Mineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth’s upper mantle.

The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the 6-fold B site holds smaller trivalent cations (Al3+, Cr3+, Fe3+). The BO6 octahedra share corners with tetrahedra forming a continuous network and the A sites are surrounded by 8 oxygens that form an irregular polyhedron.

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The general formula of garnet is A3B2(SiO4)3. However, Ca2+ cation is appreciably larger than the Mg2+, Fe2+, and Mn2+ cations (100 pm vs. 72 to 83 pm), resulting in separate mineralogical series of Ca-rich and Ca-poor garnets.

Ca-rich garnet comprise the ugrandite series; Ca-poor garnet form the pyralspite series. The ugrandite series gets is name from the main minerals in it: uvarovite, Ca3Cr2(SiO4)3grossular, Ca3Al2(SiO4)3, and andradite, Ca3Fe3+2(SiO4)3. Site occupancies are: A = Ca2+, B = Al3+, Cr3+, Fe3+. The names based upon the B site occupancy. Grossular-andradite forms a complete solid solid solution. The pyralspite series name is also based upon its minerals: pyrope, Mg3Al2(SiO4)3almandine, Fe3Al2(SiO4)3, and spessartine, Mn3Al2(SiO4)3. Site occupancies are: A = Mg2+, Fe2+, and Mn2+, B = Al3+. The names based upon the A site occupancy. Pyrope-almandine and almandine-spessartine form complete solid solutions.

Pyralspite compositions

There is limited solid solution between the pyralspite and ugrandite series as shown below.

There are many types of synthetic garnets. The gadolinium gallium garnet (GGG), Gd3Ga2(GaO4)3 is used in magnetic bubble memories, and the yttrium aluminum garnet (YAG), Y3Al2(AlO4)3 isd used for synthetic gemstones and as lasing medium in lasers (doped with Nd3+).

Some or all content above used with permission from J. H. Wittke.

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