cubic Nesosilicates
Hydrogrossular
Formula Ca3Al2(SiO4)3-x(OH)4x
Crystal Structure
#! Species: Hydrogrossular (grossular-hydrogarnet series)
#! System: Cubic (m3m)
#! Habit: Massive cryptocrystalline aggregate; green
cubic[m3m]:{110}@1.0 + {100}@0.5
cubic
m3m
{110} {100}
Quick Facts
Hardness
6.5
Specific Gravity
3.36
Refractive Index
1.71
Optical Character
Isotropic
Physical Properties
- Crystal System
- cubic
- Hardness (Mohs)
- 6.5
- Specific Gravity
- 3.36
- Cleavage
- None (garnet; granular aggregate)
- Fracture
- Splintery to uneven
- Lustre
- Waxy to sub-vitreous
Optical Properties
- Refractive Index
- 1.71
- Birefringence
- 0
- Optical Character
- Isotropic
- Pleochroism
- None (isotropic)
Characteristics
Colours
Green (Cr3+ and/or V3+ chromophores; resembles jadeite)Pink/mauve (Mn2+ substitution; Bushveld, South Africa)White (pure; colourless)
Localities
South Africa (Transvaal/Bushveld Complex; primary source of green and pink material)Pakistan (Chagai district; white material)New Zealand (minor)USA, California (minor)
Common Inclusions
Opaque black spots (chromite inclusions; common in Cr-bearing Transvaal material)Granular interlocking texture under magnification (coarser and more rounded than jadeite fabric)
Known Treatments
May be dyed to enhance colour
Crystal Forms
massivegranular
Diagnostic Features
Isotropic (singly refractive on polariscope) — jadeite and nephrite are both anisotropic aggregates; SG 3.36-3.55 (heavier than jadeite 3.34, much heavier than nephrite 2.90-3.03); FTIR shows OH absorption bands diagnostic of hydrogarnet substitution; chromite inclusions in Cr-bearing Transvaal material
Notes
Grossular garnet with OH replacing SiO4 groups (hydrogarnet substitution): Ca3Al2(SiO4)3-x(OH)4x. Cubic m3m; IMA-approved compositional variety of grossular. Trade names: "Transvaal jade," "South African jade," "African jade" — misleading; not true jade. Separation from jadeite confirmed by Hobbs 1982, Gems & Gemology 18(1):3-19 (DOI 10.5741/gems.18.1.3) [VERIFIED]: isotropic (garnet) vs anisotropic aggregate (jadeite/nephrite); SG difference. Chelsea filter: Cr-bearing hydrogrossular shows red under CCF — same as Cr jadeite; NOT diagnostic for separation. RI ~1.71-1.73 (isotropic but cryptocrystalline aggregate gives imprecise readings); FTIR OH absorption bands diagnostic for hydrogarnet substitution. Constants from Read 7th ed. (DOI 10.4324/9780080507224) [PARTIALLY_SUPPORTED].
Related Minerals
Other minerals in the cubic system
