Biotite

Mineral Name: Biotite

Chemical Composition: K(Mg,Fe2+3)(Al,Fe3+)Si3O10(OH,F)2.

Color: Biotite is most commonly dark black, but it can also be dark brown and even dark green (5).

Streak: Finding the streak of Biotite can be quite difficult due to the weakness of the mineral. When testing the streak, it is common for it to flake off and appear to produce a brown streak; however, this only occurs because of flaking. Biotite’s actual streak is either white or colorless (6).

Hardness: Biotite ranges from a 2.5 to 3; meaning your fingernail can scratch it, but a penny would not be able leave a scratch (6).

Cleavage/Fracture: Biotite has one perfect cleavage, which helps produce its thin sheets (5).

Crystal Form: The crystal form is monoclinic, meaning it has three uneven axes that form a rectangular prism. Two of the axes are perpendicular meeting at right angles and the third axes meets the other two at an angle that is not 90 degrees (4).

Luster: Vitreous to pearly (6).

Special Features: Sheets can be peeled off in thin layers. The thinner the layer, the more see through it becomes (6). Biotite is also very hard to clean. This is because if biotite is washed it will absorb water and start to break apart. The best way to clean biotite, or any mica, would be with a dry electric toothbrush (4).

Varieties: Manganophyllite, magnesium rich biotite, is the most common variation of biotite. Other variations include Annite, Siderophyllite, and Oxybiotite.

Mineral Group: Biotite is very common within the Mica group. It also belongs to the silicate group and the phyllosilicates group. Biotite is included in the silicates group because the composition contains silica. It is also apart of the phyllosilicates because it contains water, or hydroxyl atoms, that bond each layer of the sheets together (4). All together, biotite is a member of the mica group due to its one perfect cleavage that allows the mineral to peel into sheets (4).

Environment: Biotite can be found in many different rocks that have a variety of pressure and temperature (6). Biotite forms across the world in many different igneous and metamorphic rocks. It can be found in different mafic and felsic igneous rocks because it contains potassium and iron. However, it is less common in mafic rocks, such as basalt, because it isn’t stable at higher temperatures. Biotite is also very common in granite pegmatites (1). Biotite can also be found in metamorphic rocks and is one of the main components to create foliation. It can also form in carbonate rocks and metabasic rocks (1). Biotite is a common mineral in clastic sedimentary rocks, but does not significantly stand out due to weathering.

Associated Rock types: Biotite is found in so many different rocks that form at different temperatures and pressures, making it an interesting and diverse mineral. Igneous rocks like granite/rhyolite and diorite/andesite contain biotite. Metamorphic rocks that contain biotite include gneiss and schist. Sometimes, biotite can be found in sedimentary rocks, such as sandstone, because of its ability to adapt to clay minerals (6).

Occurrence in North America: Biotite can be found in the central Virginia Volcanic-Plutonic Belt as biotite granite (3). It can additionally be found in pegmatite in states such as Colorado. Due to its ability to withstand varying amounts of heat and pressure, it can be found in schist and gneiss rocks, which can be found throughout North America (2).

Economic Uses: Biotite isn’t necessarily considered a valuable mineral; however, it does complement other minerals found in various igneous and/or metamorphic rocks, such as granite. Granite is typically used in countertops and in other construction materials. Most people wouldn't know that biotite, along with hornblende, is what helps give the countertop the black speckled look. Biotite is a poor conductor of heat due to its ability to resist heat traveling through it; therefore it is used as an insulator for different electrical products and semiconductors (4).

Industrial Uses: Ground mica has many different uses. It’s most well known as a smoothing agent in wallboards when combined with gypsum (7). In this case, the ground mica creates durable wallboards by preventing cracks. The paint industry has also targeted ground mica for its pigment extension abilities in paint mixtures. In paint, ground micas, including biotite, help form a water resistant coating while also keeping the film from shrinking. In the rubber industry, sheet micas are often used as a mold release agent in products such as tires (2.) In electrical industries, micas act as an electrical and thermal insulator. For example, the use of the “glass” window connected to a modern stove. (7).

First Notable Identification: In 1816, physicist Jean Baptiste Biot discovered biotite while he was researching the different properties of the mica group, including the optical effects. JFL Hausmann named the mineral in 1847 in honor of Biot (4).

How We Identified It: Biotite is known for its black, shiny layers that peel into sheets. Although color isn't always reliable, it’s almost always a dark color. Muscovite, which is commonly mistaken for biotite, is usually lighter in color and has a more see through appearance. Biotite can also be confirmed by its low level of hardness when compared to its “look-a-likes”, such as pyrite.

Don’t Confuse It With: Biotite can easily be confused as muscovite or pyrite (fools gold). Small bits of biotite seen in beach sand can produce a color reminiscent of pyrite when struck by sunlight (2). Biotite has a hardness of 2.5, which means it can be scratched by a fingernail. Pyrite, on the other hand, has a hardness of 6.5. In other words, pyrite has the ability to cut glass, and biotite does not. More often than not, biotite is commonly mistaken for muscovite. However, biotite is usually a dark brown to black color, while muscovite is white, grey, yellow, or green in color.