Pyrope-Almandine Garnet

Category  :  Mineral: magnesium-aluminium silicate
Hardness  :  7 - 7.5
Gravity  :  3.78 (+.09 -.16)
Reflection  :  1.73 - 1.83
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.
Chemical formula  :  Mg3Al2(SiO4)3
Crystal system  :  Cubic
Major varieties  :  Pyrope, Rhodolite, Almandine

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.

Rhodolite

This is an intermediate group of garnets in the pyrope-almandine series. It is a deep pink or pinkish-red. The gems have a good transparency and are almost always faceted, generally receiving a mixed, roughly oval or round cut. As always with transparent garnets, the luster is strong. The refractive index varies from 1.755 to 1.765. Rhodolite is found in the United States, Zimbabwe, Tanzania and Sri Lanka.

Almandine

The name almandine comes from carbunculus alabandicus, after the city of Alabanda in Asia Minor. The color is red, but often a deep, violet-red. It has brilliant luster, but its transparency is frequently marred, even in very clear stones, by excessive depth of color. The cabochon cut is widely used, often being given a strongly convex shape and sometimes a concave base, in an effort to lighten the color by reducing the thickness.


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