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n. A dark-brown or dark-green to black mica, K(Mg,Fe)3(Al,Fe)Si3O10(OH)2, found in igneous and metamorphic rocks.

n. A dark brown mica; it is a mixed aluminosilicate and fluoride of potassium, magnesium and iron.

n. Mica containing iron and magnesia, generally of a black or dark green color; -- a common constituent of crystalline rocks. See mica.

n. An important member of the mica group of minerals. See mica.

n. dark brown to black mica found in igneous and metamorphic rock

"In certain minerals, notably the brown variety of mica known as biotite, the microscope reveals minute circular marks occurring here and there, quite irregularly."

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