Sillimanite mineral chemical formula.

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Sillimanite mineral

Sillimanite is a mineral species of the group of silicates under the group of the neosubsilicates of formula Al2O (SiO4) with traces of iron. The malformed crystals do not exceed 2 cm.

Described by the American mineralogist Bowen in 1824, his name comes from the American chemist and mineralogist Benjamin Silliman (1779-1864).

The stone consists of parallel chains of octahedra AlO6 and tetrahedra alternately SiO4 and AlO4.

Morphology of crystals: prismatic crystals, acicular. This morphology results from the structure of the gemstone.

A relatively stable mineral, but can be altered into kaolinite, muscovite or sericite.

It is a frequent mineral in high-grade thermal metamorphism rocks of clay rocks, by transformation of biotite or andalusite.

It can also be formed by regional metamorphism of clay rocks (gneiss) from muscovite and biotite, reactions between staurotide and biotite or between staurotide and quartz, or by polymorphic transformation of disthene.

Aluminosilicate polymorphs

Sillimanite is one of three aluminosilicate polymorphs, the other two being andalusite and kyanite. A common variety of this stone 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 crystal are common in metamorphosed sedimentary rocks. 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 in schist, gneiss, hornfels and also rarely in pegmatites.

Rocks cut into the required shape and size are used mainly in glass industries. It 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 and mullite are similar mineral species found in porcelain.

The stone has been found in Brandywine Springs, New Castle County, Delaware. It was named by the State Legislature in 1977 as the state mineral of Delaware by suggestion the Delaware Mineralogical Society.

Sillimanite cat’s eye

The stone is sometimes referred to as fibrolite because its crystals occur in groups, resembling fibers. It is found throughout the world, in Myanmar, Sri Lanka, the U.S.A, Czechoslovakia, India, Italy, Germany, and Brazil.

Sillimanite cat’s eye chemical formula is Al2SiO5 and its hardness ratio on the Mohs scale is 6-7.

Sillimanite thin section

Sillimanite thin section is colorless in plane light, although mats of fibrolite can be brownish/tan. Under crossed polars, It has second order interference colors.

Sillimanite meaning

The following section is pseudo scientific and based on cultural beliefs.

The crystal as the capacity in bringing peace, joy, and wealth to its possessor. Although it is linked with all the chakras, this stone may greatly connect with the heart chakra.


What is sillimanite uses for?

This gemstone that has a meaning and properties of making a breakthrough. People believe that it can clear unfavorable situations or energy for the owner. It is used when you are trying to achieve a higher level or when there are many rivals around you. The stone is also good to increase motivation.

How do you identify sillimanite?

Keys to identification are high relief, needle-like, fibrous or bladed habit, characteristic square cross sections with one diagonal cleavage.

Is sillimanite a good mineral to use for determining the grade of metamorphism?

Index minerals in metamorphic rocks indicate how much the original rock was metamorphosed, called the grade of metamorphism. Index Minerals minerals include chlorite, epidote, garnet, stauroliteand kyanite. Fine-grained crystals of chlorite give this slate its green color.

Where is sillimanite mineral found?

The stone is found at many points in France, Madagascar, and the eastern United States. A pale blue gem variety occurs in the gravels of Sri Lanka.

Natural sillimanite for sale in our gem shop

We make custom made sillimanite jewelry as engagement rings, necklaces, stud earrings, bracelets, pendants… Please contact us for a quote.