Description:
Group name for a family of hard silicate
Garnets as a group are relatively common in highly
metamorphosed rocks and in some igneous formations.
They form under the high temperatures and/or pressures
that those types of rocks must endure. Garnets can be
used by geologists as a gauge of how much temperature
and pressure the rock has endured. As a gemstone, garnets
have had a mixed reputation. Garnets do possess high
indices of refraction, are hard enough, have pretty
colors, are wonderfully transparent, lack cleavage and
are durable; thus making good candidates for gemstones.
However, many people consider garnets to be inferior
to other colored gems. This may be due to garnet's relative
abundance and widespread use, and therefore a (typically)
low price. Garnets are greatly variable in colors and
varieties, though, and many of these are both rare and
beautiful, producing genuinely precious gemstones. Some
garnets are truly unique in the mineral kingdom and
have much to offer as both gemstones and mineral specimens.
The general formula for most of the garnets is A3B2(SiO4)3.
The A represents divalent metals such as calcium, iron,
magnesium and/or manganese. The B represents trivalent
metals such as aluminum, chromium, iron and/or manganese
and in the rarer garnets; vanadium, titanium, zirconium
and/or silicon. The general formula for a couple of
rarer garnets (hibschite and katoite) is A3B2(SiO4)3-X(OH)4X.
The main differences in physical properties among the
members of the garnet group are slight variations in
color, density and index of refraction.
Garnets are isostructural, meaning that they share
the same crystal structure. This leads to similar crystal
shapes and properties. Garnets belong to the isometric
crystal class, which produces very symmetrical, cube-based
crystals. The most common crystal shape for garnets
however is the rhombic dodecahedron, a twelve sided
crystal with diamond-shaped (rhombic) faces. This basic
shape is the trademark of garnets, for no other crystal
shape is so closely associated with a single mineral
group like the rhombic dodecahedron is with garnets.
Most garnets are red in color, leading to the erroneous
belief that all garnets are red. In fact a few varieties,
such as grossular, can have a wide range of colors,
and uvarovite is always a bright green. As a mineral
specimen, garnets usually have well shaped and complex
crystals and their color and luster can make for a very
beautiful addition to a collection. At times, garnets
are accessory minerals to other valuable and pretty
gem minerals such as topaz, beryl, tourmaline, vesuvianite
and diopside making these specimens extra special.
The Garnet Group is actually a larger group than most
people know. In the table below are the more common
members with their chemistries and their significant
divergent properties: density (SG), index of refraction
(IR), and the typical color as well as their general
environment of formation. Under environment of formation:
metamorphic is indicated with an M and igneous with
an I. All garnets may contribute to detrital sedimentary
rocks but they are not of original sedimentary origin.
|
