Garnet
Group
Garnets (pronunciation: /ˈɡɑːrnət/) are
a group of silicate minerals that have
been used since the Bronze Age as
gemstones and abrasives. All
species of garnets possess similar
physical properties and crystal forms,
but differ in chemical composition. The
different species are pyrope, almandine,
spessartine, grossular (varieties of
which are hessonite or cinnamon-stone
and tsavorite), uvarovite and andradite.
The garnets make up two solid solution
series: pyrope-almandine-spessartine and
uvarovite-grossular-andradite.
Properties
Garnet species are found in many colors
including red, orange, yellow, green,
purple, brown, blue, black, pink, and
colorless, with reddish shades most
common.
Garnet species' light transmission
properties can range from the
gemstone-quality transparent specimens
to the opaque varieties used for
industrial purposes as abrasives. The
mineral's luster is categorized as
vitreous (glass-like) or resinous
(amber-like).
Crystal structure
Garnets are nesosilicates having the
general formula X3Y2(Si O4)3. The X site
is usually occupied by divalent cations
(Ca, Mg, Fe, Mn)2+ and the Y site by
trivalent cations (Al, Fe, Cr)3+ in an
octahedral/tetrahedral framework with
[SiO4]4− occupying the tetrahedra.[4]
Garnets are most often found in the
dodecahedral crystal habit, but are also
commonly found in the trapezohedron
habit. (Note: the word "trapezohedron"
as used here and in most mineral texts
refers to the shape called a Deltoidal
icositetrahedron in solid geometry.)
They crystallize in the cubic system,
having three axes that are all of equal
length and perpendicular to each other.
Garnets do not show cleavage, so when
they fracture under stress, sharp
irregular pieces are formed
(conchoidal).
Hardness
Because the chemical composition of
garnet varies, the atomic bonds in some
species are stronger than in others. As
a result, this mineral group shows a
range of hardness on the Mohs scale of
about 6.5 to 7.5. The harder species
like almandine are often used for
abrasive purposes.
Magnetics used in garnet series
identification
For gem identification purposes, a
pick-up response to a strong neodymium
magnet separates garnet from all other
natural transparent gemstones commonly
used in the jewelry trade. Magnetic
susceptibility measurements in
conjunction with refractive index can be
used to distinguish garnet species and
varieties, and determine the composition
of garnets in terms of percentages of
end-member species within an individual
gem.[5]
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