Garnets - X3Y2(SiO4)3

Named from granatum (a pomegranate) for its resemblance to seeds of this fruit, Garnets are nesosilicates having the general formula X3Y2(SiO4)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.

Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. 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.

Pyralspite garnets – aluminium in Y site

Pyralspite garnets show essentially complete solid solution among three end-member compositions, Pyrope-Almandine-Spessartine; minor solid solution with Ugrandite garnet may be present.

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

Almandine: Named in 1546 by Georgius Agricola for Alabanda in Turkey, an ancient gem cutting center, presumably where almandine was fashioned into gemstones. Chemically, almandine is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others.

Pyrope: From the Greek, pyropos, "fiery-eyed" in allusion to the red hue. 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. Pyrope is an indicator mineral for high-pressure rocks. The garnets from mantle-derived rocks, peridotites, and eclogites commonly contain a pyrope variety.

Spessartine: From Spessart Mountains, Bavaria, Germany is manganese aluminium garnet, Mn3Al2(SiO4)3. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites.

Optical properties of Pyralspite granet:
• Form: Garnets are high in the crystalloblastic series and appear most commonly as euhedral crystals of dodecahedron or trapozohedron habit.
• Color: Colourless to pink
• Relief: High

• Interference colors: Isotropic

Ugrandite group – calcium in X site

Extensive solid solution exist among the three and-members of the ugrandite garnet. Less solid solution exist between ugrandite and pyralspite garnet.

• Uvarovite: Ca3Cr2(SiO4)3
• Grossular: Ca3Al2(SiO4)3
• Andradite: Ca3Fe2(SiO4)3

Andradite: Named in 1868 by James Dwight Dana in honor of José Bonifácio de Andrada, Brazilian mineralogist, 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.

Grossular: Derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia, 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. Grossular is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

Uvarovite: Named after Count Sergey Semeonovich Uvarov, Russian statesman and scholar, President of the Academy of St Petersburg, 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.

Optical properties of Ugrandite granet
• Color: colourless to pale yellow (grossular), pale to deep brown (Andradite) and emerald-green (ugrandite)
• Form: the garnets are high in the crystalloblastic series and appear most commonly as euhedral crystals of dodecahedron or trapozohedron habit.
• Interference colors: isotropic but sometime ugrandite garnet show a weak birefringence
• Relief: high


Bibliography



• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.


Photo
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Garnet crystals with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)		 granatiscozia(3).jpg

Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)		 granatiscozia(4).jpg

Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)
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Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)		 granatiscozia(6).jpg

Garnet crystal with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)		 granatiscozia(7).jpg

Garnet crystals with internal foliation. Moine Schists, Glenborrodale, Ardnamurchan (Scotland). PPL image, 2x (Field of view = 7mm)
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Garnet with Staurolite (yellow) inclusions. PPL image, 2x (Field of view = 7mm)		 granatimeta(3).jpg

Garnet in a Sialic Granulite. PPL image, 2x (Field of view = 7mm)		 granatimeta(4).jpg

Garnet in a Sialic Granulite. PPL image, 2x (Field of view = 7mm)
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Garnet in a Sialic Granulite. PPL image, 2x (Field of view = 7mm)		 granatimeta(6).jpg

Garnet in a Sialic Granulite. PPL image, 2x (Field of view = 7mm)		 granatimeta(7).jpg

Garnet in a Eclogite. PPL image, 2x (Field of view = 7mm)
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Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)		 granatoconfoliazioneinterna(3).jpg

Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)		 granatoconfoliazioneinterna(4).jpg

Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)
granatoconfoliazioneinterna(5).jpg

Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)		 granatoconfoliazioneinterna(6).jpg

Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)		 granatoconfoliazioneinterna(7).jpg

Garnet with graphite or/and Rutile inclusions. PPL image, 2x (Field of view = 7mm)
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Garnet in a Eclogite. PPL image, 2x (Field of view = 7mm)		 granatimeta(9).jpg

Garnet in a Eclogite. PPL image, 2x (Field of view = 7mm)		 granatimeta(10).jpg

Garnet in a Eclogite. PPL image, 2x (Field of view = 7mm)
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Garnet with rutile inclusion. PPL image, 10x (Field of view = 2mm)		 granatimeta(12).jpg

Garnet crystals. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)		 granatimeta(13).jpg

Garnet crystal. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Garnet crystals. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)		 granatimeta(15).jpg

Garnet crystals. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)		 granatimeta(16).jpg

Garnet crystal. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)
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Garnet crystals with muscovite tails. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)		 granatimeta(18).jpg

Garnet crystals with muscovite tails. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)		 granatimeta(21).jpg

Garnet crystal with quartz inclusions. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
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Garnet crystals. Posada Asinara shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)		 granatimeta(26).jpg

Garnet crystals. Posada Asinara shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)		 granato(salarioli)(1).jpg

Garnet crystals with internal foliation. Anzasca valley, Italy. PPL image, 1x (Field of view = 9mm)
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Garnet crystals with internal foliation. Anzasca valley, Italy. XPL image, 1x (Field of view = 9mm)		 granulite0109(1).jpg

Euhedral grarnet in a granulite. PPL image, 20x (Field of view = 1mm)		 granulite0109(26).jpg

Euhedral grarnet in a granulite. PPL image, 20x (Field of view = 1mm)