What does biotite mean?
Biotite is a common rock-forming mineral belonging to the mica group, easily recognized by its black to dark brown color and its perfect cleavage, which allows it to split into thin, flexible, elastic sheets. Chemically, it is an iron- and magnesium-rich aluminosilicate, and these dark micas occur abundantly in igneous rocks such as granite, as well as in metamorphic rocks like schist and gneiss. Geologists value biotite because it forms over a wide range of temperatures and pressures, making it a useful indicator of the conditions under which a rock formed, and because its radioactive components can be dated to determine a rock's age. The mineral is named for the French physicist Jean-Baptiste Biot, who studied the optical properties of mica in the early nineteenth century. While primarily a scientific term, biotite also explains the sparkling flecks visible in ordinary building stones such as granite.
nounA common rock-forming mineral of the mica group, typically black or dark brown, characterized by perfect cleavage into thin flexible sheets.
- A dark-colored (black or dark brown) rock-forming mineral of the mica group, rich in iron and magnesium, occurring as tabular crystals with perfect basal cleavage.
"The granite was speckled with glittering flakes of biotite that caught the afternoon light."
"Thin flakes of biotite peeled away from the rock face under the geologist's knife."
"The biotite in this schist records the high temperatures the rock reached deep within the crust."
The uncountable form denotes the mineral substance in general; the plural is used when referring to multiple individual crystals, grains, or analyzed specimens.
"The thin section contained several large biotites aligned parallel to the foliation."
Biotite is named after a 19th-century French physicist who studied light and magnetism — and its tiny glittering flakes are what make many granites sparkle.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of biotite
Biotite takes its name from Jean-Baptiste Biot (1774–1862), the French physicist who conducted pioneering studies on the optical properties of mica. The mineral was given his name by other scientists in the nineteenth century, following the common practice of commemorating researchers through mineral nomenclature; it belongs to the mica group, whose name traces back through Latin to Greek. Because it honors a personal surname rather than a classical root, it has no true cognates sharing its origin.
How biotite is actually used
A technical term used chiefly in mineralogy, petrology, and earth-science education; rare in everyday conversation. It is always treated as an uncountable mass noun when referring to the mineral substance, though plural usage occurs when referring to individual crystals or grains.
Easily confused with biotite
Muscovite is the pale or colorless 'white mica', whereas biotite is the dark iron- and magnesium-rich member of the mica family.
Phlogopite is a magnesium-rich mica that is typically amber to light brown, while biotite contains significant iron and is much darker.