| Mica
var. Biotite
Biotite is a common
phyllosilicate mineral within the mica
group, with the approximate chemical
formula K(Mg,Fe)-3AlSi-3O-10(OH)
2. More generally, it refers
to the dark mica series, primarily a
solid-solution series between the
iron-endmember annite, and the magnesium
end member phlogopite; more aluminous
end-members include siderophyllite.
Biotite was named by J.F.L. Hausmann in
1847 in honor of the French physicist
Jean-Baptiste Biot, who, in 1816,
researched the optical properties of
mica, discovering many properties.
Biotite is a sheet silicate. Iron,
magnesium, aluminium, silicon, oxygen,
and hydrogen form sheets that are weakly
bound 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, and in some
instances side-by-side.
Properties
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 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. The color of biotite is usually
black and the mineral has a hardness of
2.5-3 on the Mohs scale of mineral
hardness.
Biotite dissolves in both acid and
alkaline aqueous solutions, with the
highest dissolution rates at low pH.[5]
However, biotite dissolution is highly
anisotropic with crystal edge surfaces
(h k0) reacting 45 to 132 times faster
than basal surfaces (001).
Under cross-polarized light biotite can
generally be identified by the gnarled
bird's eye extinction.
Occurrence
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. It has been estimated that
biotite comprises up to 7% of the
exposed continental crust.
The largest documented single crystals
of biotite were approximately 7 m2 (75
sq ft) sheets found in Iveland, Norway.
SOURCE
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