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(MICA GROUP) K2(Mg,Fe+2)6-4(Fe+3,Al,Ti)0-2[Si6-5Al2-3O20](OH,F)4
Crystal system: Monoclinic
Refractive index: 1.565-1.696
Birefringence: 0.04-0.08
Cleavage: perfect
Twinning: composition plane
Colour in PPL: yellow, brown or green
Pleochroism: strong
Relief: low to moderate
Distinction: brown pleochroic micas are considered to be biotite
Occurrence: wide range of environments
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Biotite - plain polarized light - Zabid petrofabric, Yemen - Field of view 0.5mm
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