11 Skarn de Granate, Fac. de Ingeniería, UNAM

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The chemistry of garnets is extremely complex. It involves the existence of intermediate chemical composition series between two extremes considered to be proper species. A given specimen receives its name from the dominant species, or if this is not known with certainty, it gets named after the series. So, we can have grossularia, andradite, or a garnet of the grossularia-andradite series. The various species of garnets are found in a wide range of colors, including red, orange, yellow, green, purple, brown, black, pink, and nearly colorless. Practically all colors except blue can be seen among garnets. Garnets are nesosilicates with the general formula X3Y2(Si O4)3. The position X is usually occupied by divalent cations such as Ca2+, Mg2+, Fe2+ and Y by trivalent cations like Al3+, Fe3+, Cr3+. This all takes place in an octahedral/tetrahedral structure with [SiO4]4− occupying the tetrahedron.