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Biotite Pictures

 

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A large "book" of cleavable biotite from Bancroft, Ontario, Canada.

biotite1.jpg (1341409 bytes) More Biotite from the Bancroft area.

Biotite cleavage fragments

 

CHEMISTRY K(Mg,Fe)3(Al3SiO10)(OH)2   Potassium, magnesium - iron aluminum silicate

CRYSTALLOGRAPHY  Monoclinic

CRYSTAL GROWTH AND HABITS  Prismatic or tabular (short prismatic) crystals.  Crystals may appear rhombohedral and are called pseudorhombohedral.  Normally biotite occurs as foliated masses or as scaly aggregates.

COLOR AND OTHER OPTICAL PROPERTIES  Dark  brownish black, greenish black to black, rarely light yellow.  Thin sheets may have a brownish or smoky color.

HARDNESS  2.5 - 3       

SPECIFIC GRAVITY  2.8 - 3.2      

LUSTER  Vitreous or splendent (very high vitreous luster)

STREAK  Brownish

BREAKABILITY  Perfect "micaceous" cleavage

OCCURRENCE  Biotite is a widespread mineral found in igneous rocks, especially granite and syenite.  It is also found in large sheets in pegmatites and in felsic lavas as phenocrysts.