Andradite

Red andradite garnets from the Wessels Mine, South Africa.
Photograph by Rob Lavinsky, iRocks.com -released under CC-BY-SA-3.0 license

Andradite from the Kayes Region, Mali.
Photograph by Rob Lavinsky, iRocks.com -released under CC-BY-SA-3.0 license

Andradite Kayes Region, Mali (about 3cm across).
Photograph by Rob Lavinsky, iRocks.com -released under CC-BY-SA-3.0 license

Andradite cluster

Andradite is a type of garnet. [1] It can come in different varieties - including Demantoid (green), Melanite (opaque black), Topazolite (yellow) and, rarely, in red as seen in the top photo. It was discovered in Norway in 1868 and named after J.B. d'Andrada e Silva (1763-1838), the famous Brazilian geologist and statesman, [2] and as can be seen, grows in fascinating shapes and forms.

Andradite is considered possibly to be the rarest of the garnets - but is nevertheless widespread and has been found in Italy, Romania, Switzerland, Norway, Russia, California, Arizona, Alaska, Mexico and the Ukraine. It is a nesosilicate of the garnet group, which also includes Pyrope, almandine, spessartine, grossular, uvarovite and hydrogrossular. [3]

Andradite has the chemical formula Ca3Fe2(SiO4)3 and a hardness of 6.5-7 on the Mohs scale. It has the highest luster of all the garnets.

Andradite comes in four varieties: Demantoid, which is green and transparent; Topazolite which is lemon yellow to yellow-green; Melanite - the black opaque variety seen in two of the pictures above; and Schloromite, which is a titanium-rich variety that is classified as a separate mineral. The Demantoid variety is considered the most valuable. Garnets are isomorphous, meaning that they have the same mineral structure only with different elements substituted in certain positions. For example, grossular has aluminium instead of iron, uvarovite has chromium. Different garnet species may sometimes occur together.[4]

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Sources:

[1] http://en.wikipedia.org/wiki/Andradite
[2] http://rruff.geo.arizona.edu/doclib/hom/andradite.pdf
[3] http://en.wikipedia.org/wiki/Silicate_minerals#Nesosilicates
[4] http://www.minerals.net/mineral/silicate/neso/garnet/andrdite.htm