Hydrogrossular is just a calcium aluminum variety of Grossular Garnet with hydroxide (OH) partially replacing silica (SiO4). It is a calcium aluminum garnet series (formula: Ca3Al2(SiO4)3-x(OH)4x, with hydroxide (OH) partially replacing silica (SiO4)). It is white to light green in color and may be colored in a white, green, and pink combination.
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.
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. It differs from the other garnets in that it is never transparent. It is typically translucent to opaque, rarely clear, and found in different tones of green to bluish-green, red, gray, and white. It ranges from translucent to opaque. The most common color is bluish-green, but they are also found in pink, white, and gray.
- Cleavage: Indistinct
- Fracture: conchoidal
- Hardness (Mohs): 7–7.5
- Density: 3.57 to 3.73
- Color: green to bluish-green, pink, white, gray
- Transparency: Transparent to opaque
- Luster: vitreous
The cause of the green color is a chromium and possibly iron. Pink hydrogrossular is caused by the presence of manganese. Green Hydrogrossular may contain small dark gray to inclusions that are black. Hydrogrossular may have dark gray to black small inclusions. It has similarities to jade and has the misnomers Transvaal jade, and African jade. Sources for pink and green hydrogrossular are Canada, Russia, South Africa, and the United States. White Hydrogrossular is available in Burma and China.
Hydrogrossular is sometimes used as a gemstone, being cabochon cut, or made into beads. These stones are sourced from New Zealand, and Africa, however, it’s likely that hydrogrossular garnet can be found in most places garnets are sourced from. Sources for green and pink hydrogrossular are South Africa, Canada, and the United States. White hydrogrossular is sourced from Burma and China.