File:Almandine garnet (Madison County, North Carolina, USA) 1.jpg

English: Almandine garnet from the Paleozoic of North Carolina, USA.

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 about 5400 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). Almandine is the most common type of garnet - it is commonly encountered 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. Initially, the development of large, undeformed garnets in metamorphic rocks may seem odd. However, some metamorphic minerals ignore external pressures as they grow. Staurolite and pyrite, both common metamorphic minerals, do the same thing.

Grossular is a calcium-aluminum garnet (ideally Ca3Al2Si3O12 - calcium aluminum silicate). It typically forms after argillaceous limestones have been contact metamorphosed or regionally metamorphosed.

Spessartine is a manganese-aluminum garnet (ideally Mn3Al2Si3O12 - manganese aluminum silicate). It is typically reddish to brownish in color. It is often reported in skarns (contact metamorphosed rocks) and rocks enriched in manganese.

Andradite is the most common variety of calcium garnet. Andradite is a calcium-iron garnet (Ca3Fe2Si3O12 - calcium iron silicate). It varies in color from yellowish to greenish to brownish to blackish. Green, chromium-bearing andradite is called demantoid. Black, titanium-bearing andradite is called melanite.

The specimen seen here is a well-formed, dodecahedral crystal of deep reddish almandine garnet that is mostly coated by greenish chlorite from the chlorite schist host rock.

Provenance: the original Albert Edward Foote of Philadelphia mineral dealer label is attached to one of the crystal's faces.

Locality: unrecorded locality in Madison County, western North Carolina, USA (but likely from the Little Pine Garnet Mine along Caney Fork Road, ~~3.2 air-miles southwest of the town of Marshall in southern Madison County)

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Photo gallery of almandine: www.mindat.org/gallery.php?min=452

Photo gallery of grossular: www.mindat.org/gallery.php?min=1755

Photo gallery of spessartine: www.mindat.org/gallery.php?min=3725

Photo gallery of andradite: www.mindat.org/gallery.php?min=223

Photo gallery of melanite:

www.mindat.org/gallery.php?min=7443