Garnet

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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 slightly 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)

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

According to the tendency to form isomorphous series between members, we can divide the above members of the garnet family into two groups (although these two groups can mix).

  • PyrAlSpites (Pyrope, Almandine, Spessartite)
  • UGrAndites (Uvarovite, Grossular, Andradite)