Stilpnomelane
The Mica Minerals
Stilpnomelane is a black, iron-rich mineral of the phyllosilicate family with the formula K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)36∙nH2O. It forms at high pressures and low temperatures in metamorphic rocks. Its flaky crystals are brittle rather than flexible. Its name means "shining black" in scientific Greek.
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)
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