|Chemical composition|| L3M2(SiO4)3
|Lustre||Vitreous to sub-adamantine|
Garnet is the family name given to a group 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)
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 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.
Then there is a 6th series that hardly produces any minerals large enough to be cut to gemstones.
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)
Valency in isomorphous replacement
The chemical formula of garnet is L3M2(SiO4)3, which means that the first element has a valency of 2+. 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.
- Naming Gem Garnets (2000) - W.Wm. Hanneman, Ph.D