, distinguished by their variation in color, fusibility, and behavior:

However, enough iron is present in many garnets to make the globule magnetic anyway.

The garnet varieties are so generally crystallized and so typical in their occurrence that the group is very easily recognized. Red grains seen embedded in metamorphic and igneous rocks are most likely to be garnets. Apatite is softer and does not melt. Altered pyrite gives a limonite streak; zircons often fluoresce and will not fuse. Short tourmalines can look like dodecahedrons, but tourmaline will not fuse like garnet.

Garnet is one of the commonest of all minerals, and ordinary localities are far too numerous to list. Special localities might be worth mentioning. Pyrope is found in transparent grains in Arizona and New Mexico, in Bohemia, and in the South African diamond pipes. Large almandine crystals are found in the Adirondacks at North Creek, New York, where it is mined for garnet paper. Almandine is the most widely used of the jewelry stones and comes, for this use, from Madagascar and India. Spessartine is less common; it is usually pegmatitic or associated with metamorphic manganese deposits. It is sometimes gemmy, a bright orange has been found in Brazil (Golconda Mine and Fortaleza Brazil) and in Namibia. At Nathrop, Colorado, and in the Thomas Range, Utah, it occurs in brown-black crystals in gas cavities in a light-colored rhyolitic lava flow.

Uvarovite occurs mainly as green crusts often associated with other minerals of chromium, usually on seams and in fissures in that mineral. The largest crystals, to 1 in. (2 cm), have been found at a Finnish copper mine -- an untypical occurrence. Grossular has been found in light-colored crystals most often in contact-metamorphic deposits, as in Morelos and Lake Jaco, Mexico, where it forms pale greenish, light pink, and white crystals associated with vesuvianite. Massive white grossular has been found with jade in Myanmar and has been carved by the Chinese. Green grossular garnet occurs in Africa in a solid vein, and some has been carved and sold as "South African jade." A Tanzanian emerald green chrome grossular rivals demantoid as a gemstone though it lacks fire (tsavorite). Andradite is probably the rarest of the garnets, and may form on metamorphic rock crevices as crusts of lustrous crystals; yellow-green topazolite and emerald green demantoid are varieties. Black melanite andradite is found in San Benito Co., California.

Some varieties are important gemstones; pyrope is used in garnet paper, a variety of sandpaper esteemed for its better cutting qualities.

The noun has one meaning:

Meaning #1: a garnet consisting of calcium iron silicate and having any color ranging from yellow and green to brown and black; used as gemstone

Andradite
The black mineral is Andradite, (Melanite)
General
Category	Mineral
Chemical formula	Ca3Fe2(SiO4)3
Identification
Color	yellow, green, brown, black
Crystal system	cubic [1]
Cleavage	none
Fracture	conchoidal to uneven [1]
Mohs Scale hardness	6.5 to 7 [1]
Luster	vitreous
Polish luster	vitreous to subadamantine [1]
Refractive index	1.888 (+.007 -.033) [1]
Optical Properties	Single refractive, often anomalous double refractive [1]
Birefringence	none [1]
Dispersion	.057 [1]
Pleochroism	none [1]
Ultraviolet fluorescence	inert [1]
Absorption spectra	demantoid - 440nm band or complete absorption at 440nm and below, may also have lines at 618, 634, 685, 690nm [1]
Specific gravity	3.84 (+/-.03); melanite - 3.90 (+/- .20) [1]
Major varieties
Demantoid	transparent light to dark green to yellow-green
Melanite	opaque black
Topazolite	transparent to translucent yellow, may show chatoyancy

Andradite is a species of the Garnet Group. It is a nesosilicate, with formula Ca₃Fe₂Si₃O₁₂. ^[1]

Andradite includes 3 varieties:

• Melanite: Black in color, usually not cut as a gemstone.
• Demantoid: Vivid green in color, one of the most valuable and rare stones in the gemological world.
• Topazolite: Yellow-green in color and sometimes of high enough quality to be cut into a faceted gemstone.
• Andradite also is found in rarely colorless.

Andradite was named after the Brazilian mineralogist José Bonifácio de Andrade e Silva (1763-1838). It has a hardness of 6.5 to 7.^[2][1] Associated minerals are micas, diopside, chlorite, and serpentine. It is found in Italy, the Ural Mountains of Russia, Arizona and California.

External links

• Mindat page about andradite

References

1. ^ ^{a b c d e f g h i j k l m n}
2. ^ Andradite Webmineral website, accessed January 23, 2007

Black crystals of andradite garnet

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