Almandine garnet (North Creek area, New York State, USA) 1
Almandine garnet from the Precambrian of New York State, USA. (~10.5 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 large garnet shown above is almandine from New York State's Adirondack Mountains. In the eastern Adirondacks, "big garnets" are famously known from the North Creek and Warrensburg areas (e.g., see Hollocher et al., 2008). This specimen was likely derived from a Precambrian amphibolite host rock. The closely-spaced fractures are parting planes.
Locality: unrecorded/undisclosed site at or near the town of North Creek, eastern Adirondack Mountains, northern Warren County, northern New York State, USA
Photo gallery of almandine:
Photo gallery of grossular:
Photo gallery of spessartine:
Photo gallery of andradite:
Photo gallery of melanite:
Hollocher et al. (2008) - Petrology of big garnet amphibolites, North Creek-Warrensburg area, Adirondacks, NY. Geological Society of America Abstracts with Programs 40(2): 21.