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

Basic

Garnet is the family name given to a group of members, with a common crystal habit and slightly different chemical makeup (isomorphous). The following are the 6 members of the garnet family:

In total there are 15 members of the garnet group, in gemology we traditionally disregard the other 9 because they do not produce gem quality minerals.

All the above members are rarely found with an ideal chemical makeup, 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

Then there is a 6th serie that hardly produces any minerals large enough to be cut to gemstones.

  • Uvarovite-Grossular

According to their 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)

Physical and optical properties

The physical and optical properties of the members of the garnet group vary greatly and we refer to the individual members for details.

Valency in isomorphous replacement

The chemical formula of garnet is L3M2(SiO4)3, which means that the first element has a valency of 2+ and the second element has a valency of 3+. Elements with the same valency can easily replace each other to form new chemical bonds, as in the case of garnet. One should not confuse the presence of trace elements with isomorphous replacement. Trace elements are not part of the "ideal" chemical makeup.

Related topics

References

  • Naming Gem Garnets (2000) - W.Wm. Hanneman, Ph.D