Hydrogrossular

Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the degree of substitution (x):[1]

  • grossular: x = 0
  • hibschite: 0.2 < x < 1.5
  • katoite: 1.5 < x < 3.
Hydrogrossular
General
CategoryNesosilicate
Formula
(repeating unit)
Ca3Al2(SiO4)3−x(OH)4x
Crystal systemCubic
Identification
Colorgreen to bluish green, pink, white, gray
Crystal habitMassive
Cleavagenone
Fractureconchoidal
Mohs scale hardness7–7.5
Lustervitreous
Specific gravity4.15+0.05
−0.03
Polish lustervitreous to subadamantine
Optical propertiesSingle refractive, anomalous aggregate reaction
Refractive index1.810+0.004
−0.020
Birefringencenone
Pleochroismnone
Ultraviolet fluorescenceinert
Absorption spectradark green hydrogrossular often shows cutoff below 460nm. Other color stones may show line around 463nm, indicating some idocrase content
References[1]

Hydrogrossular is a garnet variety in which a Si4+ is missing from a tetrahedral site. Charge balance is maintained by bonding a H+ to each of the four oxygens surrounding the vacant site.

Hydrogrossular is found in massive crystal habit, sometimes grown in with idocrase.[1]

Hydrogrossular is translucent to opaque, and found in green to bluish green, pink, white, and gray. The cause of the green color is chromium, and possibly iron. Pink hydrogrossular is caused by the presence of manganese. Hydrogrossular may have dark gray to black small inclusions.[1] It has similarities to jade, and has the misnomers Transvaal jade, and African jade.[1]

Hydrogrossular is sometimes used as a gemstone, being cabochon cut, or made into beads. Sources for green and pink hydrogrossular are South Africa, Canada, and the United States. White hydrogrossular is sourced from Burma and China.[1]

References

  1. Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6


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