Biotite

Biotite Common Facts: common phllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe) 3 (AlSi 3 O 10 )(F,OH) 2. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderphyllite. Biotite was named by [|J.F.L. Hausmann] in 1847 in honour of the French physicist, Jean-Baptiste Biot , who, in 1816, researched the optical properties of mica, discovering many unique properties. Biotite is a sheet silicate. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" ( muscovite) – both form in some rocks, in some instances side-by-side.

Properties
Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae which easily flake off. It has a monoclinic crystal system, with tabular [|r] to pismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. When biotite is found in large chunks, they are called “books” because it resembles a book with pages of many sheets. Under cross polarized light biotite can generally be identified by the gnarled bird's eye extinction.

** Characteristics: **
Biotite can be brown, yellow, or white. It has pearly luster and a white streak. Its density is between 2.8 and 3.4. Its molar mass is 433.53.

**Common Uses**
-Surface treatment in concrete, plastic, and construction materials -Used in method of dating igneous rocks -electrical devices, such as, capacitors and thermionic valves.

Mica for Dating:
Biotite has been used quite a bit to age igneous rocks. two methods exist for doing so, potassium-argon dating and argon-argon dating. due to the quick dissipation of argon from the biotite crystal structure at high temperatures, this method only gives the minimum age for a rock. Biotite can also work to assess the temperature history of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is temperature sensitive google image

Annite= KFe3AlSi3O10(OH)2 Phlogopite= KMg3AlSi3O10(OH)2 Siderophyllite KFe2AL(Al2Si2O10)(OH)2 Unnamed end member (formerly known as eastonite)= KMg2Al(Al2Si2O10)(OH)2 The variation in biotite usually occurs in what elements occupy the octahedral sites. However, variations can also occur in what elements are found in the tetrahedral and hydroxyl sites. Examples of these are siderophyllite, oxybiotite, and ferriannite. ||
 * **Biotite** ||
 * **Property** || **Value** || **Comments** ||
 * **Formula** || K(Mg,Fe)3AlSi3O10(OH,O,F)2 || Shows substantial variability in composition that can be represented with the four end members:
 * **Crystal System** || Monoclinic (2/m) || Beta = 99.3o ||
 * **Crystal Habit** || Pseudo-hexagonal prisms or lamellar plates without crystal outline. || Micaceous or tabular grains, also irregular grains that may be bent (especially in metamorphic rocks). Thin folia are elastic. ||
 * **Physical Properties** || H = 2.5 - 3

G = 2.7 - 3.3 The color of biotite in hand sample is brown to black (sometimes greenish). Its streak is white or gray, and it has a vitreous luster. || The physical properties of biotite are affected by the amount of iron present. The specific gravity (G) is greater with increasing iron content, while the color in hand sample is darker with an increase in iron. || Z^a = 0 - 9o X^c = 0 - 9o optic plane (010) ||  || alpha = beta = gamma = || 1.522-1.625 1.548-1.672 1.549-1.696 || Refractive indices increase with increasing iron content. The diversity in biotite's composition makes it hard to use optical properties as an indicator for composition. Ferriannite may have alpha and gamma indices as high as 1.677 and 1.721 respectively. ||
 * **Cleavage** || (001) perfect || Easily seen in thin section ||
 * **Color/Pleochroism** || Typically brown, brownish green or reddish brown || Usually strongly pleochroic so grains are darker when the trace of cleavage is parallel to the lower polarizer. Colors are X = colorless, light tan, pale greenish brown, pale green; Y~Z = brown, olive brown, dark green, dark red-brown. Intensity of color generally increases with increase in iron content. Parallel to (001) yields darker colors with little pleochroism. Pleochroic halos around radioactive minerals (zircon or allanite) are common. ||
 * **Optic Sign** || Biaxial (-) || Some biotite may have a 2V of 0o, is sensibly uniaxial. ||
 * **2V** || 0-25o ||  ||
 * **Twinning** || None || Twins with 001 composition planes are possible, but usually not observed ||
 * **Optic Orientation** || Y=b
 * **Refractive Indices**
 * **Max Birefringence** || 0.03-0.07 || Strong mineral color will often mask the interference colors (3rd to 4th order). Flakes on the cleavage and sections that are cut parallel with {001}show low birefringence. ||
 * **Elongation** || Yes || Along cleavage ||
 * **Extinction** || Parallel or close to parallel || Has a "birds eye" texture seen at extinction. Bent grains show wavy extinction. ||
 * **Dispersion** || v > r (weak) || Less commonly, r > v for Mg rich varieties. ||
 * **Distinguishing Features** |||| Resembles muscovite but has smaller 2V and darker color. Mottled "birds-eye" extinction helps distinguish from similar minerals outside the mica family. Has moderate to high relief in thin section. Has micaceous habit and dark color. ||
 * **Occurrence** |||| Biotite is common in a variety of igneous and metamorphic rocks. In igneous rocks, it is found more commonly in silicic and alkalic rocks, e.g. granties, diorites, gabbros and peridotites. It is important in metamorphic rocks including schists, gneisses, phyllites, and hornfels. Also found in immature sedimentary rocks, but will alter to clay minerals when weathered. ||
 * **Editors** |||| Mary Hawkins (01J), Raycine Hodo (02J), Lisa Berrios (02), Jennifer Fitzsimmons (03), Rachel Grandpre (05), Lily Seidman (11), Ngozika Onuzo (12) ||