| General Formula: K2(Mg,Fe)3AlSi3O10(OH,O,F2)2 |
Sample: PT-104 | System: Monoclinc |
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| 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 |
