Biotite

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