Sillimanite is an aluminum silicate, related to both andalusite and kyanite. In fact these three minerals are polymorphs -- they share the same chemical composition but different crystal structures. Kyanite is formed in a lower temperature/higher pressure environment, while andalusite forms in lower temperature/lower pressure conditions and sillimanite under higher temperatures/higher pressures.
Sillimanite is named after the American chemist Benjamin Silliman (1779-1864). Silliman was one of the first American professors of science and taught at Yale University. He was originally trained as a lawyer and taught law at Yale. The president of the university proposed that Silliman study chemistry and natural history and assume a new professorship in the sciences at Yale. Silliman became an important chemist and geologist and discovered the constituent elements of many minerals. He was also the very first person to distill petroleum.
Gemologically, sillimanite has a hardness of 6.5 to 7.5 on the Mohs scale with a vitreous pearly luster. It has a refractive index of 1.655 to 1.684, approximately the same as spodumene. Its specific gravity is 3.23-3.27, placing it between apatite and diopside on the density scale. Like diamond, topaz and fluorite, sillimanite has perfect cleavage.
Sillimanite occurs in a number of colors, ranging from colorless to gray-white, yellowish, brownish and bluish. Sillimanite is rarely transparent. Sillimanite also occurs in a fibrous form sometimes known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together. Some of these fibrous sillimanite display chatoyancy or the cat's eye effect, often in an attractive violet-brown shade.
Historically, sillimanite has been a rare gemstone, known mainly to collectors. Deposits have been found in Burma, Sri Lanka, Kenya and the USA. Recent finds in India have made sillimanite more widely available, though it is still classified as a collector's gem.
- First Published: February-10-2009
- Last Updated: October-06-2010
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