Sillimanite or fibrolite is an
aluminosilicate mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
with the
chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
Al
2SiO
5. Sillimanite is named after the American chemist
Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in
Chester, Connecticut.
[
]
Occurrence
Sillimanite or fibrolite is one of three aluminosilicate polymorphs, the other two being andalusite and kyanite. A common variety of sillimanite is known as ''fibrolite'', so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rock
Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s. It is an index mineral indicating high temperature but variable pressure. Example rocks include gneiss and granulite. It occurs with andalusite, kyanite, potassium feldspar, almandine, cordierite, biotite and quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
in schist
Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...
, gneiss, hornfels and also rarely in pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...
s.[ Dumortierite and mullite are similar mineral species found in ]porcelain
Porcelain (), also called china, is a ceramic material made by heating Industrial mineral, raw materials, generally including kaolinite, in a kiln to temperatures between . The greater strength and translucence of porcelain, relative to oth ...
.[Klein, Cornelis and Cornelius S. Hurlbut, Jr., ''Manual of Mineralogy,'' 1985, Wiley 20th ed., p. 380 ]
Sillimanite has been found in Brandywine Springs, New Castle County, Delaware
New Castle County is the northernmost of the three List of counties in Delaware, counties of the U.S. state of Delaware (New Castle, Kent County, Delaware, Kent, and Sussex County, Delaware, Sussex). As of the 2020 United States census, 2020 ce ...
. It was named by the State Legislature in 1977 as the state mineral of Delaware by the suggestion of the Delaware Mineralogical Society.
Uses
Natural sillimanite is used in the manufacture of high alumina refractories or 55–60% alumina bricks. However, it has mostly been replaced by the other aluminosilicate polymorphs, andalusite and kyanite, for this purpose. , sillimanite was just 2% of all aluminosilicate mineral production in the western world.
Gallery
File:Sillimanite.jpg, Silimanite crystal from Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...
File:Fibrolite.JPG, Fibrolite micrograph
See also
* List of minerals
* List of minerals named after people
* Asbestos cement, which is also called "fibrolite"
References
{{reflist
Aluminium minerals
Nesosilicates
Orthorhombic minerals
Minerals in space group 62
Industrial minerals
Symbols of Delaware