Garnet


Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular, uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine and uvarovite-grossular-andradite.

Etymology

The word garnet comes from the 14th‑century Middle English word gernet, meaning 'dark red'. It is borrowed from Old French grenate from Latin granatus, from granum. This is possibly a reference to mela granatum or even pomum granatum, a plant whose fruits contain abundant and vivid red seed covers, which are similar in shape, size, and color to some garnet crystals.

Physical properties

Properties

Garnet species are found in many colours including red, orange, yellow, green, blue, purple, pink, brown, black and colourless, with reddish shades most common.
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's luster is categorized as vitreous or resinous.

Crystal structure

Garnets are nesosilicates having the general formula X3Y23. The X site is usually occupied by divalent cations 2+ and the Y site by trivalent cations 3+ in an octahedral/tetrahedral framework with 4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit as well as the hexoctahedral habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other, but are never actually cubic because despite being isometric because the and families of planes are depleted. Garnets do not have any cleavage planes, 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 6.0 to 7.5. The harder species like almandine are often used for abrasive purposes.

Magnetics used in garnet series identification

For gem identification purposes, a pick-up response to a strong neodymium 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.

Garnet group end member species

Pyralspite garnets – aluminium in ''Y'' site


Almandine

Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle. The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. 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 Fe3Al23; the deep red transparent stones are often called precious garnet and are used as gemstones. 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 is red in color and chemically an aluminium silicate with the formula Mg3Al23, 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 diamondiferous kimberlites in Colorado, from the Bishop Conglomerate and in a Tertiary age lamprophyre at Cedar Mountain in Wyoming.
A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, 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. 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-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 or spessartite is manganese aluminium garnet, Mn3Al23. 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 an orange-yellow is found in Madagascar. Violet-red spessartines are found in rhyolites in Colorado and Maine.

Pyrope–spessartine (blue garnet or color-change garnet)

Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily, Madagascar. This type has also been found in parts of the United States, Russia, Kenya, Tanzania, and Turkey. It changes color from blue-green to purple depending on the color temperature of viewing light, as a result of the relatively high amounts of vanadium.
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.
This is the rarest type of garnet. Because of its color-changing quality, this kind of garnet resembles Alexandrite.

Ugrandite group – calcium in ''X'' site

Andradite is a calcium-iron garnet, Ca3Fe23, is of variable composition and may be red, yellow, brown, green or black. The recognized varieties are topazolite, demantoid and melanite. 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 Ca3Al23, 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, 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 skarns, contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.
Grossular garnet from Kenya and Tanzania has been called tsavorite. Tsavorite was first described in the 1960s in the :Category:Tsavo National Park|Tsavo area of Kenya, from which the gem takes its name.

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca3Cr23. 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. Uvarovite is named for Count Uvaro, a Russian imperial statesman.

Less common species

Knorringite is a magnesium-chromium garnet species with the formula Mg3Cr23. Pure endmember knorringite never occurs in nature. Pyrope rich in the knorringite component is only formed under high pressure and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.

Garnet structural group

IMA/CNMNC
Nickel-Strunz
Mineral class		Mineral nameFormulaCrystal systemPoint groupSpace group
04 OxideBitikleite-Ca3SnSb3isometricmmIad
04 OxideBitikleite-Ca33isometricmmIad
04 OxideBitikleite-Ca3SbZr3isometricmmIad
04 TellurateYafsoaniteCa3Zn32isometricmm
or 432		Iad
or I4132
08 ArsenateBerzeliiteNaCa2Mg23isometricmmIad
08 VanadatePalenzonaiteNaCa2Mn2+23isometricmmIad
08 VanadateSchäferiteNaCa2Mg23isometricmmIad

Mineral nameFormulaCrystal systemPoint groupSpace group
AlmandineFe2+3Al23isometricmmIad
AndraditeCa3Fe3+23isometricmmIad
CalderiteMn+23Fe+323isometricmmIad
GoldmaniteCa3V3+23isometricmmIad
GrossularCa3Al23isometricmmIad
HenritermieriteCa3Mn3+224tetragonal4/mmmI41/acd
HibschiteCa3Al24x isometricmmIad
KatoiteCa3Al24x isometricmmIad
KerimasiteCa3Zr22isometricmmIad
KimzeyiteCa3Zr22isometricmmIad
KnorringiteMg3Cr23isometricmmIad
MajoriteMg33tetragonal4/m
or 4/mmm		I41/a
or I41/acd
Menzerite-Y2CaMg23isometricmmIad
MomoiiteMn2+3V3+23isometricmmIad
MorimotoiteCa33isometricmmIad
PyropeMg3Al23isometricmmIad
SchorlomiteCa3Ti4+22isometricmmIad
SpessartineMn2+3Al23isometricmmIad
ToturiteCa3Sn22isometricmmIad
UvaroviteCa3Cr23isometricmmIad

The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formula A3B23. Besides silicon, a large number of elements have been put on the C site, including,,, and.
Yttrium aluminium garnet, Y3Al23, is used for synthetic gemstones. Due to its fairly high refractive index, YAG was used as a diamond simulant in the 1970s until the methods of producing the more advanced simulant cubic zirconia in commercial quantities were developed. When doped with neodymium, these YAl-garnets may be used as the lasing medium in lasers.
Interesting magnetic properties arise when the appropriate elements are used. 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 behavior. YIG is a ferrimagnetic material having a Curie temperature of 550 K.
Another example is gadolinium gallium garnet, 323 which is synthesized for use as a substrate for liquid-phase epitaxy of magnetic garnet films for bubble memory and magneto-optical applications.

Geological importance

The mineral garnet is commonly found in metamorphic and to a lesser extent, igneous rocks. Most natural garnets are compositionally zoned and contain inclusions. Its crystal lattice structure is stable at high pressures and temperatures and is thus found in green-schist facies metamorphic rocks including gneiss, hornblende schist, and mica schist. The composition that is stable at the pressure and temperature conditions of Earth's mantle is pyrope, which is often found in peridotites and kimberlites, as well as the serpentines that form from them. 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 of geothermobarometry which determines "P-T Paths", Pressure-Temperature Paths. Garnets are used as an index mineral in the delineation of isograds in metamorphic rocks. Compositional zoning and inclusions can mark the change from growth of the crystals at low temperatures to higher temperatures. Garnets that are not compositionally zoned more than likely experienced ultra high temperatures that led to diffusion of major elements within the crystal lattice, effectively homogenizing the crystal or they were never zoned. Garnets can also form metamorphic textures that can help interpret structural histories.
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-Pb geochronometer, to date the age of crystallization as well as a thermochronometer in the /He system to date timing of cooling below a closure temperature.
Garnets can be chemically altered and most often alter to serpentine, talc, and chlorite.

Uses

Gemstones

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 Roman 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 Hoo, to the Black Sea. Thousands of Tamraparniyan gold, silver and red garnet shipments were made in the old world, including to Rome, Greece, the Middle East, Serica and Anglo Saxons; recent findings such as the Staffordshire Hoard and the pendant of the Winfarthing Woman skeleton of Norfolk confirm an established gem trade route with South India and Tamraparni, known from antiquity for its production of gemstones.
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange. In the US it is known as the birthstone for January. It is the state mineral of Connecticut, New York's gemstone, and star garnet is the state gemstone of Idaho.

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 favored by cabinetmakers for finishing bare wood.
Garnet sand is also used for water filtration media.
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 are normally used for sand blasting. The pieces between 60 mesh and 200 mesh are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh 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 is
garnet-rich beach sand which is quite abundant on Indian and Australian coasts and the main producers today are 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 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 the Tuticorin beach in south India 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. Gore Mountain Garnet from Warren County, New York, USA is a significant source of rack garnet for use as an industrial abrasive.

Cultural significance

Garnet is the birthstone of January. It is also the birthstone of Aquarius in tropical astrology.

United States

has garnet as its state gemstone, Connecticut has almandine garnet as its state gemstone, Idaho has star garnet as its state gemstone, and Vermont has grossular garnet as its state gemstone.