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Geology is the Way

Garnet

Cubic

X3Y2Si3O12

Garnet is a common silicate mineral that occurs in many metamorphic rocks and that is present as an accessory in igneous rocks and as a detrital mineral in sedimentary rocks. The name ‘garnet’ derives from the Latin granatus, and it is possibly related to the remembrance of this mineral to the seeds of pomum granatum (pomegranate), since many garnets are, indeed, red and crystallize with a distinctive rhombic dodecahedron habit (resembling small, equant ‘seeds’).

Structure and chemistry
Garnets are a group of nesosilicates with general formula X3Y2Si3O12. Their structure contains a series of stacked SiO4 tetrahedrons (blue in the image below) and YO6 octahedrons (violet in the image below) that surround distorted cubes with 8 oxygens on the apices containing X cations – the largest site in garnet (light blue circles in the image).

Garnet crystal structure
3D view of the crystal structure of garnet (pyrope). Blue tetrahedrons: Si [Z-site]. Violet octahedrons: Y-sites. Light blue spheres indicate X-cations (distorted cubes are not shown for simplicity). Image by Joe Smyth via ruby.colorado.

These sites contain six major elements (Mg, Al, Fe, Mn, Ca, Cr) that define the principal end-members in the main garnet series, which are:

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

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

Properties
Habit: rhombic dodecahedron, trapezohedron
Hardness: 6 – 7.5
Density: 3.58 – 4.32 g/cm3
Cleavage: none, {110} parting uncommon
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
Pleochroism: none
Birefringence (δ): 0.00 always extinct (isotropic), very rare weak birefringence
Relief: high
[Mindat]

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 the ionic radius of Ca2+ is larger than that of Mg2+, Mn2+, and Fe2+and can be accommodated in a different structure, with larger X-sites. Pure end-member compositions are rare in nature and most garnets contain appreciable proportions of three to four components.
Other important substitutions may occur in the garnet group. Knorringite [Mg3Cr2Si3O12] can occur as a component in kimberlitic garnets and many garnets experience some degree of substitution of Si by P and (Al, Fe3+) by Ti, which can be balanced by the coeval substitution of Si by Al. Garnets can also incorporate yttrium (Y) and other Rare Earth Elements. This allows to date the timing of garnet growth with techniques such as Lu-Hf radiometric dating, which utilizes the radioactive decay system of lutetium-176 into hafnium-176. Other elements that have been reported in garnets are vanadium (V) and zirconium (Zr).
SiO2 can be replaced by H2O in the garnet structure: this substitution produces hydrogarnets like the relatively common

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

which forms a solid solution series with grossular.

garnet classification diagram
Chart showing the chemical variability of common members of the garnet group. © Lina Jakaite via strike-dip.com.

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 occur in metapelitic rocks in regionally metamorphosed terranes, but are rarely this big. In this example, garnet crystals show dark red color and an equant shape with characteristic six- to eight-sided sections. Garnet mica schist from Syros, Greece. Photo © Graeme Churchard.
garnet mica schist
Almandine garnets in a mica schist, North Tyrol, Austria. Photo © Didier Descouens.
garnet mylonite
Mylonitic gneiss with porphyroblasts of garnet (red) and plagioclase (white). Darker bands contain phyllosilicates. Otrøy, Western Gneiss Region, Norway. Photo © Woudloper.
garnet biotite skarn
Archean garnet (red) biotite (black) metasomatic rock. 3.8 cm across. Beartooth Mountains, southern Montana, USA. Photo © 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 fracture. 

CPL
CPL
CPL
PPL
PPL
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.

CPL
CPL
CPL
PPL
PPL
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.

CPL
CPL
CPL
PPL
PPL
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.

Examples of garnet-bearing rocks

Garnet from a Barrovian metamorphic terrane (NE Sardinia)
Garnet produced by medium-grade metamorphism at amphibolite-facies conditions.
Samples: micaschist, paragneiss
Metamorphic zones: garnet + albite; garnet + oligoclase; garnet + staurolite.
Other minerals: muscovite, chlorite, biotite, quartz, ilmenite, hematite, rutile.
Locality: 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.
Duchêne, S., Blichert-Toft, J., Luais, B., Télouk, P., Lardeaux, J. M., & Albarède, F. (1997). The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism. Nature387(6633), 586-589.
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.

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