GARNET
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 purples, oranges, olive
greens, and deep greens. Garnets form 12-sided crystals (dodecahedrons),
or crystals with even more faces on them. The crystals become more &
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.
The six dark red garnets shown below are almandine.
Almandine is an iron-aluminum garnet (ideally Fe3Al2Si3O12
- iron aluminum silicate). Almandine is the most common garnet - it 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. 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 mineals, do the same thing.
Almandine garnets (each is ~8 mm across) derived from Cretaceous
garnet-muscovite schist at Garnet Ledge, Alaska, USA.
The four garnets shown below are grossular (a.k.a.
grossularite). Grossular is a calcium-aluminum garnet (ideally Ca3Al2Si3O12
- calcium aluminum silicate). It typically forms after argillaceous
limestones have been contact metamorphosed or regionally metamorphosed.
Grossular garnets from Mexico. Red grossular is 1.4 cm
across. Light tan grossular is 1.7 cm across. Olive-green
grossulars are 2.4 & 2.6 cm across.
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.
Spessartine garnet in matrix from Avondale, Pennsylvania, USA (CM
specimen, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA).
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.
Demantoid andradite garnets (green) on matrix.
Melanite andradite garnet (black) (crystal is 8.5 mm across) in
garnet-pseudoleucite syenite matrix, from the Diamond Joe Quarry in Arkansas'
Magnet Cove Complex (Albian-Cenomanian, mid-Cretaceous).