andradite

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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.

• Gemstones

Andradite
single crystal (4.2cm) - Diakon, Nioro du Sahel Circle, Kayes Region, Mali
General
Category	Garnet group
Chemical formula	Ca3Fe2(SiO4)3
Crystal symmetry	Isometric 4/m 3 2/m
Unit cell	a = 12.056 Å; Z = 8
Identification
Color	Yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, black; may be sectored
Crystal habit	Commonly well-crystallized dodecahedra, trapezohedra, or combinations, also granular to massive
Crystal system	Cubic - Hexoctahedral
Cleavage	none
Fracture	conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	6.5 to 7
Luster	Adamantine to resinous, dull
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.859 calculated; 3.8 - 3.9 measured
Optical properties	Isotropic, typically weakly anisotropic
Refractive index	n = 1.887
Absorption spectra	demantoid - 440nm band or complete absorption at 440nm and below, may also have lines at 618, 634, 685, 690nm [1]
References	[1][2][3][4]
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.

Andradite includes three varieties:

• Melanite: Black in color, referred to as "titanian andradite".^[5]
• 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.

It was first described in 1868 for an occurrence in Drammen, Buskerud, Norway.^[3] Andradite was named after the Brazilian mineralogist José Bonifácio de Andrade e Silva (1763–1838).^[2]

Occurrence

Black crystals of andradite : melanite

It occurs in skarns developed in contact metamorphosed impure limestones or calcic igneous rocks; in chlorite schists and serpentinites and in alkalic igneous rocks (typically titaniferous). Associated minerals include vesuvianite, chlorite, epidote, spinel, calcite, dolomite and magnetite.^[2] It is found in Italy, the Ural Mountains of Russia, Arizona and California and in Dnipropetrovsk Oblast in Ukraine.

As the other garnets andradite crystallizes in the cubic space group [[Ia3d]], with unit-cell parameter of 12.051 Å at 100 K.^[6]

The spin structure of andradite contains two mutually canted equivalent antiferromagnetic sublattices ^[7] below the Néel temperature (T_N=11 K ^[8]).

References

1. ^ ^{a b} Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2. ^ ^{a b c} Handbook of Mineralogy
3. ^ ^{a b} Mindat.org
4. ^ Webmineral data
5. ^ http://www.mindat.org/min-7443.html Mindat
6. ^ Thomas Armbruster and Charles A. Geiger (1993): Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets. European Journal of Mineralogy v. 5, no. 1, p. 59-71.
7. ^ Danylo Zherebetskyy (2010). Quantum mechanical first principles calculations of the electronic and magnetic structure of Fe-bearing rock-forming silicates, PhD Thesis, Universal Publishers/Dissertation.com, Boca Raton, Florida, USA, p. 136. ISBN 1-59942-316-2.
8. ^ Enver Murad (1984): Magnetic ordering in andradite. American Mineralogist 69, no. 7-8; p. 722-724.

v t e Titanium minerals Oxide minerals Simple Anatase Brookite Geikielite Rutile Mixed Aeschynite-(Y) Armalcolite Betafite Euxenite Freudenbergite Haggertyite Ilmenite Keilhauite Latrappite Perovskite Polycrase Zimbabweite Zirconolite Zirkelite Silicate minerals Aenigmatite Keilhauite Lorenzenite Melanite Nenadkevichite Titanite Zircophyllite Other Sabinaite (carbonate mineral) Warwickite (borate mineral)

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