16.55: Pouretteite
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|---|---|
| Chemical composition | KNa2B3Si12O30 Borosilicate |
| Crystal system | Hexagonal |
| Habit | Hexagonal and barrel-shaped etched prisms |
| Cleavage | None |
| Fracture | Conchoidal, splintery |
| Hardness | 5-6 |
| Optic nature | Uniaxial + |
| Refractive index | 1.510 - 1.532 |
| Birefringence | 0.016 - 0.021 |
| Dispersion | Unknown |
| Specific gravity | 2.50 - 2.55 |
| Lustre | Vitreous |
| Pleochroism | Colorless to pink, depending on body color |
Figure \(\PageIndex{1}\): Podreitteite
Photo courtesy of Scott Davies, americanthai.com
A very rare gem, first found in Mont St. Hilaire, Quebec, Canada. Named after the Poudrette family who were the operators of the quarry where it was first discovered.
Color
Colorless to pink, purple.
Transparency
Transparent.
Crystal habit
As stubby, barrel-shaped prismatic or bipyramidal crystals.
Chemical composition
KNa2B3Si12O30 Potassium sodium boron silicate.
Diagnostics
Poudretteite could be mistaken for feldspar and scapolite.
Feldspar, however, has a biaxial optical nature and scapolite is uniaxial with a negative optic sign.
Specific gravity
2.51(Measured), 2.53 (Calculated)
Scapolite has a SG range of 2.634 to 2.74 and feldspar has a range of 1.52 to 1.57.
Tenacity
Brittle.
Refractive Index
nω = 1.516, nε = 1.532
Poudretteite has a uniaxial optical nature with a positive optical sign.
- Scapolite (n = 1.545 - 1.580) also has a uniaxial character, but has a negative optical sign. Although the readings are somewhat high to indicate poudretteite, the birefringence is similar.
- Feldspars (n = 1.52 - 1.57) are biaxial with a positive or negative optical sign.
Birefringence
The birefringence varies from 0.016 to 0.021 and produces a noticeable doubling of facets.
Pleochroism
Colorless to pink.
Spectroscope
No known diagnostic spectrum in the visible range.
Luminescence
None observed.
Magnification
Inclusions may include veils of two-phase (liquid + gas) inclusions and localized parallel flat tubes.
Treatments
None known.
Synthetics
None known.
Imitations
None known.
Care
Poudretteite is a very rare gem and no special vulnerabilities are reported, besides its brittleness.
Phenomenon
None known.
Occurrence
Poudretteite was originally described in 1987 as minute crystals of no gemological interest at Mont St. Hilaire; the samples had been collected in the 1960’s. In 2000, rough was discovered in the Pain Pyit district, Mogok, Myanmar that cut a 3ct stone that was later determined to be poudretteite. As of 2004, 10 examples of poudretteite had been identified amoung Mogok stones, the largest being a 22ct crystal that was cut into a 9.4ct gem.
Facet grade poudretteite is also found in Mogok, Burma. As of mid-2007, about 30 small pieces of rough have been recovered. Most produce small gems, and clean stones weighing in excess of 1 carat remain elusive. Some gems are pink in tone, but these also are not common.
Sources
- AIGS Laboratory Report on 9.41 ct faceted Poudretteite
- Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C. (1990): Handbook of Mineralogy: Mineral Data Publishing, Tucson, Arizona
- Grice, J D, Ercit, T. S., Van Velthuizen, J. and Dunn P. J. (1987) The Canadian Mineralogist Vol. 25 (1987) pp 763-766 Poudretteite, KNa2B3Si12O30, a new member of the osumilite group from Mont Saint-Hilaire, Quebec, and its crystal structure
- Smith, Christopher P., Bosshart, G., Graeser, S., Hanni, H. and Gunther, D. (2003) Gems and Gemology Vol 39 No. 1 Poudretteite: A Rare Gem Species from the Mogok Valley