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Andradite

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.

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN:0-87311-019-6
^ Andradite Webmineral website, accessed January 23, 2007