BIOTITE

General Formula:
K2(Mg,Fe)3AlSi3O10(OH,O,F2)2
Sample: PT-104 System:
Monoclinc
Biotite Porphyrobalsts
Randomly oriented biotite porphyroblasts, exhibiting a range of pleochroic colours, in a pelite collected from the Meguma Group, Nova Scotia. Note the higher relief garnet grain in the lower left corner
Field of view = 4 mm, plane light
Biotite Porphyroblasts
The biotite porphyroblasts display a range of interference colours, and "bird's eye" extinction where the cleavge traces are parallel or nearly parallel to the polars (NS and EW).
Field of view = 4 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
typically brown, browhish green or reddish brown
distinctly pleochroic
Form tabular crystals parallel to {001} with a rough hexagonal shape, also as micaceous or tabular grains or grains with irregular outlines
Relief
RI
moderate to moderately high positive
nalpha = 1.522-1.625
nbeta = 1.548-1.672
ngamma = 1.549-1.696
Cleavage perfect cleavage on {001}
Birefringence
Interference Colours
0.03-0.07
up to third or fourth order, a strong mineral colour may mask the interference colour
Twinning rarely visible
Interference Figure
Optic Sign
2V
biaxial
negative
0-25°
Optic Orientation extinction is parallel or nearly parallel, with a maximum extinction angle of a few degrees. Cleavage traces are length slow.
Composition variable composition Alteration alters to chlorite, clay minerals, and/or sericite, iron-titanium oxides, epidote, calcite, and sulphides
Occurrence common in a wide range of igneous and metamorphic rocks and may be an important detrital mineral in sediments Distinguishing Features colour, pleochroism, "birds-eye" extinction, nearly parallel extinction


Back to: