| The Garnet Mineral Group |
The Garnet mineral group is compromised of isomorphous minerals identical in crystal structure and in many cases difficult to determine one from another. The members of this group intermingle with one another, within limits.
The simple chemical formula of the group is:
X2+3Y3+2Si3O12
X most often represents Ca, Fe2+, Mn, or Mg
Y most often represents Al, Cr, or Fe3+
Some rarer varieties of Garnets, called Hydrogarnets (or (Hydrogrossular) contain some hydroxyl (OH) replacing some silica (Si3O12) [or a reduced empirical formula of SiO4]. This alters the expanded chemical formula of the Garnet group to:
X2+3Y3+2SiO3-Z(OH)4Z
Z represents an indefinite amount of silicate (SiO) that is replaced by hydroxyl (OH).
In most references guides, the Hydrogarnets are usually not recorded in the formula for garnet, since they are too rare and not significant enough.
The most common Garnet members are:
Lesser known members of the expanded Garnet Group include:
| Knorringite |
Mg3Cr2Si3O12 |
| Majorite |
Mg3(Fe,Al,Si)Cr2Si3O12 |
| Calderite |
(Mn,Ca)3(Fe3+,Al)2Si3O12 |
| Goldmanite |
Ca3(V,Al,Fe3+)2Si3O12 |
| Schorlomite |
Ca3(Fe3+,Ti)2(Si3O12 |
| Kimzeyite |
Ca3(Zr,Ti)2(Si,Al,Fe3+)3O12 |
| Morimotoite |
Ca3TiFe3+Si3O12 |
The Hydrogarnets are:
| Hibschite |
Ca3Al2(SiO4)3-x(OH)4x
(where x is 0.2 to 1.5) |
Katoite |
Ca3Al2(SiO4)3-x(OH)4x
(where x is 1.5 to 3) |
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