Description:
Biotite is a common rock forming mineral, being present
in at least some percentage in most igneous and both
regional and contact metamorphic rocks. The typical
black to brown color of biotite is characteristic although
it is difficult to distinguish brown biotite from dark
brown phlogopite. The two are actually end members in
a series that is dependent on the percentage of iron.
Phlogopite is iron poor and biotite is iron rich. The
darker color and density increase with an increase in
the iron content. Biotite tends to form in a wider range
of conditions than phlogopite which is limited mostly
to ultramafic rocks and magnesium rich marbles and pegmatites.
Biotite, like other micas, has a layered structure of
iron magnesium aluminum silicate sheets weakly bonded
together by layers of potassium ions. These potassium
ion layers produce the perfect cleavage. Biotite is
rarely considered a valuable mineral specimen, but it
can accompany other minerals and compliment them. In
Bancroft, Ontario Biotite forms large crystals with
green apatite and hornblende. Single large plates or
"books" of biotite can grow to considerable
size and can make impressive mineral specimens. Weathered
tiny crystals of biotite can appear golden yellow with
a nice sparkle producing a "fool's Gold" that
has fooled many. |
