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Optical Mineralogy, Geology
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BiotiteMica; Sheet Silicates
Monoclinic (-)
Relief - moderate
Colour - brown or green
RI - 1.53-1.696
Birefringence - 0.028-0.08
Interference colours - 2nd-3rd order, masked by the body colour
Extinction - straight, mottled
Comments - alters to green chlorite, often contains inclusions of accessory phases
c-axis parallel section (left) and subhedral basal section (right) of brown biotite. Note the presence of the one perfect cleavage on the c-axis parallel section and its absence on the basal section. The colourless inclusions in the basal section are apatite.
PPL
Fish Canyon Tuff
crystals of biotite showing colour variation by pleochroism.
PPL
Fish Canyon Tuff
c-axis parallel section of biotite, showing the perfect cleavage. This crystal contains euhedral inclusions of apatite.
PPL
Fish Canyon Tuff
pleochroism in biotite: the same crystal rotated through 90° to show the change in body colour
PPL
Fish Canyon Tuff
"shocked" crystal of biotite, showing kink bands, formed during an explosive eruption.
PPL
Fish Canyon Tuff
Pleochroic halo around a crystal of zircon in biotite.
PPL
Fish Canyon Tuff
Biotite corona around magnetite. NB this is the Fe-rich variety called lepidomelane.
PPL
Larvikite, S. Norway
Top left, green biotite. Top right, the same group of crystals rotated to show the intense pleochroism.
Above the same group of crystals under crossed polars.
Amphibolite
Subhedral basal sections of biotite
PPL
granite, NW Spain