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):
- grossular: x = 0
- hibschite: 0.2 < x < 1.5
- katoite: 1.5 < x < 3.
It is often regarded as a variety of Grossular, but it is scientifically regarded as two distinct individual mineral species that have a slightly different chemical formula than true Grossular.
- Category: Nesosilicate
- Formula: Ca3Al2(SiO4)3-x(OH)4x
- Crystal system Cubic
Fig: Hydrogrossular – a calcium aluminum garnet series
Hydrogrossular is found in massive crystal habit, sometimes grown in with idocrase. It is translucent to opaque and found in green to bluish green, pink, white, and gray. It may have dark gray to black small inclusions. It has similarities to jade and has the misnomers Transvaal jade, and African Jade.
- Color: green to bluish green, pink, white, gray
- Crystal habit: Massive
- Cleavage: none
- Fracture: conchoidal
- Mohs scale hardness: 7 – 7.5
- Luster: vitreous
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.