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Geosciences LibreTexts

16.8.3: Grossular

  • Page ID
    4134
  • Grossular Garnet
    Chemical composition Ca3Al2(SiO4)3

    Calcium aluminum silicate

    Crystal system Cubic
    Habit Dodecahedra
    Cleavage None
    Hardness 7.0-7.5
    Optic nature Isotropic
    Refractive index 1.738-1.745
    Birefringence None
    Specific gravity 3.60-3.68
    Lustre Vitreous

    Diagnostics

    Color

    Green, yellowish-green, yellow-orange and pastel.

    Inclusions

    Hessonite: granular internal structure from numerous crystal inclusions, giving it an oily appearance.
    Green grossular: rod-like or needle-like crystals.
    Hydrogrossular: granular structure often with irregular black inclusions.

    Refractometer

    Hessonite and green grossular: 1.73-1.76 (singly refractive)
    Hydrogrossular garnet: 1.70-1.73 (singly refractive)

    Specific gravity

    Grossular garnets sink in all common heavy liquids. Hydrogrossular garnet may sink slowly in methylene iodide.

    Hessonite and green grossular garnet: 3.60-3.70
    Hydrogrossular garnet: 3.35-3.55

    Chelsea Colour Filter

    Green stones may appear pinkish to red.

    Varieties

    Hessonite

    Hessonite is the brownish-red to yellowish-orange variety.

    Green grossular garnet

    File:Merelani-mint-garnet-ben.jpg

    Figure \(\PageIndex{1}\): Faceted Mint Garnet
    Photo courtesy of Ben Pfeiffer

    Green grossular garnet is the bluish-green to yellowish-green variety. Within this group, Merelani Mint garnet is a grossular variety that is found in the Merelani Hills of Tanzania. It has emerged in the last few years with a bright pastel mint green color. Due to its limited availability, it has remained mainly as a collector stone.

    Hydrogrossular garnet

    Hydrogrossular garnet is the green, pink, grayish-white to bluish variety of grossular.

    Chrome grossular

    Chrome grossular is a green variety of grossular.

    Tsavorite

    Tsavorite (or tsavolite) is a grossular that, like the Meralani mint, is green in colour. It is coloured by chromium and/or vanadium. The distinction between a 'regular' green grossular and tsavorite is commonly thought to be a function of saturation and tone. However, according to the Bridges' family (tsavorite was discovered by the late Campbell Bridges), the distinction is chemical. The chromophore for green grossular is iron, as opposed to chromium and/or vanadium.

    Mali


    Mali garnet is a yellow-green variety of grossular with a slight overlap in the grossular-andradite series.