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Garnet thin section

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About Garnet
Garnet – (mineral group name), from Latin granatum (= pomegranate), the seeds of which it was thought to resemble.
Pyralspite garnets – aluminium in Y site: Almandine – locality at Alabanda, in Asia Minor, where garnets were cut and polished in ancient times. Pyrope – from Greek fiery, in allusion to its fire-red color. Sperssartine – locality at Spessart in northwestern Bavaria.
Ugrandite group – calcium in X site: Andradite – for J. B. d’Andrada e Silva (1763-1838), Brazilian mineralogist, who examined and described a variety of this mineral. Grossular – from Latin grossularium (= gooseberry), in allusion to the pale green color of some of the specimens. Uvarovite – for Count Sergei Semeonovich Uvarov (1786-1855), Russian nobleman, Imperial Academy of St. Petersburg.
Garnet hand-specimen
Formula: X3Y2(SiO4)3
X = Mg, Ca, Fe2+, Mn2+, etc.
Y = Al, Fe3+, Cr3+, V3+, etc.

Almandine – Fe2+3Al2(SiO4)3
Pyrope – Mg3Al2(SiO4)3
Spessartine – Mn2+3Al2(SiO4)3
Andradite – Ca3Fe3+2(SiO4)3
Grossular – Ca3Al2(SiO4)3
Uvarovite – Ca3Cr2(SiO4)3

System: Cubic (Isometric)
Color: Deep red, brownish red, etc.
Lustre: Adamantine, Resinous
Hardness: 6½–7½
Density: 3.582–4.32

XPL
XPL
XPL
PPL
PPL
Garnet #1 thin section (hFOV 2mm)
XPL
XPL
XPL
PPL
PPL
Garnet #2 thin section (hFOV 2mm)
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Garnet PPL properties
Relief: High positive
Habit/Form: Garnets commonly occur as euhedral to subhedral dodecahedral {110} or trapezohedral {112} crystalss, which in thin section yield six- or eight-sided cross sections. Garnet also occurs as granular or irregular masses.
Color: Garnet group: nearly colorless to a pale-brown/pink/green.

Almandine: nearly colorless, pink, light reddish-brown.
Pyrope: colorless, pink.
Spessartine: pale orange, pale pink, pale brown.

Andradite: light green, light brown.
Grossular: colorless, very pale green.
Crystals are often zoned.
Pleochroism:
Cleavage: Absent; irregular fractures are characteristic.

Garnet XPL properties
Isotropy/Anisotropy: Isotropic, some varieties may show weak birefringence. Spessartine may be distinctly anisotropic; grossular not uncommonly is birefringent in low-order grays and is commonly twinned in ‘pie-cut’ sectors.
Interference color: The garnets are isometric, and pyrope and almandine are truly isotropic. Spessartine, grossular and andradite may show weak anisotropism.
Extinction angle:
Twins: Absent
Uniaxial/Biaxial: Isotropic (anomalous Biaxial)
Optic axial angle (2V):
Garnet distinguishing features under the microscope
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References
  • Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
  • mindat.org – The Mineral Database