English: Uvarovite garnets on chromitite from Russia. (~2.6 cm across at its widest)

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 deep green-colored crystals shown above are a scarce type of chromium garnet called uvarovite (Ca3Cr2(SiO4)3 - calcium chromium silicate). The black-colored host rock is chromitite, a crystalline-textured intrusive igneous rock almost exclusively composed of chromite (FeCr2O4 - iron chromium oxide). The uvarovite garnets are fracture-filling and hydrothermal in origin.

The chromitite host rock is from an ultramafic igneous intrusion called the Saranovskiy Massif, which contains gabbro, gabbronorite, and serpentinitzed chromititic peridotite. The intrusion is hosted in Precambrian-aged metamorphic rocks (schists and quartzites). I have not seen published isotopic ages for the Saranovskiy Massif. The only hint of a geologic age that I have encountered is the non-informative term "Caledonian", which may refer to the Silurian and Devonian.

Locality: Sarany Chromite Mine (Saranovskiy Mine), near the village of Sarany (Saranovskaya), ~12 kilometers north of the Biserskoye Chromite Deposit, Permaskaya District (Perm District), Middle Ural Mountains, western Russia

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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

Photo gallery of uvarovite:

www.mindat.org/gallery.php?min=4125