Almandine, Fe2+Al2(SiO4)3, is classified as a garnet Nesosilicate, of the Pyralspite Group. It exhibits a cubic crystal system, with cations in cubic or octahedral coordination, or a combination thereof. Crystals are rounded with 12 rhombic (dodecahedron) or 24 trapezoidal (trapezohedron) faces, or combinations of these, as well as some other forms. Fe2+ occupies the large, eight-coordinated site. The iron is frequently replaced by magnesium and manganese. There seems to be a triangular solid solution among the three members of the Pyralspite garnet group, which includes Almandine [Fe2+Al2(SiO4)3], Pyrope [Mg3Al2(SiO4)3] and Spessartine [Mn2+3Al2(SiO4)3].

Distribution

Almandine, the most common of the garnets, is deep red to brownish-red and is found in metamorphic rocks, such as schists and gneisses, in association with micas, quartz, and feldspars. Gem quality almandine is found mainly in India, Sri Lanka, and Brazil. Finer crystals are found in Alaska and Idaho. Almandine is industrially important for use as Garnet paper, an important abrasive.

Importance in Soils

Inherited from parent materials, garnets in general are widely distributed in soils and contain trace elements which may provide micronutrients for plant growth. Garnets are often important in provenance (genesis) studies, where differences in garnet composition and trace element content may be useful indicators of parent rock source. Because of their relative resistance to weathering, garnets become enhanced in soils with time. Changes in the ratio of garnet to more weatherable minerals can be used to study the relative age of different parent materials (for instance, two different tills.)

American Mineralogist