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Andradite

The Garnet Minerals

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Andradite garnet from the Idria region of central California. (more below)
Green garnet of the ugrandite group
Photo (c) 2004 Andrew Alden, licensed to About.com (fair use policy)

Andradite is one of the calcium garnets, along with grossular and uvarovite (hence the name "ugrandite" for this group). All three are typically green, but andradite may also be red, brown, black or yellow. Black andradite is called melanite, yellow andradite is called topazolite, brownish andradite is called aplome, and gemmy green andradite is known in the trade as demantoid. The formula is Ca3Fe23+(SiO4)3. Grossular and uvarovite have aluminum and chromium, respectively, in place of the iron (Fe).

This small andradite specimen has about 20 crystals, the largest nearly 5 millimeters across. It comes from the Idria area in central California and is associated with serpentine, metamorphosed oceanic mantle rock, which accounts for the high iron content. Andradite is also well known in limestone that has undergone contact metamorphism.

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