Garnet
From The Gemology Project
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--Doos 06:18, 8 December 2006 (PST) |
Garnet | |
---|---|
Chemical composition | L3M2(SiO4)3
Isomorphous series |
Crystal system | Cubic |
Habit | Dodecahedra |
Cleavage | None |
Hardness | 6.5-7.5 |
Optic nature | Isotropic |
Refractive index | 1.74-1.89 |
Birefringence | None |
Specific gravity | 3.60-4.20 |
Lustre | Vitreous to sub-adamantine |
Garnet is the family name given to a series of members, with a common crystal habit and slighly different chemical make-up (isomorphous). The following are the 6 members of the garnet family:
- Pyrope (magnesium aluminum silicate)
- Almandine (iron aluminum silicate)
- Spessartite (manganese aluminum silicate)
- Uvarovite (calcium chromium silicate)
- Grossular (calcium aluminium silicate)
- Andradite (calcium iron silicate)
According to similarities in the chemical composition, we can divide the above members of the garnet family into two groups.
- PyrAlSpites (Pyrope, Almandine, Spessartite) - the aluminium garnets
- UGrAndites (Uvarovite, Grossular, Andradite) - the calcium garnets
All the above members are rarely found with an ideal chemical make-up, instead they form isomorphous series. Most gem quality garnets belong to either of the following 5 isomorphous series and their chemical composition is an intermediate between the two endmembers mentioned.
- Pyrope-Almandine
- Pyrope-Spessartite
- Spessartite-Almandine
- Pyrope-Grossular
- Grossular-Andradite