Sillimanite is an alumino-silicate mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 with the chemical formula

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 Al₂SiO₅. Sillimanite is named after the American chemist Benjamin Silliman

Benjamin Silliman

Benjamin Silliman was an American chemist, one of the first American professors of science , and the first to distill petroleum.-Early life:...

 (1779-1864). It was first described in 1824 for an occurrence in Chester

Chester, Connecticut

Chester is a town in Middlesex County, Connecticut, United States. The population was 3,743 at the 2000 census. The town center is also defined by the U.S. Census Bureau as a census-designated place .-Geography:...

, Middlesex County, Connecticut

Middlesex County, Connecticut

Middlesex County is a county located in the south central part of the U.S. state of Connecticut. It was created in 1785 from portions of Hartford and New London counties. As of 2010, the population was 165,676....

, USA.

Occurrence

Sillimanite is one of three alumino-silicate polymorphs

Polymorphism (materials science)

Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

, the other two being andalusite

Andalusite

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....

 and kyanite

Kyanite

Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...

. 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

Thin section

In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...

 or even by the naked eye. Both the fibrous and traditional forms of sillimanite are common in metamorphosed

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

 sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s. It is an index mineral

Index mineral

An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith , chemical reactions between minerals in the solid state produce new minerals...

 indicating high-temperature metamorphism. It occurs with andalusite, kyanite, potassium feldspar, almandine

Almandine

Almandine , also known incorrectly 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 Asia Minor. Almandine is an iron alumina garnet,...

, cordierite

Cordierite

Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...

, biotite

Biotite

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...

 and quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 in schist

Schist

The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

, gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

, hornfels

Hornfels

Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...

 and also rarely in pegmatite

Pegmatite

A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....

s.

Sillimanite has been found in Brandywine Springs

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....

, New Castle County, Delaware

New Castle County, Delaware

New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of 2010 its population was 538,479, an increase of 7.6% over the previous decade. The county seat is Wilmington. The center of population of Delaware is located in New Castle County, in the town of...

, USA. It was named by the State Legislature in 1977 as the state mineral of Delaware by suggestion of the Delaware Mineralogical Society

Delaware Mineralogical Society

The Delaware Mineralogical Society, Inc., also known as DMS, is a U. S. registered tax-deductible 503 non-profit organization located in Wilmington, Delaware, USA. Its primary purpose is to promote education in the earth sciences...

, Inc.

Natural sillimanite rocks cut into the required shape and size are used mainly in glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 industries. Sillimanite is the best raw material for the manufacture of high alumina refractories or 55-60 % alumina bricks. But its use on large scale is not possible due to its fine grading and high cost.
Dumortierite

Dumortierite

Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, Al7BO33O3. Dumortierite crystallizes in the orthorhombic system typically forming fibrous aggregates of slender prismatic crystals. The crystals are vitreous and vary in color from brown, blue, and green to more rare violet...

 and mullite

Mullite

Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present...

 are similar mineral species used in quality porcelain

Porcelain

Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

.

See also

• List of minerals
• List of minerals named after people
• kyanite
  Kyanite
  Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than...
  
  
• andalusite
  Andalusite
  Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....