A black, dark brown, or greenish black micaceous mineral, occurring as a constituent of many igneous and metamorphic rocks.
- Although dark minerals such as biotite and hornblende are usually present, giving the rock a speckled appearance, they are never abundant.
- Quartz, feldspar, white mica and biotite are all major constituents.
- All four samples are medium-grained, and comprise variable proportions of hornblende, feldspar and quartz with accessory biotite and titanite.
Mid 19th century: named after J.-B. Biot (1774–1862), French mineralogist.
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