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The Mineral Sillimanite

sillimanite mineral

Sillimanite, an alumino-silicate mineral, is a variety of aluminum silicate, which is linked to both Kyanite and Andalusite. Actually, the three minerals form polymorphs, meaning they have the same chemical composition but diverse crystal structures. While Kyanite gets formed under high pressure, low temperatures, Andalusite are formed under low pressure coupled with low temperature surroundings. Sillimanite is formed under conditions of high pressure along with high temperatures.

Chemical Formula  
Al2SiO5

Derivation of Name  
This mineral gets its name from the American chemist Benjamin Silliman (1779–1864). Its first occurrence was recorded during 1824 in Chester, Middlesex County, Connecticut, USA.

Occurrence         
Sillimanite is among the three aluminosilicate polymorphs. Kyanite and Andalusite are the other two. Fibrolite is a widespread variety of Sillimanite. The mineral is so named as it looks like a cluster of fibers coiled together when seen in slim fragments or even when examined by the naked eye. Both traditional and fibrous forms of Sillimanite are widespread in metamorphosed sedimentary rocks. It is classified as an index mineral, signifying high temperature but inconsistent pressure. It is found along with Kyanite, Andalusite, Almandine, Potassium Feldspar, Biotite, Cordierit and Quartz in schist, hornfels, gneiss, and not often in pegmatites.            

Gemology  
Historically, Sillimanite has continued to be treated as an exceptional gemstone, which is largely known to collectors. Of late, it has been found in India, making its availability rather easier, though it continues to be classified among collector’s gems.

Source 
It has been located in Brandywine Springs, New Castle County, Delaware, USA. In 1977, the State government named it as the state mineral of Delaware as proposed by the Delaware Mineralogical Society, Inc.

Mineralogy 
Sillimanite, also known as Bucholzite, is a polymorph, with the two other already named minerals: Andalusite and Kyanite. A mineral having the same chemical composition as other mineral, but the dissimilar crystal structure is called a polymath. Of all the three polymorphs, Sillimanite is the rarest. "Fibrolite" is a type of Sillimanite that is widespread in the huge fibrous form. 

Color           
It occurs in white, green and brown color.

Density     
3.24 g/cm3

Mohs hardness         
7

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