SILLIMANITE: Fibrolite, Kenya (0.34), Burma (2.44). Photo © Joel E. Arem, PhD, FGA. Used with permission.
The fibrolite from Burma and Sri Lanka is well known to gem collectors, and highly prized because of its great scarcity. Blue and greenish gems are lovely, although very difficult to cut. Chatoyant material sometimes yields catseye fibrolites, which are also very rare. The material from Kenya is just as attractive as Burmese fibrolite but seems to be somewhat smaller in size.
| Data | Value |
|---|---|
| Name | Sillimanite |
| Varieties | Fibrolite |
| Crystallography | Orthorhombic. Crystals prismatic, rare; usually fibrous masses. |
| Refractive Index | 1.654-1.683 |
| Colors | Colorless, white, gray, yellowish, brownish, greenish, bluish, violet-blue. |
| Luster | Vitreous to silky. |
| Hardness | 6.5-7.5 |
| Fracture | Uneven |
| Specific Gravity | 3.23-3.27; compact varieties 3.14-3.18. |
| Birefringence | 0.020 |
| Cleavage | Perfect 1 direction |
| Dispersion | 0.015 |
| Stone Sizes | Faceted gems are usually small (under 5 carats) and quite rare. Catseye gems are generally in the same size range, up to 10 carats, may be black, yellow, or grayish green. Smithsonian Institution (Washington, D.C): 5.9 (black catseye, South Carolina). British Museum (Natural History) (London England): 35 (fibrolite). Geology Museum, London: 17 (fibrolitel) |
| Luminescence | Weak reddish fluorescence in blue Burmese material. None observed in Sri Lankan stones. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent |
| Transparency | Transparent to opaque. |
| Absorption Spectrum | Distinct lines (Sri Lanka) at 4620 and 4410, weak at 4100. |
| Phenomena | Chatoyancy (rare). |
| Formula | Al2SiO3. |
| Pleochroism |
May be strong: α: pale brown, pale yellow to green; β: brown or greenish: y: dark brown or blue, violet blue. |
| Optics | a = 1.654-1.661 β = 1.658-1.662; γ = 1.673-1.683. Biaxial (+), 2V = 21-30°. |
| Optic Sign | Biaxial + |
SILLIMANITE (=FIBROLITE) Trimorphous with Kyanite, Andalusite.
Optics: a =1.654-1.661 β= 1.658-1.662; γ= 1.673-1.683.
Biaxial (+), 2V: 21—30°.
Occurrence: A mineral of metamorphic rocks, such as schists and gneiss; also granites.
Idaho; South Dakota; Oklahoma; Pennsylvania; New York; Connecticut; Delaware; North Carolina; South Carolina.
Canada; Ireland; Scotland; France: Germany; Czech Republic; Slovakia; Brazil; India; Madagascar; North Korea; South Africa; Tanzania.
Sri Lanka and Burma: green, blue. violet—blue facetable material; also from Sri Lanka, grayish green, chatoyant fibrolite.
Kenya: facetable crystals, pale bluish color to colorless, S.G. 3.27.
Comments: The fibrolite from Burma and Sri Lanka is well known to gem collectors, and highly prized because of its great scarcity. Blue and greenish gems are lovely, although very difficult to cut. Chatoyant material sometimes yields catseye fibrolites, which are also very rare. The material from Kenya is just as attractive as Burmese fibrolite but seems to be somewhat smaller in size.
Name: After Benjamin Silliman, mineralogist, of Yale University. Fibrolite is in allusion to the fibrous nature of this variety.
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