Pyrope

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Pyrope
A gem-quality, emerald cut pyrope crystal.
General
Category	Mineral: magnesium-aluminium silicate
Chemical formula	Mg3Al2(SiO4)3
Identification
Color	Red. Some varieties are very dark, almost black, while others can take tones of purple. Some chromium-rich pyropes are thermochromic, becoming green when heated[1]
Crystal habit	Euhedra typically display rhombic dodecahedral form, but trapezohedra are not uncommon, and hexoctahedra are seen in some rare samples. Massive and granular forms also occur.
Crystal system	Cubic
Cleavage	None
Fracture	Conchoidal
Mohs Scale hardness	7 - 7.5
Luster	greasy to vitreous [2]
Polish luster	vitreous[2]
Refractive index	1.74 normal, but ranges from 1.714 to over 1.742 [2]
Optical Properties	Single refractive, often anomalous double refractive [2]
Birefringence	Isotropic, appears black in cross-polarized light
Pleochroism	none
Ultraviolet fluorescence	inert
Absorption spectra	broad band at 564nm with cutoff at 440 to 445nm. Fine gem quality pyropes may show chromium lines in the red end of the spectrum
Streak	White
Specific gravity	3.78 (+.09 -.16) [2]
Solubility	Insoluble in water, weakly soluble in HF
Mineral association	Olivine, pyroxene, hornblende, biotite, diamond

This article is about the mineral. For the bird sometimes placed in the monotypic genus Pyrope, see Fire-eyed Diucon.

The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek for fire and eye. Despite being less common than most garnets, it is a widely used gemstone with numerous alternate names, some of which are misnomers. Chrome pyrope, and Bohemian garnet are two alternate names, the usage of the later being discouraged by the Gemological Institute of America.^[2] Misnomers include Colorado ruby, Arizona ruby, California ruby, Rocky Mountain ruby, Elie Ruby, Bohemian carbuncle, and Cape ruby.

The composition of pure pyrope is Mg₃Al₂(SiO₄)₃, although typically other elements are present in at least minor proportions -- these other elements include Ca, Cr, Fe and Mn. Pyrope forms a solid solution series with almandine and spessartine, which are collectively known as the pyralspite garnets (pyrope, almandine, spessartine). Iron and manganese substitute for the magnesium in the pyrope structure. The resultant, mixed composition garnets are defined according to their pyrope-almandine ratio. The semi-precious stone rhodolite is a garnet of ~70% pyrope composition.

The origin of most pyrope is in ultramafic rocks, typically peridotite from the Earth's mantle: these mantle-derived peridotites can be attributed both to igneous and metamorphic processes. Pyrope also occurs in ultrahigh-pressure (UHP) metamorphic rocks, as in the Dora-Maira massif in the western Alps. In that massif, nearly pure pyrope occurs in crystals to almost 12 cm in diameter; some of that pyrope has inclusions of coesite, and some has inclusions of enstatite and sapphirine.

Pyrope is common in peridotite xenoliths from kimberlite pipes, some of which are diamond-bearing. Pyrope found in association with diamond commonly has a Cr₂O₃ content of 3-8%, which imparts a distinctive violet to deep purple colouration (often with a greenish tinge) and because of this is often used as a kimberlite indicator mineral in areas where erosive activity makes pin pointing the origin of the pipe difficult. These varieties are known as chrome-pyrope, or G9/G10 garnets.

[edit] Mineral identification

In hand specimen, pyrope is very tricky to distinguish from almandine, however it is likely to display fewer flaws and inclusions. Other distinguishing criteria are listed in the table to the right. Care should be taken when using these properties as many of those listed have been determined from synthetically grown, pure-composition pyrope. Others, such as pyrope's high specific gravity, may be of little use when studying a small crystal embedded in a matrix of other silicate minerals. In these cases, mineral association with other mafic and ultramafic minerals may be the best indication that the garnet you are studying is pyrope.

In petrographic thin section, the most distinguishing features of pyrope are those shared with the other common garnets: high relief and isotropy. Garnets tend to be less strongly coloured than other silicate minerals in thin section, although pyrope may show a pale pinkish-purple hue in plane-polarized light. The lack of cleavage, commonly euhedral crystal morphology, and mineral associations should also be used in identification of pyrope under the microscope.

[edit] References

1. ^ Thermochromic Cr-rich Pyrope Garnets
2. ^ ^{a b c d e f} (Gia), Gemological. Gem Reference Guide. City: Gemological Institute of America (GIA), 1988. ISBN 0-87311-019-6