GARNET

General Formula:
X3Y2(SiO4)3 where
  • X = Ca, Mg, Fe2+, Mn
  • Y = Al, Fe3+, Cr
Sample: PT-104 System:
Isometric
Garnet Porphyroblasts
Three anhedral garnet porphyroblasts associated with biotite, staurolite, and muscovite from a Meguma Group pelite.
Field of view = 2.7 mm, plane light
Garnet Porphyroblasts
The isotropic nature of the garnet is obvious as are the inclusions of quartz, the trails of which are approximately parallel to the fabric (running from top right to lower left).
Field of view = 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
commonly colourless or a pale version of the hand sample
non pleochroic
Form euhedral to subhedral dodecahedral or trapezohedral grains which display 6 to 8 sides and as anhedral granular masses
Relief
RI
high positive
n = 1.675-1.890, highly variable, depending on composition
Cleavage none, show irregular fractures
Birefringence
Interference Colours
NA
NA
Twinning not shown
Interference Figure
Optic Sign
2V
isotropic - no interference figure Optic Orientation NA
Composition highly variable Alteration alters to chlorite, commonly involved in a variety of mineral reactions in a variety of metamorphic rocks
Occurrence found in metamorphic rocks of a wide range of compositions, in some Al-rich granitic rocks and as a detritial mineral in sediments Distinguishing Features isotropic character, high relief, and grain shape


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