Grossular Garnet from Stanley Butte, Arizona.
Uvarovite from Russia, note the diamond
shaped (dodecahedron) face that is brightly reflecting light in the upper right.
Spessartine from China
GARNET is a family name.
CHEMISTRY - Members of the family can be expressed using this formula:
A3B2Si3O12
Pyrope Mg3Al2Si3O12
Almandine Fe3Al2Si3O12
Spessartine Mn3Al2Si3O12
Uvarovite Ca3Cr2Si3O12
Grossular Ca3Al2Si3O12
Andradite Ca3Fe2Si3O12
CRYSTALLOGRAPHY Isometric (Cubic)
CRYSTAL GROWTH AND HABITS Garnets typically form dodecahedrons and trapezohedrons and combinations of these forms. They can also be found as granular to massive.
COLOR AND OTHER OPTICAL PROPERTIES Garnets are commonly red but also brown, yellow, white, green or black. They are typically transparent to translucent.
HARDNESS 6.5 - 7.5
SPECIFIC GRAVITY 3.5 - 4.3
LUSTER Vitreous to resinous
STREAK White
BREAKABILITY The fracture of garnets is conchoidal to uneven and parting is occasionally observed. They are brittle.
OCCURRENCE One of the most common minerals and found as accessories in igneous rocks. They are commonly found in schists and gneisses and also as a resistant constituent in sandstones.
SPECIES DESCRIPTION
ALMANDINE This is the most abundant garnet, typically found in mica schists and gneisses. It is usually produced by metamorphism of sedimentary rocks containing an abundance of clay. Hardness 7 - 7.5 Specific Gravity 4.3 Color Deep red, brown red, red violet, black
ANDRADITE Typically formed by the metamorphism of impure limestones, calcium rich metamorphic rocks and in serpentines and chlorite schists. Hardness 6.5 - 7 Specific Gravity 3.8 - 3.9 Color Yellow, green, brown to black
GROSSULAR Normally found in and around metamorphic calcareous rocks like marble. Hardness 6.5 -7 Specific Gravity 3.5 - 3.6 Color Yellow green to dark green are the most common colors.
PYROPE Found in ultramafic rocks like peridotite, eclogites or kimberlite Hardness 7 - 7.5 Specific Gravity 3.5 - 3.6 Color Purple to red, pinkish, orangish red to nearly black
SPESSARTINE Usually found in rhyolites, granites or in granitic pegmatites Hardness 7 - 7.5 Specific Gravity 4.2 Color Red, orange, yellow, and in various intermediate colors as well as brown to black
UVAROVITE Found in metamorphosed limestones or dolomites containing chrome. Also produced by the alteration of chromite bearing serpentines. Hardness 6.5 - 7 Specific Gravity 3.7 - 3.8 Color Green, emerald green to dark green