Biotite is a common phyllosilicate mineral
within the mica group, with the
approximate chemical formula K(Mg, Fe)3AlSi3O10(F,
OH)2. More generally, it refers to the dark mica series, primarily a solid-solution series between the
iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous
endmembers include siderophyllite. Biotite was named by J.F.L. Hausmann in 1847
in honour of the French physicist Jean-Baptiste Biot, who, in 1816, researched
the optical properties of mica, discovering many unique properties.
Biotite is a
sheet silicate. Iron, magnesium,
aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bond
together by potassium ions. It is sometimes called "iron mica"
because it is more iron-rich than
phlogopite. It is also sometimes called "black mica" as opposed to
"white mica" (muscovite) -- both form in some rocks, in some
instances side-by-side.
Like other
mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which
easily flake off. It has a monoclinic
crystal system, with tabular to
prismatic crystals with an obvious pinacoid termination. It has four prism
faces and two pinacoid faces to form a pseudohexagonal crystal. Although not
easily seen because of the cleavage and sheets, fracture is uneven. It has a hardness
of 2.5–3, a specific gravity of
2.7–3.1, and an average density of
3.09 g/cm³. It appears greenish to brown or black, and even yellow when
weathered. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. When biotite is found in large chunks, they are
called “books” because it resembles
a book with pages of many sheets.
Under cross
polarized light biotite can generally be identified by the gnarled Bird's Eye Extinction.
Biotite is
found in a wide variety of igneous and metamorphic rocks. For instance, biotite
occurs in the lava of Mount Vesuvius and in the Monzoni intrusive complex of
the western Dolomites. It is an essential phenocryst in some varieties of lamprophyre.
Biotite is occasionally found in large cleavable crystals, especially in
pegmatite veins, as in New England, Virginia and North Carolina. Other notable
occurrences include Bancroft and Sudbury, Ontario. It is an essential
constituent of many metamorphic schists, and it forms in suitable compositions
over a wide range of pressure and temperature.
Biotite is
used extensively to constrain ages of rocks, by either potassium-argon dating
or argon-argon dating. Because argon escapes readily from the biotite crystal
structure at high temperatures, these methods may provide only minimum ages for
many rocks. Biotite is also useful in assessing temperature histories of
metamorphic rocks, because the partitioning of iron and magnesium between
biotite and garnet is sensitive to temperature.
Biotite is
used in electrical devices, usually as a dielectric in capacitors and
thermionic valves.
