Pyrope

Composition

Pure pyrope has the composition Mg3Al2(SiO4)3, but calcium, chromium, iron, and manganese are typically present in minor amounts. Pyrope forms a solid solution series with almandine and spessartine, known as the pyralspite garnets. Iron and manganese substitute for magnesium, and mixed-composition garnets are defined by their pyrope-almandine ratio. Rhodolite is a garnet of about 70% pyrope composition.

Distribution

Most pyrope originates in ultramafic rocks, especially peridotite from the Earth's mantle, which can result from igneous or metamorphic processes. It also occurs in ultrahigh-pressure metamorphic rocks, such as in the Dora-Maira massif in the western Alps, where nearly pure pyrope crystals reach almost 12 cm in diameter and may contain inclusions of coesite, enstatite, or sapphirine. Pyrope is common in peridotite xenoliths from kimberlite pipes, some diamond-bearing; chrome-pyrope with 3–8% Cr2O3 is used as a kimberlite indicator mineral.

Mineral identification

In hand specimens, pyrope is difficult to distinguish from almandine but tends to have fewer flaws and inclusions. Many distinguishing criteria come from synthetic pure-composition pyrope, and properties such as high specific gravity may be impractical for small crystals in a silicate matrix.

Association with mafic and ultramafic minerals is often the best clue. In thin section, pyrope shares high relief and isotropy with other garnets, may show a pale pinkish purple hue in plane-polarized light, and lacks cleavage. Euhedral crystal morphology and mineral associations aid identification.