General Information:
Chemical Formula:
K(Mg,Fe++)3[AlSi3O10(OH,F)2
Composition:
(Molecular Weight = 433.53 gm)
Potassium 9.02 % K
Magnesium 14.02 % Mg
Aluminum 6.22 % Al
Iron 6.44 % Fe
Silicon 19.44 % Si
Hydrogen 0.41 % H
Oxygen 43.36 % O
Fluorine 1.10 % F
Empirical Formula:
KMg2.5Fe2+0.5AlSi3O10(OH)1.75F0.25
Environment:
Granitic rocks. Forms a series with phlogopite. Has several polytypes (1M, 3T, 2M1)
Locality:
Common world wide.
Name Origin:
Named after the French physicist, Jean Baptiste Biot (1774 - 1862), who studied the optical properties of the micas.
Physical Properties:
Cleavage:
[001] Perfect
Color:
dark brown, greenish brown, blackish brown, yellow, or white.
Density:
2.8 - 3.4, Average = 3.09
Diaphaniety:
Transparent to translucent to opaque
Habits:
Micaceous - Platy texture with "flexible" plates., Lamellar - Thin laminae producing a lamellar structure., Pseudo Hexagonal - Crystals show a hexagonal outline.
Hardness:
2.5-3 - Finger Nail-Calcite
Luminescence:
None.
Luster:
Vitreous - Pearly
Streak:
gray
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