Smith College - Geology 222b - Petrology

Petrographic Data File

Garnet
Property
Value
Comments
Formula

Pyrope: Mg3Al2(SiO4)3
Almandine: Fe3Al2(SiO4)3
Spessartine: Mn3Al2(SiO4)3
Grossular: Ca3Al2(SiO4)3
Andradite: Ca3Fe2(SiO4)3
Uvarovite: Ca3Cr2(SiO4)3

Compositions can be expressed by the general formula A3B2(SiO4)

A may be (Mg, Mn, Ca, Fe2+)
B may be (Al, Cr, Fe3+)
Crystal System Isometric  
Crystal Habit Rounded grains, massive granular (coarse or fine)  
Hardness and Streak

H = 6.5-7.5

Streak: White

  
Cleavage No distinct cleavage Irregular or conchoidal fracture
Color/Pleochroism

Pyrope: deep red to nearly black
Almandine: deep red to brownish red
Spessartine: brownish to red
Grossularite: white, green, yellow, brown, pale red
Andradite: yellow, green, brown, black
Uvarovite: emerald-green
Hydrogrossular: green, colorless, pink, brown

Luster Vitreous to greasy or silky Different types of garnet may be translucent or slightly opaque, leading to variable lusters.
Specific Gravity G = 3.1-4.2 Density varies depending on the type of garnet, with Ca and Mg rich garnets having lower density than the Fe and Mn garnets
Refractive Index
 		 n = 1.714-1.887 Refractive index varies depending on type of garnet
Distinguishing Features High relief in thin section. Color in thin section is a pale version of the hand sample color, and is often more lightly colored than spinel.
Occurrence

Pyrope: ultramafic igneous rocks
Almandine: typical garnet in mica schist and gneiss
Spessartine: felsic igneous rocks
Grossularite and Andradite: both are rare and sometimes occur with contact metamorphism.

References Nesse, W.D. (2012) “Garnet” in Introduction to Mineralogy 2nd ed. Oxford University Press, New York. p. 341-344.
Editors Sarah Vorhies ('06), Kelsey Moore ('13), Emma Roth ('17)

 
Photomicrograph of a mostly almandine garnet in plane polarized light. Click on the thumbnail to see a larger ppl/xpl rollover image.
Photomicrograph of garnet from a California metabasalt in plane polarized light. To see larger plane polarized and cross polarized rollover image, click on thumbnail. (Image uses 1.5 objective and field of view of 7750 microns by 5750 microns)
A typical garnet crystal showing high relief, a light pink color, a hexagonal shape. Inclusions are present. Click on the thumbnail to see a larger ppl/xpl rollover image.
The garnet crystal is not a distinctive polygonal shape, however it is still easily identified by the high relief, light pink color, and extinction. Inclusions are present. Click on the thumbnail to see a larger ppl/xpl rollover image.

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