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Geosciences LibreTexts

16.8.2: Andradite

  • Page ID
    4133
  • Andradite Garnet
    Chemical composition Ca3Fe2(SiO4)3

    Calcium Iron Silicate

    Crystal system Cubic
    Habit Dodecahedra
    Cleavage None
    Hardness 6.5
    Optic nature Isotropic
    Refractive index 1.85-1.89
    Birefringence None
    Specific gravity 3.85
    Lustre Vitreous to adamantine

    Diagnostics

    Color

    Green, yellow-green and black.

    Inclusions

    • Fine curving asbestos fibres in a radiating pattern ("horse-tail") in demantoid.

    Refractometer

    With an RI of about 1.89, the readings are outside the limit of the standard gemological refractometer.

    Specific gravity

    Sinks in all common heavy liquids.

    Chelsea Color Filter

    Demantoid appears reddish or pinkish.

    Spectroscope

    Demantoid has a specific pattern due to iron and chromium.

    Dispersion

    The dispersion in andradite garnets is high (0.057) and is partially masked by body color.

    Varieties

    Demantoid

    Demantoid is the green variety of andradite.

    Melanite

    Melanite is the black, opaque variety which is mostly used in mourning jewelry.

    Topazolite

    Topazolite is the lemon yellow variety of andradite.

    Grandite

    Grandite is a brownish variety of grossular-andradite and has lower values compared to the other varieties of andradite. Its name is often used to indicate the whole grossular-andradite intermediate series. Refractive index: 1.780-1.82