English: A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 6100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

Garnet is a group of silicate minerals. Garnets are expected to be red to dark red in color - many of them are, but several garnet varieties can be other colors, including purple, orange, olive green, deep green, and black. Garnets form 12-sided crystals (dodecahedrons) or crystals with even more faces on them. The crystals become more and more rounded as the crystal face number increases. Garnet has a nonmetallic, glassy luster, whitish streak, is quite hard (H = 7), has no cleavage, and has conchoidal fracture.

Common examples of garnet include almandine, grossular, spessartine, and andradite.

Almandine is an iron-aluminum garnet (ideally Fe3Al2Si3O12 - iron aluminum silicate) - it is the most common type of garnet. This variety is commonly found as well formed crystals in schists. It is also found in some igneous rocks. Almandine is classically used as a mineral indicator of regional metamorphism. Prima facie, the development of large, undeformed garnets in metamorphic rocks seems odd. However, some metamorphic minerals ignore external pressures as they grow. Staurolite and pyrite, both common metamorphic minerals, do the same thing.

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Photo gallery of almandine garnet:

www.mindat.org/gallery.php?min=452