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16.54: Phenakite

  • Page ID
    5090
  •  

    Phenakite
    Chemical composition Beryllium sillicate

    Be2SiO4

    Crystal system Trigonal
    Habit Tabular to prismatic
    Fracture Conchoidal (brittle)
    Cleavage Indistinct (one direction)
    Hardness 7.5 - 8
    Optic nature uniaxial +
    Refractive index 1.651 - 1.670
    Birefringence 0.016
    Dispersion Low, 0.015
    Luster Vitreous
    Specific gravity 2.92 - 2.97
    Pleochroism Seen in strongly coloured stones
    Fluorescence Sometimes pale greenish / blue or pale rose

    Figure \(\PageIndex{1}\): A faceted colorless gem-quality phenakite.
    Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur

     

    Crystallography

    Trigonal system.
    Habit: Tabular, prismatic, rhombohedral, acicular.

    Diagnostics

     

    Diaphaneity

     

    Transparent.

     

    Color

     

    Colorless; also yellow, pink, brown, pinkish red due to surface stains.

     

    Refractometer

     

    Phenakite is uniaxial with a positive optic sign.
    Refractive index range: nω = 1.651-1.654 and nε = 1.663-1.670.
    The full birefringence is 0.016.

     

    Polariscope

     

    Not many gemstones are uniaxial + (zirconquartz, phenakite and some idocrase). Finding the optic sign with the use of a retardation filter should narrow down the possibilities.
    Quartz will almost always show a typical bull's-eye pattern (or Airy spirals).

     

    Spectrum

     

    Nothing diagnostic.

     

    Fluorescence

     

    Phenakite sometimes shows pale greenish / blue or pale rose fluorescence in UV light.
    Under X-ray it fluoresces blue.

     

    Pleochroism

     

    Dichroism in observed in strongly colored stones.

     

    Magnification

     

    May have many needle inclusions; 2-phase; fingerprints with phase inclusions.

    Simulants

    Name Key separation
    Spodumene Magnification features, UV, biaxial figure, cleavage and S.G
    Sillimanite S.G., R.I., birefringence, biaxial Figure
    Rock Crystal Lower S.G. (floats 2.88), bulls eye optic figure, R.I.
    Topaz More heft (sinks 3.33), biaxial figure, R.I.
    Zircon S.G., R.I., birefringence

    Occurrence

    Geological occurrence: Commonly in pegmatites, mica schists, in granite drusses.

    Geographical locations: Tokajawa river, Ural Mountains with emerald and chrysoberyl; Miask district, Rissia; large colourless crystals at San Miguel di Praciavaba, Minas Gerais; Tanzania; Maine, New Hampshire & Colarado, USA; Switzerland; Austria; Alsace, France; Namibia; Norway and Sri Lanka (large stone).