a brown form of mica consisting of hydrous silicate of potassium and magnesium and aluminum
source: WordNet 3.0
A kind of mica having generally a peculiar bronze- red or copperlike color and a pearly luster. It is a silicate of aluminia, with magnesia, potash, and some fluorine. It is characteristic of crystalline limestone or dolomite and serpentine. See Mica.