Garnet

Garnet
General
Category	Nesosilicate
Formula (repeating unit)	The general formula X3Y2(SiO4)3
Identification
Color	virtually all colors
Crystal habit	Rhombic_dodecahedron or cubic
Crystal system	Cubic rhombic dodecahedron, icositetrahedron
Cleavage	Indistinct
Fracture	conchoidal to uneven
Mohs scale hardness	6.5–7.5
Luster	vitreous to resinous
Streak	White
Specific gravity	3.1–4.3
Polish luster	vitreous to subadamantine[1]
Optical properties	Single refractive, often anomalous double refractive[1]
Refractive index	1.72–1.94
Birefringence	None
Pleochroism	None
Major varieties
Pyrope	Mg3Al2Si3O12
Almandine	Fe3Al2Si3O12
Spessartine	Mn3Al2Si3O12
Andradite	Ca3Fe2Si3O12
Grossular	Ca3Al2Si3O12
Uvarovite	Ca3Cr2Si3O12

Garnets /ˈɡɑrnət/ are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.^{[note 1]}
Garnets possess similar physical properties and crystal forms but different chemical compositions. The different species 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.

Contents 1 Physical properties 1.1 Properties 1.2 Crystal structure 1.3 Hardness 1.4 Magnetics used in garnet series identification 2 Garnet group endmember species 2.1 Pyralspite garnets – aluminium in Y site 2.1.1 Almandine 2.1.2 Pyrope 2.1.3 Spessartine 2.2 Ugrandite group – calcium in X site 2.2.1 Andradite 2.2.2 Grossular 2.2.3 Uvarovite 2.3 Less common species 2.3.1 Knorringite 3 Garnet structural group 4 Synthetic garnets 5 Geological importance of garnet 6 Uses of garnets 6.1 Gemstones 6.2 Industrial uses 7 See also 8 Notes 9 References 10 Further reading 11 External links

Physical properties

Properties

Garnet 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, Madagascar. It is also found in parts of the United States, Russia, Kenya, Tanzania, 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.

A sample showing the deep red color garnet can exhibit.

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 (glass-like) or resinous (amber-like).

Crystal structure

Crystal structure model of garnet/

Garnets are nesosilicates having the general formula X₃Y₂(Si O₄)₃. The X site is usually occupied by divalent cations (Ca²⁺, Mg²⁺, Fe²⁺) and the Y site by trivalent cations (Al³⁺, Fe³⁺, Cr³⁺) in an octahedral/tetrahedral framework with [SiO₄]⁴⁻ occupying the tetrahedra.^[4] Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. (Note: the word "trapezohedron" as used here and in most mineral texts refers to the shape called a Deltoidal icositetrahedron in solid geometry.) They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed.

Hardness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are often used for abrasive purposes.

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.^[5] See http://gemstonemagnetism.com.

Garnet group endmember species

Pyralspite garnets – aluminium in Y site

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

Almandine

Almandine in metamorphic rock

Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. The name Almandine is a corruption of 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 rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.

Pyrope

Pyrope (from the Greek pyrōpós meaning "fire-eyed") is red in color and chemically a magnesium aluminium silicate with the formula Mg₃Al₂(SiO₄)₃, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to 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.^[6]
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.^{[citation needed]}
Pyrope is an indicator mineral for high-pressure rocks. The garnets from mantle-derived rocks, peridotites, and eclogites commonly contain a pyrope variety.

Spessartine

Spessartine (the reddish mineral)

Spessartine or spessartite is manganese aluminium garnet, Mn₃Al₂(SiO₄)₃. Its name is derived from 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.

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. The recognized varieties are topazolite (yellow or green), demantoid (green) and melanite (black). Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the 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 on display at the U.S. National Museum of Natural History. The green gem at right is a type of grossular known as tsavorite.

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 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 Tsavo area of Kenya, from which the gem takes its name.^[7]

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca₃Cr₂(SiO₄)₃. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia and Outokumpu, Finland.

Less common species

• Calcium in X site
  • Goldmanite: Ca₃V₂(SiO₄)₃
  • Kimzeyite: Ca₃(Zr,Ti)₂[(Si,Al,Fe³⁺)O₄]₃
  • Morimotoite: Ca₃Ti⁴⁺Fe²⁺(SiO₄)₃
  • Schorlomite: Ca₃(Ti⁴⁺,Fe³⁺)₂[(Si,Ti)O₄]₃
• Hydroxide bearing – calcium in X site
  • Hydrogrossular: Ca₃Al₂(SiO₄)_3-x(OH)_4x
    • Hibschite: Ca₃Al₂(SiO₄)_3-x(OH)_4x (where x is between 0.2 and 1.5)
    • Katoite: Ca₃Al₂(SiO₄)_3-x(OH)_4x (where x is greater than 1.5)
• Magnesium or manganese in X site
  • Knorringite: 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.

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	m3m	Ia3d
04 Oxide	Bitikleite-(SnFe)	Ca3(SnSb5+)(Fe3+O)3	isometric	m3m	Ia3d
04 Oxide	Bitikleite-(ZrFe)	Ca3SbZr(Fe3+O4)3	isometric	m3m	Ia3d
04 Tellurate	Yafsoanite	Ca3Zn3(Te6+O6)2	isometric	m3m or 432	Ia3d or I4132
08 Arsenate	Berzeliite	NaCa2Mg2(AsO4)3	isometric	m3m	Ia3d
08 Vanadate	Palenzonaite	NaCa2Mn2+2(VO4)3	isometric	m3m	Ia3d
08 Vanadate	Schäferite	NaCa2Mg2(VO4)3	isometric	m3m	Ia3d

• IMA/CNMNC – Nickel-Strunz – Mineral subclass: 09.A Nesosilicate
  • Nickel-Strunz classification: 09.AD.25

Mineral name	Formula	Crystal system	Point group	Space group
Almandine	Fe2+3Al2(SiO4)3	isometric	m3m	Ia3d
Andradite	Ca3Fe3+2(SiO4)3	isometric	m3m	Ia3d
Calderite	Mn+23Fe+32(SiO4)3	isometric	m3m	Ia3d
Goldmanite	Ca3V3+2(SiO4)3	isometric	m3m	Ia3d
Grossular	Ca3Al2(SiO4)3	isometric	m3m	Ia3d
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	m3m	Ia3d
Katoite	Ca3Al2(SiO4)(3-x)(OH)4x (x= 1.5-3)	isometric	m3m	Ia3d
Kerimasite	Ca3Zr2(Fe+3O4)2(SiO4)	isometric	m3m	Ia3d
Kimzeyite	Ca3Zr2(Al+3O4)2(SiO4)	isometric	m3m	Ia3d
Knorringite	Mg3Cr2(SiO4)3	isometric	m3m	Ia3d
Majorite	Mg3(Fe2+Si)(SiO4)3	tetragonal	4/m or 4/mmm	I41/a or I41/acd
Menzerite-(Y)	Y2CaMg2(SiO4)3	isometric	m3m	Ia3d
Momoiite	Mn2+3V3+2(SiO4)3	isometric	m3m	Ia3d
Morimotoite	Ca3(Fe2+Ti4+)(SiO4)3	isometric	m3m	Ia3d
Pyrope	Mg3Al2(SiO4)3	isometric	m3m	Ia3d
Schorlomite	Ca3Ti4+2(Fe3+O4)2(SiO4)	isometric	m3m	Ia3d
Spessartine	Mn2+3Al2(SiO4)3	isometric	m3m	Ia3d
Toturite	Ca3Sn2(Fe3+O4)2(SiO4)	isometric	m3m	Ia3d
Uvarovite	Ca3Cr2(SiO4)3	isometric	m3m	Ia3d

• 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

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 Ge, Ga, Al, V and Fe.^[8]
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), Y₃Fe₂(FeO₄)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, 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.