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16.8.1: Almandine

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    4131
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    Almandine Garnet
    Chemical composition Fe3Al2(SiO4)3ron aluminum silicate
    Crystal system Cubic
    Habit Dodecahedra
    Cleavage None
    Hardness 7.5
    Optic nature Isotropic
    Refractive index 1.78-1.81
    Birefringence None
    Specific gravity 3.95 - 4.20
    Lustre Vitreous

    Diagnostics

    File:Almandine.jpg

    Figure \(\PageIndex{1}\): Oval, faceted Almandine garnet

    Color

    Brownish red, red to violet/purple

    Spectroscope

    Almandine has a typical iron spectrum.

    File:Almandine spectrum.jpg

    Figure \(\PageIndex{2}\): usually at 504, 520, and 573nm, may also have faint lines at 423, 460, 610 and 680-690nm

    Inclusions

    • Short rutile needles in 3 directions
    • Crystals surrounded by stress cracks
    • Irregular and rounded crystals

    Polariscope

    Almandine is isotropic but may have a small DR.

    Specific gravity

    Almandine sinks in all common heavy liquids.

    Optical effects

    Almandine may show asterism and chatoyancy.

    Varieties

    Rhodolite

    Rhodolite is the purple (pinkish/violet) variety of pyrope-almandine.

    Localities

    India, USA, Brazil

    Occurrence

    Metamorphic rocks


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