Hydrogrossular

Hydrogrossular
General
Category	Nesosilicate
Formula (repeating unit)	Ca3Al2(SiO4)3−x(OH)4x
IMA symbol	Hgrs[1]
Crystal system	Cubic
Identification
Color	green to bluish green, pink, white, gray
Crystal habit	Massive
Cleavage	none
Fracture	conchoidal
Mohs scale hardness	7–7.5
Luster	vitreous
Specific gravity	4.15+0.05 −0.03
Polish luster	vitreous to subadamantine
Optical properties	single refractive, anomalous aggregate reaction
Refractive index	1.810+0.004 −0.020
Birefringence	none
Pleochroism	none
Ultraviolet fluorescence	inert
Absorption spectra	dark green hydrogrossular often shows cutoff below 460 nm. Other color stones may show line around 463 nm, indicating some idocrase content
References	Gemological Institute of America[2]

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):^[2]

• grossular: x = 0
• hibschite: 0.2 < x < 1.5
• katoite: 1.5 < x < 3.

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.^[2]

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.^[2] It has similarities to jade, and has the misnomers Transvaal jade, and African jade.^[2]

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.^[2]