Garnet

The Garnet group of minerals show crystals with a habit of dodecahedrons and trapezohedrons. They are nesosilicates with the same general formula, A₃B₂(SiO₄)₃. Many different chemical elements are included in the several varieties of garnet, including calcium, magnesium, aluminium, iron²⁺, iron³⁺, chromium, manganese and titanium. Garnets show no cleavage, but do show a dodecahedral parting. Fracture is conchoidal to uneven; some varieties are very tough and are valuable for abrasive purposes. Hardness is 6.5-7.5, specific gravity is 3.1-4.3, lustre is vitreous to resinous, and they can be transparent to opaque. Garnets come in a wide variety of colours including red, yellow, brown, black, green, or colorless. The only colour not reported for garnet is purple. The name "garnet" comes from the latin granatus, a grain possibly in reference to malum garanatum (pomegranate) a plant with red seeds similar in shape, size and colour to some garnet crystals.

Six common varieties of garnet are recognized, based on their chemical composition they are; pyrope, almandine or carbuncle, spessartite, grossularite (also called hessonite or cinnamon-stone), uvarovite and andradite. The garnets make up two solid solution series; 1) pyrope-almandine-spessarite and 2) uvarovite-grossularite-andradite.

Grossularite is a calcium-aluminium garnet with the formula Ca₃Al₂(SiO₄)₃, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossularite is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown, red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossularite is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

Pyrope, from the latin pyropos meaning similar to fire. Sometimes called Cape ruby, is ruby-red in color and chemically a magnesium aluminium silicate with the formula Mg₃Al₂(SiO₄)₃, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to almost black. Transparent pyropes are used as gemstones. The name pyrope is derived from the Greek word meaning "fire-like." A variety of pyrope from Macon County, North Carolina is of a violet-red shade and has been called rhodolite, from the Greek meaning "a rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the proportion of two molecules pyrope to one molecule almandite.

Almandite, sometimes called almandine, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the latin meaning "little spark." The name Almandite is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandite is an iron-aluminium garnet with the formula Fe₃Al₂(SiO₄)₃; the deep red transparent stones are often called precious garnet and are used as gemstones. Almandite occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, etc.

Spessartite is manganese aluminium garnet, Mn₃Al₂(SiO₄)₃. It?s name is derived from Spessart in Bavaria. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartite of a beautiful orange-yellow is found in Madagascar. Violet-red spessartites are found in rhyolites in Colorado and Maine.

Uvarovite is a calcium chromium silicate with the formula Ca₃Cr₂(SiO₄)₃. It is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite and serpentinite or sometimes in crystalline marbles and schists. It is found in the Urals of Russia and Outukompu, Finland. Knorringite is a rare variety in which magnesium replaces calcium. It is often found in kimberlites and used as an indicator mineral in the search for diamonds.

Andradite is a calcium-iron garnet, Ca₃Fe₂(SiO₄)₃, is of variable composition and may be red, yellow, brown, green or black. The recognized subvarieties are topazolite (yellow or green), demantoid (green) and melantite (black). Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the Urals" from its occurrence there.

Gadolinium gallium garnet, Gd₃Ga₂(GaO₄)₃, is synthesized for use in magnetic bubble memory. Yttrium aluminium garnet, Y₃Al₂(AlO₄)₃, is used for synthetic gemstones. When doped with neodymium (Nd³⁺), these YAl-Garnets are useful as the lasing medium in lasers.

• Dana's Manual of Mineralogy ISBN 0471032883
• mindat.org
• Minerals.net
• USGS Garnet locations - USA