Encyclopedia
| Garnet | | General | Category | Mineral | Chemical formula ' | nesosilicates with the general formula X3Y23 | Identification | Color | virtually all colors | Crystal habit | rhombic dodecahedra or cubic | Crystal system | Isometric | Twinning | ? | Cleavage | None | Fracture | conchoidal to uneven | Mohs Scale hardness | 6.5 - 7.5 | Luster | vitreous to resinous | Refractive index | 1.72 - 1.94 | Pleochroism | only in certain rare varieties | Streak | White | Specific gravity | 3.1 - 4.3 | Major varieties | Pyrope | Mg3Al2Si3O12 | Almandine | Fe3Al2Si3O12 | Spessartite | Mn3Al2Si3O12 | Andradite | Ca3Fe2Si3O12 | Grossularite | Ca3Al2Si3O12 | Uvarovite | Ca3Cr2Si3O12 |
The
garnet group of
minerals show
crystals with a habit of
rhombic dodecahedrons and
trapezohedrons. They are nesosilicates with the same general formula,
X3Y23 in which the
X site is usually occupied by divalent cations and the
Y site by trivalent cations . The
chemical elements in garnet include calcium,
magnesium,
aluminium,
iron2+, iron
3+,
chromium,
manganese, and
titanium. Garnets show no cleavage and a dodecahedral parting. Fracture is conchoidal to uneven; some varieties are very tough and are valuable for abrasive purposes. hardness is 6.5-7.5, specific gravity is 3.1-4.3, lustre is vitreous to resinous, and they can be transparent to opaque. The name "garnet" comes from the
Latin granatus , possibly a reference to the
malum granatum , a plant with red seeds similar in shape, size, and color to some garnet crystals.
Garnets are most commonly red in color but can be found in a variety of colors, including purple,
red, orange,
yellow,
green, brown, black, or colorless. The lack of a
blue garnet was remedied in 1990s following the discovery of color-change blue to red/pink material in Bekily,
Madagascar but these stones are very rare. Color-change garnets are by far the rarest garnets except uvarovite, which does not come in cuttable sizes. In daylight, their color can be shades of green, beige, brown, gray and rarely blue, to a reddish or purplish/pink color in incandescent light. By composition, these garnets are a mix of spessartine and pyrope, as are
Malaya garnets. The color change of these new garnets is often more intense and more dramatic than the color change of top quality
Alexandrite which is frequently disappointing, but still sells for many thousands of U.S. dollars per carat. It is expected that blue color-change garnets will match
Alexandrite prices or even exceed them as the color change is often better and these garnets are much rarer. The blue color-change type is mainly caused by relatively high amounts of vanadium .
Six common varieties of garnet are recognized based on their chemical composition. They are pyrope, almandine or carbuncle, spessartite, grossularite , uvarovite and andradite. The garnets make up two solid solution series; 1. pyrope-almandine-spessarite and 2. uvarovite-grossularite-andradite.
Garnet is the
birthstone for January, and has been used since the
Bronze Age.
Garnet group endmembers
Pyralspite garnets - Al in B site
- Almandine: Fe3Al23
- Pyrope: Mg3Al23
- Spessartine: Mn3Al23
Almandite
Almandite, sometimes called almandine, is the modern gem known as carbuncle . The term "carbuncle" is derived from the
Latin meaning "little spark." The name Almandite is a corruption of Alabanda, a region in
Asia Minor where these stones were cut in ancient times. Chemically, almandite is an iron-aluminium garnet with the formula Fe
3Al
23; the deep red transparent stones are often called precious garnet and are used as gemstones . Almandite occurs in
metamorphic rocks like
mica schists, associated with minerals such as
staurolite,
kyanite,
andalusite, and others. Almandite has nicknames of Oriental garnet, almandine ruby, and carbuncle.
Pyrope
Pyrope, from the Latin
pyropos, means similar to fire. It is
ruby-red in color and chemically a magnesium aluminium silicate with the formula Mg
3Al
23, 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 nicknames of
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 spessatine 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.
Spessartite
Spessartite or spessartine is manganese aluminium garnet, Mn
3Al
23. 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. Spessartite of a beautiful orange-yellow is found in
Madagascar . Violet-red spessartites are found in
rhyolites in
Colorado and
Maine.
Ugrandite group - calcium in A site
- Andradite: Ca3Fe23
- Grossular: Ca3Al23
- Uvarovite: Ca3Cr23
Andradite
Andradite is a calcium-iron garnet, Ca
3Fe
23, is of variable composition and may be red, yellow, brown, green or black. The recognized subvarieties are topazolite , demantoid and melantite . 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.
Grossularite
Grossularite is a calcium-aluminium garnet with the formula Ca
3Al
23, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossularite 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 , 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. Grossularite 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 Ca
3Cr
23. 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 A site
- Goldmanite: Ca3V23
- Kimzeyite: Ca32[O4]3
- Morimotoite: Ca3Ti4+Fe2+3
- Schorlomite: Ca32[O4]3
- Hydroxide bearing - calcium in A site
- Hydrogrossular: Ca3Al23-x4x
- Hibschite: Ca3Al23-x4x
- Katoite: Ca3Al23-x4x
- Magnesium or manganese in A site
- Knorringite: Mg3Cr23
- Majorite: Mg323
- Calderite: Mn3Fe3+23
Knorringite
Knorringite is a magnesium chromium garnet with the formula Mg
3Cr
23. Pure knorringite never occurs in nature. Garnet with high knorringite content may be generated only under high pressure. Knorringite is basically pyrope with a very high chromium content and is often found in
kimberlites. It is used as an indicator mineral in the search for
diamonds.
Synthetic Garnets
In yttrium iron garnet ,
323, the five iron ions occupy two
octahedral and three
tetrahedral sites, with the yttrium 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. By substituting specific sites with
rare earth elements, for example, interesting magnetic properties can be obtained.
One example for this is gadolinium gallium garnet,
323, which is synthesized for use in magnetic
bubble memory.
Yttrium aluminium garnet , Y
3Al
23, is used for synthetic
gemstone. When doped with
neodymium , these YAl-Garnets are useful as the lasing medium in lasers.
Uses of garnets
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. Pyrope varieties are used as kimberlite indicator minerals in
diamond prospecting.
Garnets are very abundant in the lower crust and
mantle and thus play an important role in geochemical understanding of the Earth.
References
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
- Color Encyclopedia of Gemstones ISBN 0-442-20333-0
