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| <<Back to Main | ApatiteApatite | Aquamarine and BerylsAquamarine and Beryls | ChrysoberylChrysoberyl | Crystal CollectionsCrystal Collections | DanburiteDanburite | DiopsideDiopside | EnstatiteEnstatite | Feldspar and MoonstonesFeldspar and Moonstones | FluoriteFluorite | GarnetGarnet | Gemstones in matrixGemstones in matrix | Lapis LazuliLapis Lazuli | PegmatitePegmatite | PeridotPeridot | PetalitePetalite | PhenakitePhenakite | Pyrite, Hematite and Iron OresPyrite, Hematite and Iron Ores | Quartz Family: Amethyst, Citrine, Agate...Quartz Family: Amethyst, Citrine, Agate... | Rare Burmese SpecimensRare Burmese Specimens | RubyRuby | SapphireSapphire | ScapoliteScapolite | Sillimanite or FibroliteSillimanite or Fibrolite | SpinelSpinel | TopazTopaz | TourmalineTourmaline | ZirconZircon | Other Fine CrystalsOther Fine Crystals | | Our Top Sillimanite or Fibrolite Highlight | Sillimanite or Fibrolite | | | The name Sillimanite comes from the name of Benjamin Silliman, who was professor at Yale University at the 19th siecle and known for its works on oil-petrol). It is also called fibrolite, which refers to the fibrous texture of the stone. The crystal habit can present long slender prisms without distinct terminations but massive forms also occur. Its cleavage (parallel to one pair of prism faces) is perfect and its hardness varies between 6 and 7on Mohs' scale. But it has a strong resistance, toughness, to erosion. That propertie is known from prehistory, that is why sillimanite was used to make some polished axes.
This stone is usually colorless, grey to blue or yellowish. Some confusion are possible with sapphires from Ceylan and jadeite or nephrite. Some stones from Sri Lanka can show a greyish-green chatoyancy. Because it is a biaxial stone, it shows an intense pleochroism (pale green, dark green and blue).
Sillimanite is an alumino-silicate (Al2SiO5), which means that it is a (neso) silicate crystallizing in the orthorhombic system. It is so a polymorph (same chemical formula but different crystal strucure) of kyanite, or disthene, and andalusite. It is a metamorphic rock and so occurs in schists, gneisses and granites associated with andalusite and corundum. Its specific gravity is around 3,25. Sillimanite is a traditional gemstone found in the gravels of the Mogok Valley in Burma, where some beautiful blue-violet crystals are found. Most of the stones are found as slightly waterworned crystal and fine euhedral crystals a re rare in Mogok, sillimanite also occurs in Sri Lanka, Kenya and in Idaho (United States).
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