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Mica ~ variety Muscovite
Muscovite (also known as common mica,
isinglass, or potash mica) is a hydrated
phyllosilicate mineral of aluminium and
potassium with formula
KAl2(AlSi3O10)(F,OH)2, or
(KF)2(Al2O3)3(SiO2)6(H2O). It has a
highly perfect basal cleavage yielding
remarkably thin laminae (sheets) which
are often highly elastic. Sheets of
muscovite 5 m × 3 m have been found in
Nellore, India.
Muscovite
has a Mohs hardness of 2–2.25 parallel
to the [001] face, 4 perpendicular to
the [001] and a specific gravity of
2.76–3. It can be colorless or tinted
through grays, browns, greens, yellows,
or (rarely) violet or red, and can be
transparent or translucent. It is
anisotropic and has high birefringence.
Its crystal system is monoclinic. The
green, chromium-rich variety is called
fuchsite; mariposite is also a
chromium-rich type of muscovite.
Muscovite
is the most common mica, found in
granites, pegmatites, gneisses, and
schists, and as a contact metamorphic
rock or as a secondary mineral resulting
from the alteration of topaz, feldspar,
kyanite, etc. In pegmatites, it is often
found in immense sheets that are
commercially valuable. Muscovite is in
demand for the manufacture of
fireproofing and insulating materials
and to some extent as a lubricant.
The
name muscovite comes from Muscovy-glass,
a name given to the mineral in
Elizabethan England due to its use in
medieval Russia as a cheaper alternative
to glass in windows. This usage became
widely known in England during the
sixteenth century with its first mention
appearing in letters by George
Turberville, the secretary of England's
ambassador to the Russian tsar Ivan the
Terrible, in 1568.
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