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Definition of Biotite

Biotite is the most common mica mineral and also known as black mica, a silicate mineral in the common mica group. Approximate chemical formula K (Mg, Fe). Biotite can be found in massive crystal layers weighing several hundred pounds. It is abundant in metamorphic rocks (both regional and contact), pegmatites, and also in granites and other invasive magmatic rocks. Biotite usually occurs in brown to black, dark green variety.

Biotite is a name used for a range of black mica minerals with different chemical compositions but with very similar physical properties. These minerals are usually indistinguishable from each other without laboratory analysis. There is a small list of biotite minerals that were down.

Mineral Chemical Composition
Annite KFe3(AlSi3)O10(OH)2
Phlogopite KMg3(AlSi3)O10(OH)2
Siderophyllite KFe2Al(Al2Si2)O10(F,OH)2
Eastonite KMg2Al(Al2Si3)O10(OH)2
Fluorannite KFe3(AlSi3)O10F2
Fluorophlogopite KMg3(AlSi3)O10F2

Occurrence of Biotite

Biotite is found in a wide variety of magmatic and metamorphic rocks. Biotite is common in various magmatic and metamorphic rocks. In igneous rocks, more commonly in silicic and alkaline rocks, e.g. granites, diorites, gabbros and peridotites. Schists are important in metamorphic rocks including gneisses, filites and horns. It is also found in immature sedimentary rocks, but changes the clay minerals when worn. The biotite in the granite tends to be weaker than the biotite present in the riolite in the volcanic equivalent of magnesium. Biotite is sometimes found in large degradable crystals in pegmatite veins, as is sometimes the case in New England, Virginia and North Carolina USA. It is a fundamental component of many metamorphic shingers and forms in suitable compositions in a wide range of pressure and temperature. Biotite is estimated to be up to 7% of the exposed continental crust.

Physical Properties of Biotite

Physical Properties of Biotite
Chemical Classification Dark mica
Color Black, dark green, dark brown
Streak White to gray, flakes often produced
Luster Vitreous
Diaphaneity Thin sheets are transparent to translucent, books are opaque.
Cleavage Basal, perfect
Mohs Hardness 2.5 to 3
Specific Gravity 2.7 to 3.4
Diagnostic Properties Dark color, perfect cleavage
Chemical Composition K(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2
Crystal System Monoclinic
Uses Very little industrial use

Optical Properties of Biotite

Property Value
Formula K(Mg,Fe)3AlSi3O10(OH,O,F)2
Crystal System Monoclinic (2/m)
Crystal Habit Pseudo-hexagonal prisms or lamellar plates without crystal outline.
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.

Cleavage (001) perfect
Color/Pleochroism Typically brown, brownish green or reddish brown
Optic Sign Biaxial (-)
2V 0-25o
Twinning None
Optic Orientation Y=b
Z^a = 0 – 9o
X^c = 0 – 9o
optic plane (010)
Refractive Indices
alpha =
beta =
gamma =
Max Birefringence 0.03-0.07
Elongation Yes
Extinction  Parallel or close to parallel

v > r (weak)

Uses of Biotite

  • The modified biotite, produced by hydrothermal or abrasion processes, produces vermiculite, a porous material used as an insulating material in gypsum wall panels and other heat insulation applications. When heated, the vermiculite will expand to most of its initial volume at an accordion-like expansion of its layered structure. This makes it a very effective sealant.
  • In addition, soil is added to soil to improve the quality of the soil and to improve drainage as it easily absorbs liquids, fertilizers, herbicides and pesticides.
  • Biotite is commonly used to limit the age of rocks through potassium-argon escort or argon-argon escort.
  • Biotite is also useful for evaluating the temperature histories of metamorphic rocks, because the division of iron and magnesium between biotite and garnet is temperature sensitive.