Garnet is a group of minerals that have been used since the Bronze Age as gemstones
and abrasives. Garnets are most often seen in red, but are available in a wide variety of colors spanning the entire spectrum.
The name "garnet" comes from the Latin granatus ("grain"),
possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds
similar in shape, size, and color to some garnet crystals.
Six common species of garnet are recognized based on their chemical composition. They are pyrope, almandine, spessartite,
grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite
and andradite. The garnets make up two solid solution series; 1. pyrope-almandine-spessarite
and 2. uvarovite-grossular-andradite.
Physical Properties
Appearance
Garnets species are found in many colors including red, orange, yellow, green, blue, purple, brown, black, pink and colorless.
The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in parts of the United States,
Russia and Turkey. It changes color from blue-green in the
daylight to purple in incandescent light, as a result of the relatively high amounts of
vanadium (about 1 wt.% V2O3). Other varieties of color-changing garnets
exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear
a reddish or purplish/pink color. Because of their color changing quality, this kind of garnet is often mistaken for
Alexandrite.
Garnet species’s 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
Molecular model of garnet.
Garnets are nesosilicates having the general formula
X3Y2(SiO4)3. The X site is usually occupied by divalent cations
(Ca2+, Mg2+, Fe2+) and the Y site by trivalent cations (Al3+, Fe3+,
Cr3+) in an octahedral/tetrahedral framework
with [SiO4]4− providing the tetrahedra[2]. 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 isometric 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.
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.
Garnet group endmember species
Pyralspite garnets - Al in Y site
Almandine
Almandine in gneissic rock
Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle
(though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "little spark." The name Almandine is a corruption of Alabanda, a
region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an
iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones
are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandine occurs in
metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet,
almandine ruby, and carbuncle.
Pyrope
Pyrope, from the Latin pyropos, means similar to fire. It is red in color and
chemically a magnesium aluminium silicate with the formula
Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous
iron. The color of pyrope varies from deep red to almost black. Transparent pyropes are used as gemstones.
A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, from the Greek meaning "a
rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the
proportion of two parts pyrope to one part almandite. Pyrope has tradenames some of which are misnomers; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and
Bohemian garnet from the Czech Republic. Another intriguing find is the blue
color-change garnets from Madagascar, a pyrope spessartine mix. The color of these blue garnets is not like sapphire blue in subdued daylight but more reminiscent of the
grayish blues and greenish blues sometimes seen in spinel However in white LED light the color is equal to the best corn flower blue sapphire or D block tanzanite this is due to the blue garnets ability to
absorb the yellow component of the emitted light.
Pyrope is an indicator mineral for high pressure rocks. The garnets from mantle
derived rocks, peridotites and eclogites, commonly contain
a pyrope variety.
Spessartine
Spessartine (the yellow mineral)
Spessartine or incorrectly spessartite is manganese aluminium garnet,
Mn3Al2(SiO4)3. Its name is derived from Spessart in
Bavaria. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartine of a beautiful orange-yellow is found in
Madagascar (see Mandarin garnet). Violet-red
spessartines are found in rhyolites in Colorado and
Maine.
Ugrandite group - calcium in X site
Andradite
Andradite (the black mineral)
Andradite is a calcium-iron garnet,
Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or
black. The recognized varieties are topazolite (yellow or green), demantoid (green) and
melantite (black). Andradite is found both in deep-seated igneous rocks like
syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called
the "emerald of the Urals" from its occurrence there,
and is one of the most prized of garnet varieties. Topazolite is a golden yellow variety and melanite is a black variety.
Grossular
Grossular is a calcium-aluminium garnet with the formula
Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the
aluminium by ferric iron. The name grossular is derived from the botanical name for the
gooseberry, grossularia, in reference to the green garnet of this composition that is
found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow.
Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also
been called hessonite from the Greek meaning inferior. Grossular is found in
contact metamorphosed limestones with
vesuvianite, diopside, wollastonite and wernerite.
One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called
tsavorite. This garnet was discovered in the 1960s in the Tsavo
area of Kenya, from which the gem takes its name.
Uvarovite
Uvarovite is a calcium chromium garnet with the formula
Ca3Cr2(SiO4)3. This is a rather rare garnet, bright green in color, usually found as
small crystals associated with chromite in peridotite,
serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains
of Russia and Outokumpu, Finland.
Less common species
- Calcium in X site
- Goldmanite: Ca3V2(SiO4)3
- Kimzeyite: Ca3(Zr,Ti)2[(Si,Al,Fe3+)O4]3
- Morimotoite: Ca3Ti4+Fe2+(SiO4)3
- Schorlomite: Ca3(Ti4+,Fe3+)2[(Si,Ti)O4]3
- Hydroxide bearing - calcium in X site
- Hydrogrossular:
Ca3Al2(SiO4)3-x(OH)4x
- Hibschite: Ca3Al2(SiO4)3-x(OH)4x (where x is between 0.2 and 1.5)
- Katoite: Ca3Al2(SiO4)3-x(OH)4x (where x is greater than 1.5)
- Magnesium or manganese in X site
Knorringite
Knorringite is a magnesium chromium garnet species with the formula
Mg3Cr2(SiO4)3. Pure endmember
knorringite never occurs in nature. Knorringite is only formed under high pressure and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.
Synthetic Garnets
In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and
three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an
irregular cube. The iron ions in the two coordination sites exhibit different spins,
resulting in magnetic behaviour. YIG is a ferrimagnetic material having a Curie temperature of
550 K. By substituting specific sites with rare
earth elements, for example, interesting magnetic properties can be obtained.
One example for this is gadolinium gallium garnet, Gd3Ga2(GaO4)3, which is
synthesized for use in magnetic bubble memory.
Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstone. When doped with neodymium (Nd3+), these YAl-garnets are useful as the lasing medium in lasers.
Geological importance of garnet
The Garnet group is a key mineral in interpreting the genesis of many igneous and
metamorphic rocks via geothermobarometry.
Diffusion of elements is relatively slow in garnet compared to rates in many other minerals,
and garnets are also relatively resistant to alteration. Hence, individual garnets commonly
preserve compositional zonations that are used to interpret the temperature-time histories of the rocks in which they grew.
Garnet grains that lack compositional zonation commonly are interpreted as having been homogenized by diffusion, and the inferred
homogenization also has implications for the temperature-time history of the host rock.
Garnets are also useful in defining metamorphic facies of rocks. For instance,
eclogite can be defined as a rock of basalt composition, but
mainly consisting of garnet and omphacite. Pyrope-rich garnet
is restricted to relatively high-pressure metamorphic rocks, such as those in the lower crust and in the Earth's mantle. Peridotite may contain plagioclase, or aluminum-rich spinel, or pyrope-rich garnet, and the presence of each of the three minerals defines a pressure-temperature
range in which the mineral could equilibrate with olivine plus pyroxene: the three are listed in order of increasing pressure for stability of the peridotite mineral
assemblage. Hence, garnet peridotite must have been formed at great depth in the earth. Xenoliths of garnet peridotite have been carried up from depths of 100 km and greater by kimberlite, and garnets from such disaggegated xenoliths are used as a kimberlite indicator minerals in diamond prospecting. At depths of about 300
to 400 km and greater, a pyroxene component is dissolved in garnet, by the substitution of (Mg,Fe) plus Si for 2Al in the
octahedral (Y) site in the garnet structure, creating unusually silica-rich garnets that have solid solution towards
Majorite. Such silica-rich garnets have been identified as inclusions within diamonds.
Uses of garnets
Pendant in uvarovite, a rare bright-green garnet.
Pure crystals of garnet are used as gemstones. Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Mixed with very high pressure water,
garnet is used to cut steel and other materials in water jets. Garnet sand is also used
for water filtration media.
References
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