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Home / Gem Library / Gemstones Varieties / Sillimanite May 21, 2019

Natural Sillimanite

Sillimanite, also called FIBROLITE, is a brown, pale green, or white glassy silicate mineral that often occurs in long, slender, needle like crystals frequently found in fibrous aggregates. An aluminum silicate, (Al2SiO5), it occurs in high-temperature regionally metamorphosed or sedimentary clay-rich rocks (e.g., schists and gneisses). Sillimanite is found at many points in France, Madagascar, and the eastern United States and also occurs in pale sapphire-blue colors in the gravels of Sri Lanka.

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Sillimanite Profile

Color information: violet blue, grey green, colorless, brown, green,
Refractive Index: 1.658 - 1.678
Chemical Composition: Al2SiO5
Hardness: 6 - 7.5
Density: 3.25
Crystal Group: Orthorhombic
Ocurrence: France, Madagascar, U.S.A., Sri Lanka, Burma. Kenya, Myanmar.
Sign of the Zodiac: N/A
Month of the year: N/A
Sillimanite is named after the American chemist Benjamin Silliman. It was first described in 1824 as an occurrence in Chester, Connecticut, USA. A common variety of sillimanite is known as fibrolite that was so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. The hardness for the crystalline version of the mineral is 7.5 while the fibrous variety may vary between 6 and 7.

Sillimanite crystallizes in the orthorhombic system and is polymorphous with both kyanite and andalusite. All three stones share the same chemical composition but vary in crystal formation.

Natural sillimanite is the best raw material for the manufacture of high alumina refractories but its use on a large scale is not possible due to the high cost and required purification. With the advent of online selling Sillimanite has become more popular with collectors while also enjoying a resurgence among metaphysical philosophers and healers.

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