Granitic rocks. Forms a series with phlogopite. Has several polytypes (1M, 3T, 2M1). Biotite is now a group name for phlogopite, siderophyllite and eastonite.
IMA Status:
Not Approved IMA 1998 - Bitote is now the group name
Locality:
Common world wide. Link to MinDat.org Location Data.
Name Origin:
Named in 1847 after the French physicist, Jean Baptiste Biot (1774 - 1862), who studied the optical properties of the micas.
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Physical Properties of Biotite
Cleavage:
{001} Perfect
Color:
Dark brown, Greenish brown, Blackish brown, Yellow, White.
Density:
2.8 - 3.4, Average = 3.09
Diaphaneity:
Transparent to translucent to opaque
Fracture:
Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern.
Habit:
Lamellar - Thin laminae producing a lamellar structure.
Habit:
Micaceous - Platy texture with "flexible" plates.
Habit:
Pseudo Hexagonal - Crystals show a hexagonal outline.
Hardness:
2.5-3 - Finger Nail-Calcite
Luminescence:
Non-fluorescent.
Luster:
Vitreous - Pearly
Streak:
gray
Optical Properties of Biotite
Gladstone-Dale:
CI meas= 0.026 (Excellent) - where the CI = (1-KPDmeas/KC) CI calc= -0.044 (Good) - where the CI = (1-KPDcalc/KC)
KPDcalc= 0.2163,KPDmeas= 0.2016,KC= 0.2071 Ncalc = 1.58 - 1.7
Optical Data:
Biaxial (-), a=1.565-1.625, b=1.605-1.675, g=1.605-1.675, bire=0.0400-0.0500, 2V(Calc)=0, 2V(Meas)=0-25. Dispersion r > v or r < v.
Pleochroism (x):
yellow, buff, light brown, pale greenish brown, or pale greenish brown.
Pleochroism (y):
reddish brown, brown, greenish brown, bluish green, or opaque.
Calculated Properties of Biotite
Electron Density:
Bulk Density (Electron Density)=3.07 gm/cc note: Specific Gravity of Biotite =3.10 gm/cc.
Fermion Index:
Fermion Index = 0.02 Boson Index = 0.98
Photoelectric:
PEBiotite = 4.04 barns/electron U=PEBiotite x relectron= 12.42 barns/cc.
Radioactivity:
GRapi = 129.08 (Gamma Ray American Petroleum Institute Units)
Concentration of Biotite per GRapi unit = 0.77 (%)
Estimated Radioactivity from Biotite - barely detectable
Biotite Classification
Dana Class:
71.02.02b.02(71)Phyllosilicate Sheets of Six-Membered Rings
Chemical Formula: 
- barely detectable