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Geosciences LibreTexts

16.08.04: Pyrope

  • Page ID
    4135
  •  

    Pyrope Garnet
    Chemical composition Mg3Al2(SiO4)3

    Magnesium aluminum silicate

    Crystal system Cubic
    Habit Dodecahedra
    Cleavage None
    Hardness 7.0-7.5
    Dispersion Medium, 0.022
    Optic nature Isotropic
    Refractive index 1.730-1.760
    Birefringence None
    Specific gravity 3.65-3.80
    Lustre Vitreous

    Pyrope is a red, orange-red, brownish-red to pink species of the garnet group which forms isomorphous series with almandinespessartite and grossular. Pyrope, like all other species of the garnet group, is seldom seen having its ideal chemical makeup.

    Diagnostics

    Spectroscope

    Pyrope has a chromium spectrum that can be combined with the typical spectrum of almandine.

    Inclusions

    • Rounded crystals of low relief
    • Rutile needles

    Polariscope

    Pyrope (in all its isomorphous series) may show anomalous birefringence.

    Refractometer

    The refraction index of pyrope is 1.714 but, because it is never found pure, its index has a lower value of 1.730 and an upper value of 1.760.

    Pyrope is singly refractive (isotropic).

    Specific gravity

    Pure pyrope has a specific gravity of 3.58. Due to isomorphous replacement, this value is never measured. Instead, its value ranges from 3.65 to 3.80.

    Pyrope sinks in all common heavy liquids.

    Varieties

    Rhodolite

    Rhodolite is the purple variety of pyrope-almandine. The name and color come from the flower of the rhodondendron.
    The jewelry trade sometimes describes every purple garnet as a rhodolite but it should be reserved only for those having an intermediate chemical composition between pyrope and almandine.

    Malaia

    Malaia garnet (or malaya) is the red-orange variety of pyrope-spessartite discovered in the 1970's in Kenia as a by-product of rhodolite.
    The name is derived from the Swahili word for prostitute (which freely translates to "outcast" or "out of the family") [see: Swahili - English Dictionary; Rouse, 1986].

    Localities

    • South-Africa
    • USA
    • Russia

    References