| General Information |
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| Varieties/Types: | |
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| Chemical Formula | Michael O’Donoghue, Gems, Sixth Edition (2006) |
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| Physical Properties of Sillimanite |
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| Mohs Hardness | 6 to 7.5Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Specific Gravity | 3.20 to 3.26Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Cleavage Quality | PerfectUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references |
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| Optical Properties of Sillimanite |
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| Refractive Index | 1.653 to 1.685Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Optical Character | Biaxial/+Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Birefringence | 0.014 to 0.021Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Pleochroism | Strong trichroism is common for green stones: yellowish green - dark green - blueHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Chatoyancy | Common grayish green to dark brown cat's eye due to grayish hypersthene's fibers (or rutile needles)Herve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) |
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| Colour |
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| Colour (General) | Colourless, blue-green, blue, green, gray-green, brownishUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references |
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| Causes of Colour | Blue, Fe2+-O-Ti4+, charge transfer, probably similar to blue kyanite. Yellow, Fe3+ or Cr3+ in tetrahedral coordination. Brown, Fe features of yellow sillimanite plus inclusions of iron rich phaseW. William Hanneman, Pragmatic Spectroscopy For Gemologists (2011) |
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| Transparency | Transparent,Translucent,OpaqueUlrich Henn and Claudio C. Milisenda, Gemmological Tables (2004) More from other references |
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| Fluorescence & other light emissions |
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| Fluorescence (General) | May fluoresce reddishHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Crystallography of Sillimanite |
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| Crystal System | OrthorhombicHerve Nicolas Lazzarelli, Blue Chart Gem Identification (2010) More from other references |
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| Habit | Prismatic, some material is fibrousMichael O’Donoghue, Gems, Sixth Edition (2006) |
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| Geological Environment |
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| Where found: | Sillimanite is found in high-grade metamorphic schists and gneisses, sometimes in pegmatites. Probably most gem sillimanite is found in gem gravels.Michael O’Donoghue, Gems, Sixth Edition (2006) |
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| Inclusions in Sillimanite |
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Hypersthene's fibers (or rutile needles)- Blue Chart Gem Identification, Herve Nicolas Lazzarelli, 2010, p 4
Oriented needle-like crystals - Gemmological Tables, Ulrich Henn and Claudio C. Milisenda, 2004, p 18 |
| Further Information |
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| Mineral information: | Sillimanite information at mindat.org |
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| Significant Gem Localities |
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| Myanmar | | |
| [var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok | |
| Ted Themelis (2008) Gems & mines of Mogok |
- Pein-Pyit (Painpyit; Pyan Pyit)
| [var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok | |
| [var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok | |
| [var: Fibrolite] Ted Themelis (2008) Gems & mines of Mogok | | Sri Lanka | | |
| No reference listed | |
| Gems, Sixth Edition, Michael O’Donoghue, 2006, p. 451 |
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