Biotite Facts

Biotite Facts
Biotite is group of common rock-forming minerals found in igneous and metamorphic rocks. It is a name used for a large group of black mica minerals which include annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. The micas vary in chemical composition but have very similar physical properties and are all sheet silicate minerals. In entry-level geology courses and in the field, the minerals generally cannot be distinguished without chemical, optical, or x-ray analysis so the name "biotite" is used. The color of biotite may be black, dark green, or dark brown, and its streak ranges from white to gray with flakes often produced. The crystal system is monoclinic, and Its luster is vitreous, cleavge basal or perfect, on the Mohs Hardness Scale it scores between 2.5 and 3, and its specific gravity is 2.7 to 3.4.
Interesting Biotite Facts:
Biotite is a primary mineral found in several crystalline igneous rocks including granite, diorite, gabbro, peridotite, and pegmatite.
Biotite will also form under metamorphic conditions to form schist and gneiss when argillaceous rocks are exposed to heat and pressure.
Biotite is not very resistant to weathering and will transform into clay minerals, it is still sometimes found in sediments and sandstones.
It is a mineral that is easily identified and with a little experience, a person can recognize biotite on sight.
It is a black mica with perfect cleavage and a vitreous luster on the cleavage faces.
When it is separated into thin sheets of mica, the sheets are flexible but will break upon severe bending.
When biotite is held help to the light, the sheets are transparent to translucent with a brown, gray, or greenish color.
When a brown color is noticed an experienced observer may recognize phlogopite.
The black mica minerals named biotite usually cannot be distinguished from one another without laboratory analysis.
There are a small number of commercial uses for biotite. Ground mica may be used as a filler and extender in paints, an additive to drilling muds, and in rubber products as an inert filler and mold-release agent.
Biotite can also be used as a non-stick surface coating on asphalt shingles and rolled roofing, as well as in the potassium-argon and argon-argon methods of dating igneous rocks.
Biotite is sometimes called the "other fools' gold" because tiny flakes of biotite swishing in a gold pan can produce bright bronze-colored reflections in the pan when struck by sunlight. A pin test can quickly distinguish between biotite, which is soft, and gold which is a hard mineral.

Related Links:
Minerals Facts
Animals Facts
Rhyolite Facts
Amphibolite Facts
Andesite Facts
Hornfels Facts
Gneiss Facts
Schist Rock Facts