Spessartine Garnet (#92)

A pair of lustrous intergrown dodecahedral dark orange-brown crystals to 5/8" on gray rhyolite.
North America Garnet Hill, near Ely, Nevada 1 ¼" x 1" x 1"

 

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Garnets are among the common minerals of the earth's crust, and they constitute a chemically diverse family of minerals. The general chemical formula for natural garnets (there are also many synthetic "garnet" compounds) is X3Y2SI3O12, where X and Y stand for various chemical elements that can be substituted into the formula. The most abundant natural garnets are:

X Y    
Mg3 Al2 Si3O12 Pyrope
Fe3 Al2 Si3O12 Almandine
Mn3 Al2 Si3O12 Spessartine
Ca3 Al2 Si3O12 Grossular
Ca3 Fe2 Si3O12 Andradite

There are about 20 other less common minerals that belong to the garnet group. Although the January birthstone, "garnet," is usually a reddish stone in jewelry stores, natural garnets can be colorless, yellow, pink, red, orange, brown, green, and black, and any shades or intermediates of the above.

Garnets exhibit a wide range in chemical composition. For example, garnets in any composition in the pyrope-almandine-spessartine range exist. Such variability between distinct "pure" compositions is called solid solution. This particular specimen is labeled spessartine based on its color and the sort of rock in which it occurs (a volcanic rock), but it likely contains significant Fe (iron) as well as major manganese (Mn).