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16.55: Pouretteite

  • Page ID
    6280
  •  

     

    Poudretteite
    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

    File:Poudretteite.jpeg

    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

    • 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