16.54: Phenakite
- Page ID
- 5090
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|---|---|
| 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 + (zircon, quartz, 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).