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{{Infobox mineral
name = Garnet
category = nesosilicates
boxwidth =
boxbgcolor =
image = GarnetCrystalUSGOV.jpg
caption =
formula = The general formula X3Y2(SiO4)3
molweight =
Z = 8
spacegroup = Ia3d
crystalclass = 4/m\bar{3} 2/m
color = virtually all colors
habit = rhombic dodecahedra or cubic
system = Cubic
twinning =
cleavage = None
fracture = conchoidal to uneven
mohs = 6.0 - 7.5
luster = vitreous to resinous
polish = vitreous to subadamantine
refractive = 1.72 - 1.94
opticalprop = Single refractive, often anomalous double refractive
birefringence = None
pleochroism = None
streak = White
gravity = 3.1 - 4.3
melt =
fusibility =
diagnostic =
solubility =
diaphaneity =
other =
var1text = Mg3Al2Si3O12
var2text = Fe3Al2Si3O12
var3text = Mn3Al2Si3O12
var4text = Ca3Fe2Si3O12
var5text = Ca3Al2Si3O12
var6text = Ca3Cr2Si3O12}}The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or 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 by their chemical composition. They 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: 1. pyrope-almandine-spessarite and 2. uvarovite-grossular-andradite.

Physical properties

Properties

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, Madagascarmarker. It is also found in parts of the United Statesmarker, Russiamarker and Turkeymarker. 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− occupying the tetrahedra. 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.

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 - Aluminium in Y site



Almandine

Almandine in metamorphic 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 "live coal" or burning charcoal. The name Almandine is a corruption of Alabandamarker, a region in Asia Minormarker 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 Greek pyrōpós meaning "fire-eyed") 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 Countymarker, North Carolinamarker 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 almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames some of which are misnomers; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian garnet from the Czech Republicmarker. Another intriguing find is the blue color-changing 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 cornflower blue sapphire, or D block tanzanite; this is due to the blue garnet's 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 reddish mineral)


Spessartine or spessartite is manganese aluminium garnet, Mn3Al2(SiO4)3. Its name is derived from Spessartmarker in Bavariamarker. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartine of an orange-yellow is found in Madagascarmarker. Violet-red spessartines are found in rhyolites in Coloradomarker and Mainemarker.

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 melanite (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 Uralsmarker" 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 Siberiamarker. 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


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 mountainsmarker of Russiamarker and Outokumpu, Finlandmarker. Uvarovite crystals are generally too small to facet and are left attached to a matrix and incorporated into jewelry as seen below.

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.

Color change garnets

Garnet members of the pyrope-spessartine solid-solution series from Bekily in Madagascar display several colors depending on the light source. The alexandrite-like color change from blue-green in daylight to purple in incandescent light is mainly caused by relatively high amounts of vanadium. Although they look a lot like alexandrites they are different because they change color throughout the day. They are green or blue grey in the early morning and reddish in the late afternoon or in strong sunlight. Bekily garnets will appear red in the afternoon while the alexandrites remain green. Garnets from other parts of East Africa also change color but as they normally change from brown or orange to red, they don't look much like alexandrite.

Some of the stones are almost blue especially under fluorescent light but most of them are grey blue or green in daylight and change to red under incandescent or late afternoon light. The stones can show an excellent color change and can easily be confused with alexandrite.

Synthetic garnets

The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formula A3B2(C O4)3. Besides silicon, a large number of elements have been put on the C site, including , , , and .

Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstones. When doped with neodymium (Nd3+), these YAl-garnets are useful as the lasing medium in lasers.

Interesting magnetic properties arise when the appropriate elements are used. In yttrium iron garnet (YIG), 3 2(Fe 4)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.

Another example is gadolinium gallium garnet, 3 2(Ga4)3, which is synthesized for use in magnetic bubble memory.

Geological importance of garnet

Garnet var.
Spessartine, Putian City, Putian Prefecture, Fujian Province, China
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 aluminium-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.

The largest documented garnet single crystal was an isometric block measuring ~2.3 m and weighing ~37.5 tons. The news on larger garnet crystals found near Alice Springs, Northern Territory, Australia have not been confirmed.

Uses of garnets

Red garnets were the most commonly used gemstones in the Late Antique Roman world, and the Migration Period art of the "barbarian" peoples who took over the territory of the Western Empire. They were especially used inlaid in gold cells in the cloisonné technique, a style often just called garnet cloisonné, found from Anglo-Saxon England, as at Sutton Hoomarker, to the Black Seamarker.

Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow and orange. In the USAmarker it is known as the birthstone for January. It is the state mineral of Connecticutmarker. It is also New York'smarker gemstone.

Industrial uses

Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Alluvial garnet grains which are rounder are more suitable for such blasting treatments. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. For water jet cutting, garnet extracted from hard rock is suitable since it is more angular in form, therefore more efficient in cutting.

Garnet paper is favoured by cabinetmakers for finishing bare wood.

Garnet sand is also used for water filtration media.

As an abrasive garnet can be broadly divided in two categories, blasting grade and water jet grade. The garnet, as it is mined and collected, is crushed to finer grains; all pieces which are larger than 60 mesh (250 micrometres) are normally used for sand blasting. The pieces between 60 mesh (250 micrometres) and 200 mesh (74 micrometres) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 micrometres) are used for glass polishing and lapping. Regardless of the application, the larger grain sizes are used for faster work and the smaller ones are used for finer finishes.

There are different kinds of abrasive garnets which can be divided based on their origin. The largest source of abrasive garnet today isgarnet rich beach sand which is quite abundant on Indian and Australian coasts and the main producers today are seen to be Australia and India.

This material is particularly popular due to its consistent supplies, huge quantities and clean material. The common problems with this material are the presence of ilmenite and chloride compounds. Since the material is being naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at the Tuticorin beach is 80 mesh, and ranges from 56 mesh to 100 mesh size .

River garnet is particularly abundant in Australia. The river sand garnet occurs as a placer deposit. .

Rock garnet is perhaps the garnet type used for the longest period of time. This type of garnet is produced in America, China and western India. These crystals are crushed in mills and then purified by wind blowing, magnetic separation, sieving and, if required, washing. Being freshly crushed, this garnet has the sharpest edges and therefore performs far better than other kinds of garnet. Both the river and the beach garnet suffer from the tumbling effect of hundreds of thousands of years which rounds off the edges.

Garnet has been mined in western Rajasthanmarker for the past 200 years, but mainly for the gemstone grade stones. Abrasive garnet was mainly mined as a secondary product while mining for gem garnets and was used as lapping and polishing media for the glass industries. The host rock of the garnet here is garnetiferous mica schist and the total percentage of garnet is not more than 7% to 10% , which makes the material extremely costly and non economical to extract for non-gemstone applications.

See also



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


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