Royal Ontario Museum Polarised-Light Microscopy Laboratory

Garnet Group

Typically (Mg,Fe,Mn)3Al 2Si3O12 or Ca3(Al, Cr, Fe/Ti)2Si3O12

Crystal system: Cubic
Refractive index: 1.714 to 1.887
Birefringence: none
Cleavage: none
Twinning: complex and sector twinning, zoning
Colour in PPL: colourless, pink, yellow, or brown
Pleochroism: none
Relief: very high
Distinction: high relief and isotropic or very weak birefringence distinctive
Occurrence: found in some ultramafic rocks, typical in high-grade regional metamorphic rocks, Ca-rich varieties in metamorphosed limestones

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