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Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, defining distinct species. These species fall into two primary solid solution series: the pyralspite series (pyrope,almandine,spessartine), with the general formula [Mg,Fe,Mn]3Al2(SiO4)3; and the ugrandite series (uvarovite,grossular,andradite), with the general formula Ca3[Cr,Al,Fe]2(SiO4)3. Notable varieties of grossular includehessonite andtsavorite.
The wordgarnet comes from the 14th-centuryMiddle English wordgernet, meaning 'dark red'. It is borrowed from Old Frenchgrenate fromLatingranatus, fromgranum ('grain, seed').[3] This is possibly a reference tomela granatum or evenpomum granatum ('pomegranate',[4]Punica granatum), a plant whose fruits contain abundant and vivid red seed covers (arils), which are similar in shape, size, and color to some garnet crystals.[5] Hessonite garnet is also named 'gomed' in Indian literature and is one of the nine jewels in Vedic astrology that comprise theNavaratna.[6]
A sample showing the deep red color garnet can exhibit.
Garnet species are found in every colour, with reddish shades most common. Blue garnets are the rarest and were first reported in the 1990s.[7][8][9][10]
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'slustre is categorized asvitreous (glass-like) or resinous (amber-like).[3]
Garnets arenesosilicates having the general formulaX3Y2(SiO 4)3. TheX site is usually occupied by divalent cations (Ca,Mg,Fe,Mn)2+ and theY site by trivalent cations (Al, Fe,Cr)3+ in anoctahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra.[11] Garnets are most often found in thedodecahedralcrystal habit, but are also commonly found in thetrapezohedron habit as well as thehexoctahedral habit.[3] They crystallize in thecubic system, having three axes that are all of equal length and perpendicular to one another but are never actually cubic because, despite being isometric, the {100} and {111} families of planes are depleted.[3] Garnets do not have anycleavage planes, so, when they fracture under stress, sharp, irregular (conchoidal) pieces are formed.[12]
Crystal structure of pyrope garnet. White spheres are oxygen; black, silicon; blue, aluminium; and red, magnesium.
Same view, with ion sizes reduced to better show all ions
Silicon ion size exaggerated to emphasize silica tetrahedra
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 theMohs scale of about 6.0 to 7.5.[13] The harder species likealmandine are often used for abrasive purposes.[14]
For gem identification purposes, a pick-up response to a strongneodymium 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.[15]
Almandine, sometimes incorrectly called almandite, is the modern gem known ascarbuncle (though originally almost any red gemstone was known by this name).[16] The term "carbuncle" is derived from theLatin meaning "live coal" or burning charcoal. The nameAlmandine is a corruption ofAlabanda, a region inAsia 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).[17] Almandine occurs inmetamorphic rocks likemicaschists, associated with minerals such asstaurolite,kyanite,andalusite, and others.[18] Almandine has nicknames of Oriental garnet,[19] almandine ruby, and carbuncle.[16]
Pyrope (from the Greekpyrōpós meaning "firelike")[3] is red in color and chemically an aluminiumsilicate 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 black. Pyrope and spessartine gemstones have been recovered from the Sloan diamondiferouskimberlites inColorado, from the Bishop Conglomerate and in aTertiary agelamprophyre at Cedar Mountain inWyoming.[20]
A variety of pyrope fromMacon County,North Carolina is a violet-red shade and has been calledrhodolite, Greek for "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.[21] Pyrope has tradenames some of which aremisnomers;Cape ruby,Arizona ruby,California ruby,Rocky Mountain ruby, andBohemian ruby from theCzech Republic.[16]
Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily,Madagascar. This type has also been found in parts of theUnited States,Russia,Kenya,Tanzania, andTurkey. It changes color from blue-green to purple depending on thecolor temperature of viewing light, as a result of the relatively high amounts ofvanadium (about 1 wt.% V2O3).[9]
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.[25]
This is the rarest type of garnet. Because of its color-changing quality, this kind of garnet resemblesalexandrite.[26]
Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black.[3] The recognized varieties aredemantoid (green),melanite (black),[3] and topazolite (yellow or green). The red-brown translucent variety ofcolophonite is recognized as a partially obsolete name.[27] Andradite is found inskarns[3] and in deep-seatedigneous rocks likesyenite[28] as well as serpentines[29] andgreenschists.[30] Demantoid is one of the most prized of garnet varieties.[31]
Grossular garnet from Quebec, collected by Dr John Hunter in the 18th century, Hunterian Museum, GlasgowGrossular garnets on display at the U.S.National Museum of Natural History. The green gem at right is a type of grossular known astsavorite.
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 thebotanical name for thegooseberry,grossularia, in reference to the green garnet of this composition that is found inSiberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow.[3] Because of its inferior hardness tozircon, which the yellow crystals resemble, they have also been calledhessonite from theGreek meaning inferior.[32] Grossular is found in skarns,[3] contact metamorphosedlimestones withvesuvianite,diopside,wollastonite andwernerite.
Grossular garnet fromKenya andTanzania has been called tsavorite. Tsavorite was first described in the 1960s in theTsavo area of Kenya, from which the gem takes its name.[33][34]
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 withchromite inperidotite,serpentinite, and kimberlites. It is found in crystallinemarbles and schists in theUral Mountains of Russia andOutokumpu, Finland. Uvarovite is named forCount Uvaro, a Russian imperial statesman.[3]
Knorringite is a magnesium-chromium garnet species with the formula Mg3Cr2(SiO4)3. Pureendmember knorringite never occurs in nature. Pyrope rich in the knorringite component is only formed under high pressure and is often found inkimberlites. It is used as an indicator mineral in the search fordiamonds.[35]
References:Mindat.org; mineral name, chemical formula and space group (American Mineralogist Crystal Structure Database) of the IMA Database of Mineral Properties/ RRUFF Project, Univ. of Arizona, was preferred most of the time. Minor components in formulae have been left out to highlight the dominant chemical endmember that defines each species.
The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formulaA3B2(CO4)3. Besides silicon, a large number of elements have been put on theC site, includinggermanium,gallium,aluminum,vanadium andiron.[36]
Interesting magnetic properties arise when the appropriate elements are used. Inyttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy twooctahedral and threetetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit differentspins, resulting inmagnetic behavior. YIG is aferrimagnetic material having aCurie temperature of 550 K. Yttrium iron garnet can be made intoYIG spheres, which serve as magnetically tunablefilters andresonators formicrowave frequencies.[41]
Lutetium aluminium garnet (LuAG),Al5Lu3O12, is an inorganic compound with a unique crystal structure primarily known for its use in high-efficiency laser devices. LuAG is also useful in the synthesis oftransparent ceramics.[42] LuAG is particularly favored over other crystals for its high density and thermal conductivity; it has a relatively smalllattice constant in comparison to the otherrare-earth garnets, which results in a higher density producing a crystal field with narrower linewidths and greater energy level splitting in absorption and emission.[43]
Main garnet producing countriesGarnet var. Spessartine, Putian City, Putian Prefecture, Fujian Province, China
The mineral garnet is commonly found in metamorphic and to a lesser extent, igneous rocks. Most natural garnets are compositionally zoned and contain inclusions.[45] Its crystal lattice structure is stable at high pressures and temperatures and is thus found in green-schist facies metamorphic rocks includinggneiss, hornblendeschist, and mica schist.[46] The composition that is stable at the pressure and temperature conditions of Earth's mantle is pyrope, which is often found inperidotites andkimberlites, as well as theserpentines that form from them.[46] Garnets are unique in that they can record the pressures and temperatures of peak metamorphism and are used as geobarometers and geothermometers in the study ofgeothermobarometry which determines "P-T Paths", Pressure-Temperature Paths. Garnets are used as an index mineral in the delineation ofisograds in metamorphic rocks.[46] Compositional zoning and inclusions can mark the change from growth of the crystals at low temperatures to higher temperatures.[47] Garnets that are not compositionally zoned more than likely experienced ultra high temperatures (above 700 °C) that led to diffusion of major elements within the crystal lattice, effectively homogenizing the crystal[47] or they were never zoned. Garnets can also form metamorphic textures that can help interpret structural histories.[47]
In addition to being used to devolve conditions of metamorphism, garnets can be used to date certain geologic events. Garnet has been developed as a U-Pbgeochronometer, to date the age of crystallization[48] as well as athermochronometer in the (U-Th)/He system[49] to date timing of cooling below aclosure temperature.
Garnets can be chemically altered and most often alter to serpentine,talc, andchlorite.[46]
The open-pit Barton Garnet Mine, located atGore Mountain in theAdirondack Mountains, yields the world's largest single crystals of garnet; diameters range from 5 to 35 cm and commonly average 10–18 cm.[50]
Gore Mountain garnets are unique in many respects, and considerable effort has been made to determine the timing of garnet growth. The first dating was that of Basu et al. (1989), who used plagioclase-hornblende-garnet to produce a Sm/Nd isochron that yielded an age of 1059 ± 19 Ma. Mezger et al. (1992) conducted their own Sm/Nd investigation using hornblende and the drilled core of a 50 cm garnet to produce an isochron age of 1051 ± 4 Ma. Connelly (2006) utilized seven different fractions of a Gore Mountain garnet to obtain a Lu-Hf isochron age of 1046.6 ± 6 Ma. It is therefore concluded with confidence that the garnets formed at 1049 ± 5 Ma, the average of the three determinations. This is also the local age of peak metamorphism in the 1090–1040 Ma Ottawan phase of theGrenvillian orogeny and serves as a critical data point in ascertaining the evolution of the megacrystic garnet deposits.[50]
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange.[54] In the United States it is known as thebirthstone for January.[2][55][56] It is also the birthstone ofAquarius andCapricorn intropical astrology.[57][58] The garnet family is one of the most complex in the gem world. It is not a single species, but is composed of multiple species and varieties.[59]
Garnet sand is a goodabrasive, and a common replacement for silica sand in abrasive blasting operations. Alluvial garnet grains which are rounder are more suitable for such blasting treatments. Mixed with very high pressure water, garnet is used to cutsteel and other materials inwater jets. For water jet cutting, garnet extracted from hard rock is suitable since it is more angular in form, therefore more efficient in cutting.[66]
Garnet paper is favored by cabinetmakers for finishing bare wood.[67]
As an abrasive, garnet can be broadly divided into 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 micrometers) are normally used for sand blasting. The pieces between 60 mesh (250 micrometers) and 200 mesh (74 micrometers) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 micrometers) 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.[68]
There are different kinds of abrasive garnets which can be divided based on their origin. The largest source of abrasive garnet today is garnet-rich beach sand which is quite abundant onIndian andAustralian coasts and the main producers today are Australia and India.[69]
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 has been naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at theTuticorin beach in south India is 80 mesh, and ranges from 56 mesh to 100 mesh size.[citation needed]
River garnet is particularly abundant in Australia. The river sand garnet occurs as aplacer deposit.[70]
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.Gore Mountain Garnet fromWarren County, New York, USA, is a significant source of rock garnet for use as an industrial abrasive.[3]
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