Biotite K2(Mg,Fe)2(OH)2(AlSi3O10)

Optical Properties:

Biaxial negative; strong pleochroism in brown, reddish brown, and green; extinction is parallel to one excellent cleavage. Near extinction, a "crinkly", or sparkly appearance is common (upper left photo). The most distinctive properties of biotite are the combination of strong pleochroism and parallel extinction. The view upper right, in plane polarized light, shows a biotite flake lying on its cleavage, with tiny inclusions of zircon; radioactive decay of uranium in the zircon crystals has destroyed the biotite lattice near the inclusions, creating dark a dark halo around each zircon.

Although hornblende has similar pleochroic colors and can be confused with biotite, hornblende has inclined extinction, not parallel extinction.

Occurrence:

Biotite is a common mineral in siliceous intrusive rocks such as granite, quartz monzonite, granodiorite, and tonalite. It is less common in siliceous volcanic rocks. Biotite appears in shale, phyllite, and schist derived from shale protoliths.

Alteration:

In the photo above, biotite (dark mineal) is altering to chlorite.

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