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Biotite thin section

About Biotite
For Jean Baptiste Biot (1774-1862), French physicist, who studied the optical differences between the micas.
Biotite hand-specimen
Formula: K(Mg,Fe)3(AlSi3O10)(F,OH)2
System: Monoclinic
Color: Brown, gray, green, yellow, or reddish brown
Lustre: Vitreous, Pearly
Hardness: 2–3
Density: 2.7–3.3
Biotite #1 thin section (hFOV 2mm)
Biotite #2 thin section (hFOV 2mm)
Biotite PPL properties
Relief: Moderate positive
Habit/Form: Euhedral crystals are not uncommon, are usually tabular parallel to (001), and have a roughly hexagonal cross section. Also found as micaceous or tabular grains, or grains with irregular outline. Grains may be bent, particularly in metamorphic rocks.
Color: Typically brown, brownish green to olive green (due to Fe2+ content) or reddish brown (due to Ti and Fe3+ content). Other colors: gray-yellow, yellow-brown, orange-brown, dark brown, dark green, dark red-brown
Pleochroism: Intense; absent in basal section. The absorption is stronger when the cleavage traces are parallel to the vibration plane of the lower nicol.
Cleavage: Very perfect in one direction {001}; in basal sections (z-axis) has no cleavage.
Biotite XPL properties
Isotropy/Anisotropy: Anisotropic
Interference color: Order II-III bright
Extinction angle: 0 – 3° (bird’s eye extinction)
Twins: Twin-plane {110} may be present.
Uniaxial/Biaxial: Biaxial (-) (anomalous Uniaxial-)
Optic axial angle (2V): 2V 0 – 25°
Biotite distinguishing features under the microscope
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  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database