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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 formula of the group is:
X 2+3 Y 3+2 Si 3 O
12 *
X = Calcium
(Ca), Iron (Fe), Manganese (Mn), or Magnesium (Mg)
Y = Aluminum
(Al), Chromium (Cr), or Iron (Fe) **
Some rarer varieties of Garnets, called Hydrogarnets, contain
some hydroxyl (OH) replacing some silica (Si3O12) [or emperically
reduced to SiO4]. This alters
the formula for the Garnet group to:
X 2+3 Y 3+2 Si O 3 - Z (OH) 4 Z
Z signifies
that an indefinite amount of silicate (SiO) is replaced by
hydroxyl (OH)
In most references, the hydrogarnets are usually not recorded in
the formula for garnet, since they are too rare. However, they
are mentioned here for comprehensiveness.
The most common Garnet members are:
| Pyrope | Mg3Al2Si3O12 |
| Almandine | Fe2+3Al2Si3O12 |
| Spessartine | Mn3Al2Si3O12 |
| Andradite | Ca3Fe3+2Si3O12 |
| Grossular | Ca3Al2Si3O12 |
| Uvarovite | Ca3Cr2Si3O12 |
Lesser known Garnets (only Schorlomite discussed in this guide)
| Knorringite | Mg3Cr2Si3O12 |
| Majorite | Mg3(Fe,Al,Si)2Si3O12 |
| Calderite | (Mn,Ca)3(Fe3+,Al)2Si3O12 |
| Goldmanite | Ca3(V,Al,Fe3+)Si3O12 |
| Schorlomite | Ca3(Fe3+,Ti)2(Si,Ti)3O12 |
| 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) |
* The silicon (Si) may also be partially replaced by some aluminum, iron, or titanium)
** and in some rare members Titanium (Ti), Zirconium (Zr), Vanadium (V)
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