The Mogok Valley in the Shan State of upper Myanmar (Burma) is a region rich in history, tradition, and gemstones. It is commonly referred to as the premier source for ruby, sapphire, spinel, peridot, and jadeite. Similar to the Mont Saint- Hilaire region, the complex geochemistry of the Mogok stone tract is responsible for the growth of rare and unusual minerals. Contact and regional metamorphism in the Mogok area is well known to have provided for the geological processes required to enable the formation and growth of a wide variety of gem materials, including amethyst, andalusite, danburite, garnet, goshenite, scapolite, topaz, tourmaline, and zircon. The area has also produced a number of very rare gems, such as sinhalite, colorless chrysoberyl, taaffeite, painite, serendibite, poudretteite, and alexandrite.
In November of 2000, an unfamiliar gemstone was purchased by an Italian dealer buying in Mogok. This gemstone was submitted to the Gubelin Gem Lab for examination and identification and proved to be the first documented gem-quality specimen of the rare borosilicate, poudretteite. The results were also provided to R. Schlussel, who later included poudretteite in his book about Mogok. This extremely rare sample permitted the first comprehensive gemological description of this material with a variety of techniques that had not been previously available. A detailed gemological report of this unique specimen is included in spring 2003 issue of Gems and Gemology. The article presents a complete gemological description of this very rare gemstone and furthers the characterization of the mineral with advanced spectroscopic and chemical analytical techniques.
Previous to this, the Mont St. Hilaire crystals were the only specimens of poudretteite. known to exist. Those crystals occurred in marble xenoliths within a nepheline syenite breccia, associated with pectolite, apophyllite, and aegirine. They were described as colorless to very pale pink, roughly equidimensional, deeply etched, barrel-shaped hexagonal prisms measuring up to 5mm in the longest dimension. Chemical analysis indicated the formula KNa2B3Si12O30.
Poudretteite was referred to as a new member of the osumilite group. Osumilite itself was first described in 1956. Because milarite is the prototype mineral for the large structural group that includes osumilite, for historical reasons some mineralogists came to prefer the term milarite. Both group names are currently used in the mineralogical terminology, and Gems and Gemology describes poudretteite as belonging to the osumilite/milarite group. There are 17 minerals in the osumilite/milarite group. Of these, only sugilite is familiar to most gemologists. Sogdianite, another mineral in this group, has also been encountered, though rarely, in gem quality.
The RI of poudretteite ranges between 1.511 to 1.532 with a birefringence of .016 to .021. It is uniaxial positive with a reported S.G. ranging from 2.511 to 2.527. The pleochroism is strong; saturated purple pink parallel to the c axis and near colorless to pale brown perpendicular to the C axis. It is inert to UV fluorescence. No distinctive lines are apparent in the visible absorption spectrum although a faint broad band is observable centered around 530nm.
Manganese is the primary color-causing element in poudretteite and this is evidenced by the considerable variation in Mn concentration measured between the near-colorless and highly saturated areas of poudretteite. Substitution of cations by divalent and trivalent transition metal ions is the most frequent cause of color in silicates. Manganese is a common cause of pink to red and purple coloration in a variety of other gem-quality silicate materials such as red beryl and tourmaline, as well as the carbonate rhodochrosite. However, other trace elements or possibly color centers may also have an influence in producing the intense purple coloration in poudretteite.
It is likely that few gemologists will ever encounter a faceted poudretteite. The combination of color, refractive indices and specific gravity will readily separate it from amethyst, which is probably the only major commercially available gem material that it might be confused with. Poudretteite could conceivably also be confused with sugilite. Although the color of these two materials is somewhat similar, poudretteite is a transparent mineral, while sugilite is, even in its best quality, translucent. In addition, the refractive index (approximately 1.60) and the specific gravity (2-74 to 2.78) of sugilite are significantly higher than that of poudretteite.
For detailed gemological, spectrographic, and chemical analysis, please see Gems and Gemology, spring 2003.
