Garnet
Encyclopedia
The garnet (ˈɡɑrnɨt) group includes a group of minerals that have been used since the Bronze Age
as gemstone
s 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: pyrope-almandine-spessarite and uvarovite-grossular-andradite.
. 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).
Garnets are nesosilicates having the general formula X3Y2(Si
O
4)3. The X site is usually occupied by divalent cations (Ca
2+, Mg
2+, Fe
2+) and the Y site by trivalent cations (Al
3+, Fe3+, Cr
3+) 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.
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 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 rock
s like mica
schist
s, associated with minerals such as staurolite
, kyanite
, andalusite
, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.
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 almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames some of which are misnomer
s; 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, peridotite
s and eclogite
s, commonly contain a pyrope variety.
Spessartine or 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 phyllite
s. Spessartine of an orange-yellow is found in Madagascar. Violet-red spessartines are found in rhyolite
s in Colorado
and Maine
.
(green) and melanite (black). Andradite is found both in deep-seated igneous rock
s 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 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 limestone
s 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 Tsavo area of Kenya, from which the gem takes its name.
in peridotite, serpentinite
, and kimberlites. It is found in crystalline marble
s and schists in the Ural mountains
of Russia and Outokumpu, Finland
.
s. It is used as an indicator mineral in the search for diamond
s.
Yttrium aluminium garnet
(YAG), Y3Al2(AlO4)3, 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
(Nd3+), 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
(YIG), 32(Fe4)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
, 32(GaO4)3 which is synthesized for use as a substrate for liquid phase epitaxy of magnetic garnet films for bubble memory
and magneto-optical
applications.
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. Xenolith
s 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.
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 Hoo
, to the Black Sea
.
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow and orange. In the USA it is known as the birthstone for January. It is the state mineral of Connecticut
, New York's gemstone, and star garnet (garnet with rutile
asterisms) is the state gemstone of Idaho
.
, 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 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 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.
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 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 Rajasthan
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.
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
as gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus ("grain
GRAIN
GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and...
"), possibly a reference to the Punica granatum ("pomegranate
Pomegranate
The pomegranate , Punica granatum, is a fruit-bearing deciduous shrub or small tree growing between five and eight meters tall.Native to the area of modern day Iran, the pomegranate has been cultivated in the Caucasus since ancient times. From there it spread to Asian areas such as the Caucasus as...
"), 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
Pyrope
The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a...
, almandine
Almandine
Almandine , also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet,...
, spessartine
Spessartine
Spessartine previously named spessartite, is a nesosilicate, manganese aluminium garnet species, Mn3Al23.The name is a derivative of Spessart in Bavaria, Germany, the type locality of the mineral. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic...
, grossular
Grossular
Grossular or grossularite is a calcium-aluminium mineral species of the garnet group 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...
(varieties of which are hessonite
Hessonite
Hessonite or Cinnamon Stone is a variety of grossular, a calcium aluminium mineral of the garnet group with the general formula Ca3Al2Si3O12. The name is from the Greek hēssōn, inferior, in allusion to its lower hardness and density than most other garnet species varieties.It has a characteristic...
or cinnamon-stone and tsavorite
Tsavorite
Tsavorite or tsavolite is a variety of the garnet group species grossular, a calcium-aluminium garnet with the formula Ca3Al2Si3O12. Trace amounts of vanadium or chromium provide the green color....
), uvarovite
Uvarovite
Uvarovite is a chromium-bearing garnet group species with the formula: Ca3Cr23. It was discovered in 1832 by Germain Henri Hess who named it after Count Sergei Semenovitch Uvarov , a Russian statesman and amateur mineral collector....
and andradite
Andradite
Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.Andradite includes three varieties:* Melanite: Black in color, referred to as "titanian andradite"....
. The garnets make up two solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
series: pyrope-almandine-spessarite and uvarovite-grossular-andradite.
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, MadagascarMadagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
. It is also found in parts of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...
. It changes color from blue-green in the daylight to purple in incandescent light, as a result of the relatively high amounts of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
(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
Chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, meaning "a gold-white spar". Despite the similarity of their names, chrysoberyl and beryl are two completely...
.
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
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
is categorized as vitreous (glass-like) or resinous (amber-like).
Crystal structure
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
4)3. The X site is usually occupied by divalent cations (Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
2+, Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
2+, Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
2+) and the Y site by trivalent cations (Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
3+, Fe3+, Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
3+) in an octahedral
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
/tetrahedral
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
framework with [SiO4]4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....
, 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
Deltoidal icositetrahedron
In geometry, a deltoidal icositetrahedron is a Catalan solid which looks a bit like an overinflated cube. Its dual polyhedron is the rhombicuboctahedron....
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
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
, 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.Pyralspite garnets - Aluminium in Y site
- AlmandineAlmandineAlmandine , also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet,...
: Fe3Al2(SiO4)3 - PyropePyropeThe mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a...
: Mg3Al2(SiO4)3 - SpessartineSpessartineSpessartine previously named spessartite, is a nesosilicate, manganese aluminium garnet species, Mn3Al23.The name is a derivative of Spessart in Bavaria, Germany, the type locality of the mineral. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic...
: MnManganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
3Al2(SiO4)3
Almandine
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
meaning "live coal" or burning charcoal. The name Almandine is a corruption of Alabanda
Alabanda
Alabanda – also hê Alabanda, ta Alabanda, Alabandeus, Alabandensis, Alabandenus, and for a time, Antiochia of the Chrysaorians – was an ancient city of Caria, Anatolia, the site of which is now located near Doğanyurt , Aydin Province, in the Asian part of Turkey.The city is located in...
, a region in Asia Minor
Asia Minor
Asia Minor is a geographical location at the westernmost protrusion of Asia, also called Anatolia, and corresponds to the western two thirds of the Asian part of Turkey...
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 rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s like mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s, associated with minerals such as staurolite
Staurolite
Staurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak.-Properties:It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: 2Al94O204...
, kyanite
Kyanite
Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
, andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....
, 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 silicateSilicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
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
Macon County, North Carolina
- Geographic features :Of the in Macon County, are federal lands that lie within the Nantahala National Forest and are administered by the United States Forest Service. Of the of USFS land, lie in the Highlands Ranger District and the remaining lie in the Wayah Ranger District...
, North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
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 misnomer
Misnomer
A misnomer is a term which suggests an interpretation that is known to be untrue. Such incorrect terms sometimes derive their names because of the form, action, or origin of the subject becoming named popularly or widely referenced—long before their true natures were known.- Sources of misnomers...
s; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian garnet from the Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
. Another intriguing find is the blue color-changing garnets from Madagascar, a pyrope spessartine mix. The color of these blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
garnets is not like sapphire blue in subdued daylight but more reminiscent of the grayish blues and greenish blues sometimes seen in spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
. However, in white LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
light the color is equal to the best cornflower blue sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
, or D block tanzanite
Tanzanite
Tanzanite is the blue/purple variety of the mineral zoisite which was discovered in the Mererani Hills of Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. It is used as a gemstone...
; 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, peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
s and eclogite
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
s, commonly contain a pyrope variety.
Spessartine
Spessart
The Spessart is a low mountain range in northwestern Bavaria and southern Hesse, Germany. It is bordered on three sides by the Main River. The two most important towns located at the foot of the Spessart are Aschaffenburg and Würzburg....
in Bavaria
Bavaria
Bavaria, formally the Free State of Bavaria is a state of Germany, located in the southeast of Germany. With an area of , it is the largest state by area, forming almost 20% of the total land area of Germany...
. It occurs most often in granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
and allied rock types and in certain low grade metamorphic phyllite
Phyllite
Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...
s. Spessartine of an orange-yellow is found in Madagascar. Violet-red spessartines are found in rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
s in Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
and Maine
Maine
Maine is a state in the New England region of the northeastern United States, bordered by the Atlantic Ocean to the east and south, New Hampshire to the west, and the Canadian provinces of Quebec to the northwest and New Brunswick to the northeast. Maine is both the northernmost and easternmost...
.
Ugrandite group - calcium in X site
- AndraditeAndraditeAndradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.Andradite includes three varieties:* Melanite: Black in color, referred to as "titanian andradite"....
: Ca3Fe2(SiO4)3 - GrossularGrossularGrossular or grossularite is a calcium-aluminium mineral species of the garnet group 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...
: Ca3Al2(SiO4)3 - UvaroviteUvaroviteUvarovite is a chromium-bearing garnet group species with the formula: Ca3Cr23. It was discovered in 1832 by Germain Henri Hess who named it after Count Sergei Semenovitch Uvarov , a Russian statesman and amateur mineral collector....
: Ca3Cr2(SiO4)3
Andradite
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), demantoidDemantoid
Demantoid is the green gemstone variety of the mineral andradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet. The chemical formula is Ca3Fe23 with chromium substitution as the cause of the demantoid green color...
(green) and melanite (black). Andradite is found both in deep-seated igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s like syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald
Emerald
Emerald is a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the 10 point Mohs scale of mineral hardness...
of the Urals
Ural Mountains
The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...
" 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
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
name for the gooseberry
Gooseberry
The gooseberry or ; Ribes uva-crispa, syn. R. grossularia) is a species of Ribes, native to Europe, northwestern Africa and southwestern Asia...
, grossularia, in reference to the green garnet of this composition that is found in Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Because of its inferior hardness to zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
, which the yellow crystals resemble, they have also been called hessonite from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
meaning inferior. Grossular is found in contact metamorphosed limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s with vesuvianite
Vesuvianite
Vesuvianite, also known as idocrase, is a green, brown, yellow, or blue silicate mineral. Vesuvianite occurs as tetragonal crystals in skarn deposits and limestones that have been subjected to contact metamorphism...
, diopside
Diopside
Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class...
, wollastonite
Wollastonite
Wollastonite is a calcium inosilicate mineral that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolostone is subjected to high temperature and pressure sometimes in the presence of silica-bearing fluids...
and wernerite.
Grossular garnet from Kenya and Tanzania has been called tsavorite. Tsavorite was first described 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 chromiteChromite
Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...
in peridotite, serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
, and kimberlites. It is found in crystalline marble
Marble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...
s and schists in the Ural mountains
Ural Mountains
The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...
of Russia and Outokumpu, Finland
Outokumpu, Finland
Outokumpu is a town and municipality of Finland.It is located in the province of Eastern Finland and is part of the North Karelia region. The municipality has a population of and covers an area of of which is water. The population density is ....
.
Less common species
- Calcium in X site
- GoldmaniteGoldmaniteGoldmanite is a green or greenish-brown silicate mineral of the garnet group with a chemical formula of Ca323.-Discovery:It was first described in 1964 for an occurrence in the Laguna District, Cibola County, New Mexico and is named after Marcus Isaac Goldman , an American petrologist...
: Ca3V2(SiO4)3 - Kimzeyite: Ca3(ZrZirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
,TiTitaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
)2[(Si,Al,Fe3+)O4]3 - Morimotoite: Ca3Ti4+Fe2+(SiO4)3
- Schorlomite: Ca3(Ti4+,Fe3+)2[(Si,Ti)O4]3
- Goldmanite
- Hydroxide bearing - calcium in X site
- HydrogrossularHydrogrossularHydrogrossular, is a calcium aluminium garnet series . The endmembers of the hydrogarnet family depend on the degree of substitution :...
: 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)
- Hydrogrossular
- Magnesium or manganese in X site
- KnorringiteKnorringiteKnorringite is a mineral species belonging to the garnet group, and forms a series with the species pyrope. It was discovered in 1968 in the Kao kimberlite pipe in the Butha-Buthe District of Lesotho and is named after Oleg Von Knorring, a professor of mineralogy at the University of Leeds in...
: Mg3Cr2(SiO4)3 - MajoriteMajoriteMajorite is a type of mineral found in the upper mantle of the Earth. Its chemical formula is Mg323. Majorite was first described from the Coorara Meteorite of Western Australia and has been reported from various other meteorites...
: Mg3(Fe,Al,Si)2(SiO4)3 - CalderiteCalderiteCalderite is a mineral in the garnet group with the chemical formula 323.It is dark reddish brown to dark yellowish in color and generally granular massive in form....
: Mn3Fe3+2(SiO4)3
- Knorringite
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 kimberliteKimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
s. It is used as an indicator mineral in the search for diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
s.
Garnet structural group
- Formula: X3Z2(TO4)3 (X = Ca, Fe, etc., Z = Al, Cr, etc., T = Si, As, V, Fe, Al)
- All are cubic or strongly pseudocubic.
| IMA/CNMNC Nickel-Strunz Mineral class | Mineral name | Formula | Crystal system | Point group | Space group |
|---|---|---|---|---|---|
| 04 Oxide | Bitikleite-(SnAl) | Ca3SnSb(AlO4)3 | isometric | mm | Iad |
| 04 Oxide | Bitikleite-(SnFe) | Ca3(SnSb5+)Fe3+3O12 | isometric | mm | Iad |
| 04 Oxide | Bitikleite-(ZrFe) | Ca3SbZr(Fe3+O4)3 | isometric | mm | Iad |
| 04 Tellurite | Yafsoanite | Ca3Zn3(Te6+O6)2 | isometric | mm or 432 | Iad or I4132 |
| 08 Arsenate | Berzeliite | NaCa2Mg2(AsO4)3 | isometric | mm | Iad |
| 08 Vanadate | Palenzonaite | NaCa2Mn2+2(VO4)3 | isometric | mm | Iad |
| 08 Vanadate | Schäferite Schäferite Schäferite is a rare vanadate mineral with chemical formula Ca2NaMg2[VO4]3. Schäferite is isometric, which means that it has three axes of equal length and 90° angles between the axes. Schäferite belongs to the optical class Isotropic, meaning that the velocity of light is the same no matter which... |
NaCa2Mg2(VO4)3 | isometric | mm | Iad |
- IMA/CNMNC - Nickel-Strunz - Mineral subclass: 09.A Nesosilicate
- Nickel-Strunz classificationStrunz classificationNickel–Strunz classification is a scheme for categorizing minerals based upon their chemical composition, introduced by German mineralogist Karl Hugo Strunz in his 1941 Mineralogische Tabellen. The 4th edition was edited by Christel Tennyson too . It was followed by A.S...
: 09.AD.25
- Nickel-Strunz classification
| Mineral name | Formula | Crystal system | Point group | Space group |
|---|---|---|---|---|
| Almandine Almandine Almandine , also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet,... |
Fe2+3Al2(SiO4)3 | isometric | mm | Iad |
| Andradite Andradite Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.Andradite includes three varieties:* Melanite: Black in color, referred to as "titanian andradite".... |
Ca3Fe3+2(SiO4)3 | isometric | mm | Iad |
| Calderite Calderite Calderite is a mineral in the garnet group with the chemical formula 323.It is dark reddish brown to dark yellowish in color and generally granular massive in form.... |
Mn+23Fe+32(SiO4)3 | isometric | mm | Iad |
| Goldmanite Goldmanite Goldmanite is a green or greenish-brown silicate mineral of the garnet group with a chemical formula of Ca323.-Discovery:It was first described in 1964 for an occurrence in the Laguna District, Cibola County, New Mexico and is named after Marcus Isaac Goldman , an American petrologist... |
Ca3V3+2(SiO4)3 | isometric | mm | Iad |
| Grossular Grossular Grossular or grossularite is a calcium-aluminium mineral species of the garnet group 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... |
Ca3Al2(SiO4)3 | isometric | mm | Iad |
| Henritermierite | Ca3Mn3+2(SiO4)2(OH)4 | tetragonal | 4/mmm | I41/acd |
| Hibschite | Ca3Al2(SiO4)(3-x)(OH)4x (x= 0.2-1.5) | isometric | mm | Iad |
| Katoite | Ca3Al2(SiO4)(3-x)(OH)4x (x= 1.5-3) | isometric | mm | Iad |
| Kerimasite | Ca3Zr2(Fe+32Si)O12 | isometric | mm | Iad |
| Kimzeyite | Ca3(Zr,Ti)2(Si,Al,Fe3+)3O12 | isometric | mm | Iad |
| Knorringite Knorringite Knorringite is a mineral species belonging to the garnet group, and forms a series with the species pyrope. It was discovered in 1968 in the Kao kimberlite pipe in the Butha-Buthe District of Lesotho and is named after Oleg Von Knorring, a professor of mineralogy at the University of Leeds in... |
Mg3Cr2(SiO4)3 | isometric | mm | Iad |
| Majorite Majorite Majorite is a type of mineral found in the upper mantle of the Earth. Its chemical formula is Mg323. Majorite was first described from the Coorara Meteorite of Western Australia and has been reported from various other meteorites... |
Mg3(Fe2+Si)(SiO4)3 | tetragonal | 4/m or 4/mmm | I41/a or I41/acd |
| Momoite | (Mn2+,Ca)3(V3+,Al)2(SiO4)3 | isometric | mm | Iad |
| Morimotoite | Ca3(Ti,Fe2+,Fe3+)2(Si,Fe3+)3O12 | isometric | mm | Iad |
| Pyrope Pyrope The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a... |
Mg3Al2(SiO4)3 | isometric | mm | Iad |
| Schorlomite | Ca3(Ti4+,Fe3+)2[(Si,Fe3+)O4]3 | isometric | mm | Iad |
| Spessartine Spessartine Spessartine previously named spessartite, is a nesosilicate, manganese aluminium garnet species, Mn3Al23.The name is a derivative of Spessart in Bavaria, Germany, the type locality of the mineral. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic... |
Mn2+3Al2(SiO4)3 | isometric | mm | Iad |
| Toturite | Ca3Sn2Fe2SiO12 | isometric | mm | Iad |
| Uvarovite Uvarovite Uvarovite is a chromium-bearing garnet group species with the formula: Ca3Cr23. It was discovered in 1832 by Germain Henri Hess who named it after Count Sergei Semenovitch Uvarov , a Russian statesman and amateur mineral collector.... |
Ca3Cr2(SiO4)3 | isometric | mm | Iad |
- References: Mindat.orgMindat.orgMindat.org is a non-commercial online mineralogical database, claiming to be the largest mineral database and mineralogical reference website on the internet....
; 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.
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
Yttrium aluminium garnet
Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite . YAG is commonly used as a host material in various solid-state...
(YAG), Y3Al2(AlO4)3, is used for synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
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
Cubic zirconia
Cubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...
in commercial quantities were developed. When doped with neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
(Nd3+), 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
Yttrium iron garnet
Yttrium iron garnet is a kind of synthetic garnet, with chemical composition 323, or Y3Fe5O12. It is a ferrimagnetic material with a Curie temperature of 550 K....
(YIG), 32(Fe4)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
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
, resulting in magnetic behaviour. YIG is a ferrimagnetic
Ferrimagnetism
In physics, a ferrimagnetic material is one in which the magnetic moments of the atoms on different sublattices are opposed, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains...
material having a Curie temperature
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
of 550 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
.
Another example is gadolinium gallium garnet
Gadolinium gallium garnet
Gadolinium Gallium Garnet is a synthetic crystalline material of the garnet group, with good mechanical, thermal, and optical properties. It is typically colorless. It has cubic lattice, density 7.08 g/cm³ and Mohs hardness is variously noted as 6.5 and 7.5...
, 32(GaO4)3 which is synthesized for use as a substrate for liquid phase epitaxy of magnetic garnet films for bubble memory
Bubble memory
Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data...
and magneto-optical
Photomagnetism
The photomagnetic effect is the effect in which a material acquires its ferromagnetic properties in response to light. The current model for this phenomenon is a light induced electron transfer, accompanied by the reversal of the spin direction of an electron. This leads to an increase in spin...
applications.
Geological importance of garnet
Geothermobarometry
Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobarometry is a combination of geobarometry, where a pressure of mineral formation is resolved, and geothermometry where a temperature of formation is...
. Diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
of elements is relatively slow in garnet compared to rates in many other minerals, and garnets are also relatively resistant to alteration
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
. 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
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
of rocks. For instance, eclogite can be defined as a rock of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
composition, but mainly consisting of garnet and omphacite
Omphacite
Omphacite is a member of the pyroxene group of silicate minerals with formula: Si2O6. It is a variably deep to pale green or nearly colorless variety of pyroxene. Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite. It crystallizes in the monoclinic system...
. Pyrope-rich garnet is restricted to relatively high-pressure metamorphic rocks, such as those in the lower crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and in the Earth's mantle. Peridotite may contain plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, 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
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
plus pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
: 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. Xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s 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
Migration Period art
Migration Period art denotes the artwork of the Germanic peoples during the Migration period . It includes the Migration art of the Germanic tribes on the continent, as well the start of the Insular art or Hiberno-Saxon art of the Anglo-Saxon and Celtic fusion in the British Isles...
of the "barbarian
Barbarian
Barbarian and savage are terms used to refer to a person who is perceived to be uncivilized. The word is often used either in a general reference to a member of a nation or ethnos, typically a tribal society as seen by an urban civilization either viewed as inferior, or admired as a noble savage...
" peoples who took over the territory of the Western Empire. They were especially used inlaid in gold cells in the cloisonné
Cloisonné
Cloisonné is an ancient technique for decorating metalwork objects, in recent centuries using vitreous enamel, and in older periods also inlays of cut gemstones, glass, and other materials. The resulting objects can also be called cloisonné...
technique, a style often just called garnet cloisonné, found from Anglo-Saxon
Anglo-Saxon
Anglo-Saxon may refer to:* Anglo-Saxons, a group that invaded Britain** Old English, their language** Anglo-Saxon England, their history, one of various ships* White Anglo-Saxon Protestant, an ethnicity* Anglo-Saxon economy, modern macroeconomic term...
England, as at Sutton Hoo
Sutton Hoo
Sutton Hoo, near to Woodbridge, in the English county of Suffolk, is the site of two 6th and early 7th century cemeteries. One contained an undisturbed ship burial including a wealth of Anglo-Saxon artefacts of outstanding art-historical and archaeological significance, now held in the British...
, to the Black Sea
Black Sea
The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...
.
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow and orange. In the USA it is known as the birthstone for January. It is the state mineral of Connecticut
Connecticut
Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...
, New York's gemstone, and star garnet (garnet with rutile
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
asterisms) is the state gemstone of Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
.
Industrial uses
Garnet sand is a good abrasiveAbrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...
, 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
Water jet cutter
A water jet cutter, also known as a waterjet, is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance. The process is essentially the same as water erosion found in nature but greatly accelerated...
. 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 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 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.
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 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
Placer deposit
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early...
.
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 Rajasthan
Rajasthan
Rājasthān the land of Rajasthanis, , is the largest state of the Republic of India by area. It is located in the northwest of India. It encompasses most of the area of the large, inhospitable Great Indian Desert , which has an edge paralleling the Sutlej-Indus river valley along its border with...
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.
Further reading
- 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
External links
- USGS Garnet locations - USA
- http://www.gemstone.org/gem-by-gem/english/garnet.html
- http://www.mindat.org/min-10272.html
- Mineral Information Institute Garnet