cubic Nesosilicates

Hydrogrossular

Ca3Al2(SiO4)3-x(OH)4x

Crystal Structure

CDL Notation

 #! Species: Hydrogrossular (grossular-hydrogarnet series)
#! System: Cubic (m3m)
#! Habit: Massive cryptocrystalline aggregate; green
cubic[m3m]:{110}@1.0 + {100}@0.5

Crystal System cubic

Point Group m3m

Crystal Forms (2)

{110} {100}

Quick Facts

Hardness

6.5

Specific Gravity

3.36

Refractive Index

1.71

Optical Character

Isotropic

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Physical Properties

Crystal Systemcubic

Hardness (Mohs)6.5

Specific Gravity3.36

CleavageNone (garnet; granular aggregate)

FractureSplintery to uneven

LustreWaxy to sub-vitreous

Optical Properties

Refractive Index1.71

Optical CharacterIsotropic

PleochroismNone (isotropic)

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].