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Classification
Crystal System: Monoclinic
Class: Silicate
Sub Class: Phyllosilicate (layered silicate)
Group: Mica
Subgroup: Trioctahedral Mica
Series: Biotite-Phlogopite Series (dark micas)
Structure and Composition
Composition: K(Mg,Fe)2(AlSi3O10)(OH)2
Structure: biotite consists of alternating layers of tetrahedral and octahedral layers forming a sandwich-like structure. In tetrahedral layers, silicate tetrahedrons share 3 of their 4 atoms with adjacent tetrahedrons. In octahedral layers, magnesium or iron takes the place of silicon and has a coordination number of 6 with oxygen instead of 4. Cations, in this case, potassium, hold these sheets together and balance out the charges in the biotite's structure.
In the structure on the right red represents oxygen, grey represents silicon tetrahedra, orange represents iron or magnesium, and purple represents potassium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: Black, brown, gray, green, or reddish brown
Luster: Vitreous, Silky, Pearly
Hardness: 2.5
Streak: White
Specific gravity: 2.7–3.1
Common Habit: Thin and sheet-like, flakey in rocks, sometimes massive.
Cleavage: Perfect on {001} (splits into sheets)
Identification In Thin Section
PPL Properties
Relief: Moderate Positive
Habit/Form: Often appears as tabs or long skinny flakes that may be bent
Color: Typically in shades of brown, but also can be yellow, green or red
Cleavage: Very perfect in one direction {001}; in basal sections (z-axis) has no cleavage
Pleochroism: Intense.
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 (-)
Biotite PPL (photo by me)
Biotite PPL (photo by me)
Geologic Occurrence
Biotite is widespread in a wide variety of igneous and metamorphic rocks and, to a much lesser extent, in some immature sedimentary rocks. Biotite is more prevalent in mafic igneous rocks.
Significance
Less valuable varieties of phlogopite and biotite are ground up to be used as filler, lubricant, or dusting material to keep rubber and asphalt goods from sticking to one another during storage and transport. Altered biotite, produced through hydrothermal or weathering processes, produces a porous material called vermiculite. When heated, vermiculite will expand to many times its initial volume making it an effective insulating agent. Vermiculite is also commonly added to potting soil to improve soil quality and drainage.
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