Biotite
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Plane-Polarized Light
- Moderate relief
- Orange, brown or dark green (A)
- Perfect micaceous cleavage
- May be dark pleochroic halos around inclusions of zircon or other mildly radioactive minerals (B)
Crossed Polarizers
- Second-and third-order interference colors usually not strikingly evident because of the strong natural coloration.
- Mottling common, giving the mineral a gnarly or "birds-eye maple" texture (A)
- May be dark pleochroic halos around inclusions of zircon or other mildly radioactive minerals (B)
Biotite in plane polarized light. This biotite is very green. Note the mottled texture.
Same field in crossed polarizers. The appearance is dominated by the deep color and changes little.
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Plane polarized light. This biotite is very pleochroic and changes color from light greenish brown to nearly opaque. |
A plane polarized light view of biotite ranging from light yellow-green to dark green in color. Note the good cleavage.
Same field in crossed polarizers. Light biotite grains can show appreciable interference color.
Biotite is often brown. The specimen below, seen in plane polarized light, ranges from light tan to red-brown in color. Numerous pleochroic haloes around small zircon inclusions can be seen.
Same field in crossed polarizers. This relatively light biotite shows obvious second-order colors, though somewhat modified by the natural color.
The two specimens below show biotite of different shades dotted with numerous dark haloes around zircon inclusions.
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Created 22 Sept 1997, Last Update