Sillimanite is an aluminosilicate

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

with the

chemical formula In chemistry, 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, ...

Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in

Chester, Connecticut Chester is a town in Middlesex County, Connecticut, United States. The population was 3,749 at the 2020 census. The town center is defined by the U.S. Census Bureau as a census-designated place (CDP). The name is a transfer from Chester, in En ...

Occurrence

Sillimanite 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 that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s. It is an index mineral indicating high temperature but variable pressure. Example rocks include

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

and granulite. It occurs with andalusite, kyanite,

potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium: *Orthoclase ( endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock *Microcline, chemically the same as orthocla ...

almandine Almandine (), also known as almandite, is a species of mineral belonging to the garnet group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asi ...

, cordierite,

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more ...

and

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

, gneiss,

hornfels Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These pro ...

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 silici ...

s. Dumortierite and mullite are similar mineral species found in

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...

.Klein, Cornelis and Cornelius S. Hurlbut, Jr., ''Manual of Mineralogy,'' 1985, Wiley 20th ed., p. 380 Sillimanite has been found in

Brandywine Springs Brandywine Springs is a historic area near Newport, Delaware, along the Red Clay Creek. It is noted from early American history as a Revolutionary War encampment of General George Washington's army. In 1853, Captain Alden Partridge opened the N ...

New Castle County, Delaware New Castle County is the northernmost of the three counties of the U.S. state of Delaware (New Castle, Kent, and Sussex). As of the 2020 census, the population was 570,719, making it the most populous county in Delaware, with nearly 60% of th ...

. 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.

Images

File:Sillimanite.jpg, Silimanite crystal from

Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...

File:Fibrolite.JPG, Fibrolite micrograph

References

{{reflist Aluminium minerals Nesosilicates Orthorhombic minerals Minerals in space group 62 Industrial minerals Symbols of Delaware

Occurrence

Uses

Images

See also

References