| Pyrope | |
|---|---|
 | |
| General | |
| Category | Nesosilicate |
| Formula | Mg3Al2(SiO4)3 |
| IMA symbol | Prp[1] |
| Strunz classification | 9.AD.25 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H–M symbol: (4/m3 2/m) |
| Space group | Ia3d |
| Identification | |
| Color | Blood red to black red, red, orange red, pink, some varieties are very dark, almost black, while others can take tones of purple to purple red, Somechromium-rich pyropes arethermochromic, becoming green when heated.[2] |
| Crystal habit | Euhedra typically displayrhombic dodecahedral form, buttrapezohedra are not uncommon, and hexoctahedra are seen in some rare samples. Massive and granular forms also occur. |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 7.0–7.5 |
| Luster | greasy to vitreous[3] |
| Streak | White |
| Specific gravity | 3.78+0.09 −0.16[3] |
| Polish luster | vitreous[3] |
| Optical properties | Single refractive, often anomalous double refractive[3] |
| Refractive index | 1.74 normal, but ranges from 1.714 to over 1.742[3] |
| Birefringence | Isotropic, appears black in cross-polarized light |
| Pleochroism | none |
| Ultravioletfluorescence | inert |
| Absorption spectra | broad band at 564 nm with cutoff at 440 to 445 nm. Fine gem quality pyropes may showchromium lines in the red end of the spectrum |
| Solubility | Insoluble in water, weakly soluble inHF |
| Mineral association | Olivine,pyroxene,hornblende,biotite,diamond |
| References | [4] |
The mineralpyrope is a member of thegarnet 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 words forfire andeye. Despite being less common than most garnets, it is a widely usedgemstone with numerous alternative names, some of which aremisnomers.Chrome pyrope, andBohemian garnet are two alternative names, the usage of the latter being discouraged by theGemological Institute of America.[3] Misnomers includeColorado ruby,Arizona ruby,California ruby,Rocky Mountain ruby,Elie Ruby,Bohemian carbuncle, andCape ruby.
The composition of pure pyrope is Mg3Al2(SiO4)3, although typically other elements are present in at least minor proportions—these other elements includeCa,Cr,Fe andMn. Pyrope forms asolid solution series withalmandine andspessartine, which are collectively known as thepyralspite 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 stonerhodolite is a garnet of ~70% pyrope composition.
The origin of most pyrope is inultramafic rocks, typicallyperidotite from theEarth's mantle: these mantle-derived peridotites can be attributed both toigneous andmetamorphic 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 (5 in) in diameter; some of that pyrope has inclusions ofcoesite, and some has inclusions ofenstatite andsapphirine.
Pyrope is common inperidotite xenoliths fromkimberlite pipes, some of which arediamond-bearing. Pyrope found in association with diamond commonly has a Cr2O3 content of 3–8%, which imparts a distinctive violet to deep purple coloration (often with a greenish tinge) and because of this is often used as a kimberlite indicator mineral in areas where erosive activity makes pinpointing the origin of the pipe difficult. These varieties are known aschrome-pyrope, orG9/G10 garnets.
In hand specimens, 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 adjacent table. 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 othermafic andultramafic minerals may be the best indication that the garnet you are studying is pyrope.
Inpetrographicthin 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.
Gemmological classifications by E. Ya. Kievlenko (1980), updated | |||||||||
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