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16.71: Willemite

  • Page ID
    6540
  • Willemite

    Willemite
    Chemical composition Zn2SiO4
    Crystal system Trigonal
    Habit prismatic; massive
    Cleavage indistinct
    Fracture Conchoidal to irregular
    Hardness 5.5
    Optic nature Uniaxial +
    Refractive index 1.691 - 1.725
    Birefringence 0.028
    Dispersion n d
    Specific gravity 3.89 - 4.19
    Lustre Vitreous to resinous
    Fluorescence green to yellow

    Willemite was named after King Willem (William) the first (1772-1843) of The Netherlands, by the French mineralogist Armand Lévy who discovered it in 1830 on former Dutch soil (present-day Belgium).

     

    Crystal Habit

    Crystals are long to short prismatic, to 10cm in length. Also fine to coarse granular massive.

     

    Chemical Composition

    Zinc silicate, Zn2Si04. Minor to trace amounts of iron, aluminum, manganese, calcium, and magnesium may be present.

     

    Diagnostics

    Yellow to yellow-green shades of willemite can be confused with golden beryl (which possesses a lower refractive index at 1.570 - 1.575, is harder at 7.5 to 8, and has a lower specific gravity of 2.72), and chrysoberyl (which is harder at 8.5, has a higher specific gravity 3.73, and a higher refractive index at 1.746 – 1.755).

     

    Color

    Transparent rough is colorless to golden yellow, pale blue-green. A canary-yellow variety of willemite from Tsumeb, Namibia has been documented, with the color attributed to cadmium (although whether the cadmium is present at a substitute for an essential component, or as finely divided greenockite – cadmium sulfide – is uncertain). Massive material cut en cabochon may be white, apple green, amethystine, yellow to golden brown, grayish, brown. An opaque flesh-red variety is called “troostite” which consists of exsolution laminae of willemite in tephroite, a manganese silicate.

     

    Magnification

    may exhibit black inclusions of franklinite, an iron manganese zinc oxide.

     

    Specific Gravity

    3.89 – 4.19, sinks in all usual heavy liquids.

     

    Refractive Index

    nω = 1.691 – 1.694 , nε = 1.719 – 1.725

     

    Treatments

    There are reports that some rough has been heat-treated to convert it from colorless to blue.

     

    Phenomenon

    The willemite from Franklin and Ogdensburg, Sussex County, New Jersey commonly fluoresces a brilliant green under Short Wave ultraviolet. It also commonly is phosphorescent, with some specimens continuing to luminesce for hours after excitation has terminated; triboluminscence (luminescence when the specimen is abraded or broken) has been observed. Willemite with a butter-yellow fluorescence has been observed but is uncommon at both Franklin and Ogdensburg. Fluorescent response in Namibian willemite appears to be more erratic in its presence or absence, and tends to be yellow under Short Wave ultraviolet radiation and yellow to orange under Long Wave. Fluorescence must not be considered diagnostic; non-fluorescent willemite has been observed. The “troostite” variety may appear as homogenous willemite in white light, but can exhibit thin lines or streaks of fluorescent green in a non-fluorescent groundmass.

     

    Synthetics

    None known.

     

    Imitations

    None known.

     

    Occurrence

    Willemite occurs as a primary and secondary mineral in metamorphosed zinc ore deposits. It is uncommon, except at the zinc mines of Franklin and Ogdensburg in Sussex County, New Jersey, where it is one of the three primary ore minerals. Enormous tonnages were mined between circa 1830 and circa 1980. In the late 19th century, one crystal recovered yielded enough rough to cut several stones; the largest faceted stone was 8 carats and became part of the Frederick A. Canfield collection. Of willemite as a gemstone, Dunn (1995) writes: “Willemite has been cut as an uncommon, exotic gem. Fine-quality gemstones have been cut from orange-yellow willemite crystals: gems of 11.1 and 11.7 carats are in the Smithsonian Institution; one of 36.93 carats is in the Harvard Mineralogical Museum; one of 6.28 carats is in the American Museum of Natural History in New York. Massive material may be quite clean internally and translucent and has provided some gemmy material for the cutting of cabochons.”Facet-grade willemite has also been recovered from the Tsumeb (Tsumcorp) Mine in Tsumeb, Namibia.

     

    Care and Maintenance

    Willemite can be decomposed by mineral acids into a colorless silica gel; care should be exercised in the use of chemical cleaning agents.

     

    Sources

    • Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C. (1990): Handbook of Mineralogy: Mineral Data Publishing, Tucson, Arizona
    • Palache, C. (1935), Minerals of Franklin and Sterling Hill, New Jersey, USG Professional Paper 180
    • Palache, Charles, Harry Berman & Clifford Frondel (1944): The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Seventh edition