Garnet

This article's lead section may be too short to adequately summarize the key points. Please consider expanding the lead to provide an accessible overview of all important aspects of the article. (January 2024)

Garnet
Crystal class		${\displaystyle 4/m{\bar {3}}2/m}$
Specific gravity	3.1–4.3
Polish luster	vitreous to subadamantine[2]
Optical properties	Single refractive, often anomalous double refractive[2]
Refractive index	1.72–1.94
Birefringence	None
Pleochroism	None
Ultraviolet fluorescence	variable
Other characteristics	variable magnetic attraction
Major varieties
Pyrope	Mg3Al2Si3O12
Almandine	Fe3Al2Si3O12
Spessartine	Mn3Al2Si3O12
Andradite	Ca3Fe2Si3O12
Grossular	Ca3Al2Si3O12
Uvarovite	Ca3Cr2Si3O12

Garnets ( /ˈɡɑːrnɪt/) 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

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's

• 
  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

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

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.^[15]

Garnet group end member species

Pyralspite garnets – aluminium in Y site

• Almandine: Fe₃Al₂(SiO₄)₃
• Pyrope: Mg₃Al₂(SiO₄)₃
• Spessartine: Mn₃Al₂(SiO₄)₃

Almandine

Almandine, sometimes incorrectly called almandite, is the modern gem known as

Pyrope (from the Greek pyrōpós meaning "firelike")[3] is red in color and chemically an aluminium silicate with the formula Mg₃Al₂(SiO₄)₃, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to 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.^[20]

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.^[21] Pyrope has tradenames some of which are misnomers; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian ruby from the Czech Republic.^[16]

Pyrope is an indicator mineral for high-pressure rocks. Mantle-derived rocks (peridotites and eclogites) commonly contain a pyrope variety.^[22]

Spessartine

Spessartine or spessartite is manganese aluminium garnet, Mn₃Al₂(SiO₄)₃. Its name is derived from Spessart in Bavaria.^[3] It occurs most often in skarns,^[3] granite pegmatite and allied rock types,^[23] and in certain low grade metamorphic phyllites. Spessartine of an orange-yellow is found in Madagascar.^[24] Violet-red spessartines are found in rhyolites in Colorado^[21] and Maine.^{[citation needed]}

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 (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 resembles alexandrite.^[26]

Ugrandite group – calcium in X site

• Andradite: Ca₃Fe₂(SiO₄)₃
• Grossular: Ca₃Al₂(SiO₄)₃
• Uvarovite: 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.

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

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.^[32][33]

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

• Calcium in X site
  • Goldmanite: Ca₃(V³⁺,Al,Fe³⁺)₂(SiO₄)₃
  • Kimzeyite: Ca₃(Zr, Ti)₂[(Si,Al,Fe³⁺)O₄]₃
  • Morimotoite: Ca₃Ti⁴⁺Fe²⁺(SiO₄)₃
  • Schorlomite: Ca₃Ti⁴⁺₂(SiO₄)(Fe³⁺O₄)₂
• Hydroxide bearing – calcium in X site
  • Hydrogrossular: Ca₃Al₂(SiO₄)_3−x(OH)_4x
    • Hibschite: Ca₃Al₂(SiO₄)_3−x(OH)_4x (where x is between 0.2 and 1.5)
    • Katoite: Ca₃Al₂(SiO₄)_3−x(OH)_4x (where x is greater than 1.5)
• Magnesium or manganese in X site
  • Knorringite: Mg₃Cr₂(SiO₄)₃
  • Majorite: Mg₃(Fe²⁺Si)(SiO₄)₃
  • Calderite: Mn₃Fe³⁺₂(SiO₄)₃

Knorringite

Knorringite is a magnesium-chromium garnet species with the formula Mg₃Cr₂(SiO₄)₃. 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.^[34]

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 subclass: 09.A Nesosilicate
  • Nickel-Strunz classification
    : 09.AD.25

• 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.

Synthetic garnets

Also known as rare-earth garnets.

The crystallographic structure of garnets has been expanded from the prototype to include chemicals with the general formula A₃B₂(CO₄)₃. Besides silicon, a large number of elements have been put on the C site, including

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 of transparent ceramics.^[41] LuAG is particularly favored over other crystals for its high density and thermal conductivity; it has a relatively small lattice constant in comparison to the other rare-earth garnets, which results in a higher density producing a crystal field with narrower linewidths and greater energy level splitting in absorption and emission.^[42]

Terbium gallium garnet (TGG), Tb₃Ga₅O₁₂, is a Faraday rotator material with excellent transparency properties and is very resistant to laser damage. TGG can be used in optical isolators for laser systems, in optical circulators for fiber optic systems, in optical modulators, and in current and magnetic field sensors.^[43]

Another example is gadolinium gallium garnet (GGG), Gd₃Ga₂(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.^{[citation needed]}

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.

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^[47] as well as a thermochronometer in the (U-Th)/He system^[48] to date timing of cooling below a closure temperature.

Garnets can be chemically altered and most often alter to serpentine, talc, and chlorite.^[45]

Uses

Gemstones

Red garnets were the most commonly used gemstones in the

Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange.[52] In the US it is known as the birthstone for January.^[2] 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.^[53] It is the state mineral of Connecticut,^[54] New York's gemstone,^[55] and star garnet (garnet with rutile asterisms) is the state gemstone of Idaho.^[56]

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.^[57]

Garnet paper is favored by cabinetmakers for finishing bare wood.^[58]

Garnet sand is also used for

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.^{[citation needed]}

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.^[59]

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.^{[citation needed]}

River garnet is particularly abundant in Australia. The river sand garnet occurs as a placer deposit.^[60]

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 rock garnet for use as an industrial abrasive.^[3]

Cultural significance

Garnet is the

United States

Garnet is

Since 2003 New York State has ranked first in industrial garnet-production in the United States. Since there are just a few companies that account for all U.S. industrial garnet production, published detailed production statistics for New York State are not available. However, generally speaking, Barton Mines in Warren County is the largest U.S. garnet producer.[69]

Collections

The New York State Museum in Albany, NY houses specimens from significant sites across the state, including 93 mineral species from the Balmat-Edwards mining district in St. Lawrence, super garnets from the Barton Mine in the Adirondack Mountains, and Herkimer diamonds from Herkimer County, New York^[70]

Oldest garnet mine

The largest garnet mine in the world is located Near North Creek New York and is operated by Barton Mines Corporation who supplies about 90% of the world's garnet.^[71] Barton Mines Corporation is the first and oldest industrial garnet mining operation in the world and the second oldest continuous mining operation in the United States under the same management and mining the same product throughout its history. The Gore Mountain Mine of the Barton Mines Corporation was first mined under the direction of H. H. Barton Sr. in 1878 to produce garnet as the primary product.^[71]

Largest garnet crystal

The open-pit Barton Garnet Mine, located at Gore Mountain in the Adirondack Highlands, yields the world's largest single crystals of garnet; diameters range from 5 to 35 cm and commonly average 10–18 cm.^[72]

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 the Grenvillian orogeny and serves as a critical data point in ascertaining the evolution of the megacrystic garnet deposits.^[72]

See also

• Abrasive blasting

References

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

Wikimedia Commons has media related to:
Garnet (category)

• http://www.gemstonemagnetism.com contains a comprehensive section about garnets and garnet magnetism.
• USGS Garnet locations – USA
• http://gemstone.org/education/gem-by-gem/154-garnet
• http://www.mindat.org/min-10272.html
• Blog post on garnets on the Law Library of Congress's blog
• https://www.birthstone.guide/garnet-birthstone-meaning Garnet birthstone stories