Minerals & Gemstone 480x104
Minerals & Gemstone 480x104
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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:

Pyrope Mg3Al2Si3O12
Almandine Fe2+3Al2Si3O12
Spessartine Mn3Al2Si3O12
Andradite Ca3Fe3+2Si3O12
Grossular Ca3Al2Si3O12
Uvarovite Ca3Cr2Si3O12


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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