Pyrope
Encyclopedia : P : PY : PYR : Pyrope
| Pyrope | |
|---|---|
| Category | Mineral: magnesium-aluminium silicate |
| Chemical formula | Mg3Al2(SiO4)3 |
| Physical and Optical Properties | |
| Colour | 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[link]. |
| 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. |
| Lustre | Vitreous |
| Crystal system | Cubic |
| Cleavage | None |
| Fracture | Conchoidal |
| Hardness | 7 - 7.5 |
| Streak | White |
| Specific gravity | 3.582 |
| Solubility | Insoluble in water, weakly soluble in HF |
| Birefringence | Isotropic, appears black in cross-polarized light. |
| Refractive index | 1.73 |
| Mineral association | Olivine, pyroxene, hornblende, biotite, diamond |
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 known variously as Bohemian Garnet, Cape Ruby, or other locality-specific names.
The origin of most pyrope is igneous, rather than metamorphic, and is most frequently found in rocks of ultramafic origin. Their standard chemical composition is Mg3Al2(SiO4)3, although this can vary with increasing Cr, Fe and Mn content. In fact, pure pyrope garnets are almost unknown in nature, as 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.
Pyrope is common in diamond-bearing kimberlite pipes. 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.
Mineral identification
In hand specimen, pyrope is very tricky to distinguish from almandine, however it is likely to display fewer flaws and inclusions. Other distingishing 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.
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