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Geology is the Way

Garnet

Cubic

X3Y2Si3O12

The members of the garnet group represent typical metamorphic minerals, but they are present also as accessories in igneous rocks and as detrital grains in sediments. The name ‘garnet’ originates from the Latin granatus, and it is possibly related to the remembrance of this mineral to the seeds of pomum granatum (pomegranate). All members of the garnet group are, indeed, cubic minerals with distinctive rhombic dodecahedron habit.

Structure and chemistry
Garnets constitute a group of nesosilicates with general formula X3Y2Si3O12. Their structure contains a series of stacked SiO4 tetrahedrons and YO6 octahedrons that surround distorted cubes with 8 oxygens on the apices that contain X cations – the largest site in garnets. This group of minerals is characterized by seven major elements (Mg, Al, Fe, Mn, Ca, Cr, and Ti) that define the names of the principal end-members and may also contain a wide range of trace elements. The major mineral species are

Pyrope: Mg3Al2Si3O12
Almandine: Fe3Al2Si3O12
Spessartine: Mn3Al2Si3O12
Grossular: Ca3Al2Si3O12
Andradite: Ca3(Fe3+,Ti)2Si3O12
Uvarovite: Ca3Cr2Si3O12

These six garnet species are grouped into two main series, depending on the content of the X and Y sites: PyrAlSpite (pyrope, almandine, and spessartine) and UGrAndite (uvarovite, grossular, andradite) garnets. The solid solution is complete between members of the same series, and only partial between pyralspite and ugrandite garnets, because Ca2+ has larger radius than Mg, Mn, and Fe2+and it is accommodated in a slightly different structure. Pure end-member compositions are rare in nature and most garnets contain appreciable proportions of three to four components.

Garnet var. andradite from Antetezambato, Antsiranana Province, Madagascar. Photo by Robert M. Lavinsky, IRocks.

Properties
Habit: rhombic dodecahedron, cubic
Hardness: 6 – 7.5
Cleavage: none
Twinning: complex sector twinning
Color: red, brown, black, green, yellow, pink or white
Luster: vitreous, resinous
Streak: white
Alteration: chlorite, serpentine, talc
In thin section…
Refraction index (n): 1.675-1.887
Color: colorless to pink, yellow, and brown
Birefringence (δ): always extinct (isotropic), very rare weak birefringence
Relief: high
[Mindat]

SiO2 can be replaced by H2O in the garnet structures producing hydrogarnets like the relatively common

Hydrogrossular: Ca3(Fe3+,Ti)2(SiO4)1-m(OH)4m

which forms a solid solution series with grossular.

garnet classification diagram

This beautiful diagram by Lina Jakaite shows the chemical variation and most common colors of the end members of the Garnet Group. Visit strike-dip.com for more!

Field features

garnet crystal morphology

Common habits observed in garnet crystals (modified after Goldschmidt, 1918).

Garnet forms characteristic equant crystals with dodecahedron or trapezohedron habit, commonly showing reddish to brownish color. Outcrop sections show 6-sided shapes (hexagon-like) to 4- and 8-sided ones. Crystals may show conchoidal fracture and cleavage is very rare. Garnet is relatively hard and generally more resistant on streak than metal. Its vitreous lustre is well visible on fracture surfaces.

garnet schist

Garnet grains are relatively common in metapelitic rocks in regionally metamorphosed terranes. Note the reddish colors and characteristic equant grains with 4- to 6-sided shapes. Garnet mica schist from Syros, Greece. Photo by Graeme Churchard.

garnet mica schist

Almandine garnets in a mica schist, North Tyrol, Austria. Photo by Didier Descouens.

garnet mylonite

Mylonitic gneiss with porphyroblasts of garnet (red) and plagioclase (white). Otrøy, Western Gneiss Region, Norway. Photo by Woudloper.

garnet biotite skarn

Archean garnet (red) biotite (black) metasomatic rock. 3.8 cm across. Beartooth Mountains, southern Montana, USA. Photo by James St. John.

Garnet in thin section
The identification of garnet in thin section is generally straightforward, thanks to its unique features. It shows a very high relief at PPL and it is always extinct at CPL. Some varieties (e.g. hydrogrossular) may show a very weak birefringence. Colors are pale in thin section, ranging from transparent to pale pink to yellowish (grossular), brownish (andradite), and green (ugrandite garnets). Compared to spinel, garnet shows much paler colors. Crystals are very commonly euhedral, showing 6-sided sections and conchoidal fractures. 

Above: large garnet crystal in a paragneiss, fractured and partially replaced by phyllosilicates with yellowish to brownish color. Width: 10 mm. Posada Valley, NE Sardinia, Italy.

Above: garnet grains in quartz-rich layer within a mica schist. Note the very high relief compared to the surrounding quartz (grey). Width: 5 mm. Posada Valley, NE Sardinia, Italy.

Above: garnet grains with asymmetric pressure shadows surrounded by white mica (high interference colors) in a mylonitic paragneiss. Width: 5 mm. Posada Valley, NE Sardinia, Italy.

Occurrence
Garnets are relatively common in metamorphic rocks and very rare in igneous rocks. However, under special conditions, they can be found in some plutonic and volcanic rocks. Pyrope-rich compositions may store Al and Mg in ultrabasic rock types, such as kimberlite and peridotite, and may originate from the metamorphism of basic rocks, for example at eclogite-facies. Almandine is a typical product of regional and contact metamorphism in metapelites, which are generally rich in Al and Fe compared to Mg. In these rocks it can be found associated with white mica, chlorite, biotite, staurolite, or Al-silicates. Almandine to pyrope compositions are found in granulitic rocks and blueschists. Al-rich granitic rocks may also -very very rarely – contain almandine garnets. Spessartine is uncommon in rocks and present only as solid solution end-member. It can be, however, found in some metasomatic rocks like skarn and other mineralizations. Ca-bearing garnets like grossular and andradite may occur in Ca-rich rocks like metamorphosed or metasomatized impure metacarbonates, in vesicles in basaltic rocks, sometimes even in pegmatites. These garnets may be associated with other Ca-silicates like vesuvianite, diopside, or wollastonite. Uvarovite is very rare and found only in Cr-rich rocks like metamorphosed serpentinite or some skarns, where it may originate due to Cr-metasomatism of limestones. Hydrogrossular can be found in altered basic rocks and metamorphosed marls. All garnets may occur in sedimentary rocks in the form of detrital minerals.

References
Baxter, E. F., Caddick, M. J., & Ague, J. J. (2013). Garnet: Common mineral, uncommonly useful. Elements9(6), 415-419.
Cesare, B., Nestola, F., Johnson, T., Mugnaioli, E., Della Ventura, G., Peruzzo, L., … & Erickson, T. (2019). Garnet, the archetypal cubic mineral, grows tetragonal. Scientific reports9(1), 1-13.
Grew, E. S., Locock, A. J., Mills, S. J., Galuskina, I. O., Galuskin, E. V., & Hålenius, U. (2013). Nomenclature of the garnet supergroup. American Mineralogist98(4), 785-811.
Novak, G. A., & Gibbs, G. V. (1971). The crystal chemistry of the silicate garnets. American Mineralogist: Journal of Earth and Planetary Materials56(5-6), 791-825.

        

See also
Alexstrekeisen.it – Garnet.
Garnet gallery by James St. John.
Gery Parent on Flickr.
Garnet by D. Nishio-Hamane.

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