Garnet

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, defining distinct species. These species fall into two primary solid solution series: the pyralspite series, with the general formula ₃Al₂₃; and the ugrandite series, with the general formula Ca₃₂₃. Notable varieties of grossular include hessonite and tsavorite.
Although garnets are often associated with metamorphism, they can also occur in volcanic rocks on rare occasions.

Etymology

The word "garnet" comes from the 14th-century Middle English word gernet, meaning dark red. It is borrowed from Old French grenate from Latin granatus, from granum. This is possibly a reference to mela granatum or even pomum granatum, a plant whose fruits contain abundant and vivid red seed covers, which are similar in shape, size, and color to some garnet crystals. Hessonite garnet is also named gomed in Indian literature and is one of the nine jewels in Vedic astrology that comprise the Navaratna.

Physical properties

Properties

Garnet species are found in every colour, with reddish shades most common. Blue garnets are the rarest and were first reported in the 1990s.
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 lustre is categorized as vitreous or resinous.

Crystal structure

Garnets are nesosilicates having the general formula X₃Y₂₃. The X site is usually occupied by divalent cations ²⁺ and the Y site by trivalent cations ³⁺ in an octahedral/tetrahedral framework with ⁴⁻ occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit as well as the hexoctahedral habit. They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to one another but are never actually cubic because, despite being isometric, the and families of planes are depleted. Garnets do not have any cleavage planes, so when they fracture under stress, sharp, irregular pieces are formed.

Hardness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.0 to 7.5. The harder species like almandine are often used for abrasive purposes.

Magnetics used in garnet series identification

For gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, and determine the composition of garnets in terms of percentages of end-member species within an individual gem.

Garnet group end member species

Pyralspite garnets – aluminium in ''Y'' site

Almandine: Fe₃Al₂₃
Pyrope: Mg₃Al₂₃
Spessartine: Mn₃Al₂₃
Almandine

Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle. The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. The name "almandine" is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminium garnet with the formula Fe₃Al₂₃; the deep red transparent stones are often called precious garnet and are used as gemstones. Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.

Pyrope

Pyrope is red in color and chemically an aluminium silicate with the formula Mg₃Al₂₃, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to black. Pyrope and spessartine gemstones have been recovered from the Sloan diamondiferous kimberlites in Colorado, from the Bishop Conglomerate and in a Tertiary age lamprophyre at Cedar Mountain in Wyoming.
A variety of pyrope from Macon County, North Carolina, is a violet-red shade and has been called "rhodolite", Greek for rose. In chemical composition, it may be considered as essentially an isomorphous mixture of pyrope and almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames some of which are misnomers: Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian ruby'from the Czech Republic.
Pyrope is an indicator mineral for high-pressure rocks. Mantle-derived rocks commonly contain a pyrope variety.

Spessartine

Spessartine or spessartite is manganese aluminium garnet, Mn₃Al₂₃. Its name is derived from Spessart in Bavaria. It occurs most often in skarns, 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

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

Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily, Madagascar. This type has also been found in parts of the United States, Russia, Kenya, Tanzania, and Turkey. It changes color from blue-green to purple depending on the color temperature of viewing light, as a result of the relatively high amounts of vanadium.
Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color.
This is the rarest type of garnet. Because of its color-changing quality, this kind of garnet resembles alexandrite.

Ugrandite group – calcium in ''X'' site

Andradite: Ca₃Fe₂₃
Grossular: Ca₃Al₂₃
Uvarovite: Ca₃Cr₂₃
Andradite

Andradite is a calcium-iron garnet, Ca₃Fe₂₃, is of variable composition and may be red, yellow, brown, green, or black. The recognized varieties are demantoid, melanite, and topazolite. The red-brown translucent variety of colophonite is recognized as a partially obsolete name. Andradite is found in skarns and in deep-seated igneous rocks like syenite as well as serpentines and greenschists. Demantoid is one of the most prized of garnet varieties.

Grossular

Grossular is a calcium-aluminium garnet with the formula Ca₃Al₂₃, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical former generic name for the gooseberry, Grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown, red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossular is found in skarns, contact metamorphosed limestones with vesuvianite, diopside, wollastonite, and wernerite.
Grossular garnet from Kenya and Tanzania has been called tsavorite. Tsavorite was first described in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca₃Cr₂₃. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural Mountains of Russia and Outokumpu, Finland. Uvarovite is named for Count Uvaro, a Russian imperial statesman.

Less common species

Calcium in X site
*Goldmanite:
*Kimzeyite:
*Morimotoite:
*Schorlomite:
Hydroxide bearing – calcium in X site
*Hydrogrossular:
**Hibschite:
**Katoite:
Magnesium or manganese in X site
*Knorringite:
*Majorite:
*Calderite:
Knorringite

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

Garnet structural group

Formula: X₃Z₂₃
*All are cubic or strongly pseudocubic.

IMA/CNMNC Nickel-Strunz Mineral class	Mineral name	Formula	Crystal system	Point group	Space group
04 Oxide	Bitikleite-	Ca₃SnSb₃	isometric	mm	Iad
04 Oxide	Bitikleite-	Ca₃₃	isometric	mm	Iad
04 Oxide	Bitikleite-	Ca₃SbZr₃	isometric	mm	Iad
04 Tellurate	Yafsoanite	Ca₃Zn₃₂	isometric	mm or 432	Iad or I4₁32
08 Arsenate	Berzeliite	NaCa₂Mg₂₃	isometric	mm	Iad
08 Vanadate	Palenzonaite	NaCa₂Mn²⁺₂₃	isometric	mm	Iad
08 Vanadate	Schäferite	NaCa₂Mg₂₃	isometric	mm	Iad

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⁺₃Al₂₃	isometric	mm	Iad
Andradite	Ca₃Fe³⁺₂₃	isometric	mm	Iad
Calderite	Mn⁺²₃Fe⁺³₂₃	isometric	mm	Iad
Goldmanite	Ca₃V³⁺₂₃	isometric	mm	Iad
Grossular	Ca₃Al₂₃	isometric	mm	Iad
Henritermierite	Ca₃Mn³⁺₂₂₄	tetragonal	4/mmm	I4₁/acd
Hibschite	Ca₃Al₂_4x	isometric	mm	Iad
Katoite	Ca₃Al₂_4x	isometric	mm	Iad
Kerimasite	Ca₃Zr₂₂	isometric	mm	Iad
Kimzeyite	Ca₃Zr₂₂	isometric	mm	Iad
Knorringite	Mg₃Cr₂₃	isometric	mm	Iad
Majorite	Mg₃₃	tetragonal	4/m or 4/mmm	I4₁/a or I4₁/acd
Menzerite-	Y₂CaMg₂₃	isometric	mm	Iad
Momoiite	Mn²⁺₃V³⁺₂₃	isometric	mm	Iad
Morimotoite	Ca₃₃	isometric	mm	Iad
Pyrope	Mg₃Al₂₃	isometric	mm	Iad
Schorlomite	Ca₃Ti⁴⁺₂₂	isometric	mm	Iad
Spessartine	Mn²⁺₃Al₂₃	isometric	mm	Iad
Toturite	Ca₃Sn₂₂	isometric	mm	Iad
Uvarovite	Ca₃Cr₂₃	isometric	mm	Iad

References: Mindat.org; mineral name, chemical formula and space group 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.