Aluminium silicate explained

Aluminum silicate (or aluminium silicate) is a name commonly applied to chemical compounds which are derived from aluminium oxide, Al₂O₃ and silicon dioxide, SiO₂ which may be anhydrous or hydrated, naturally occurring as minerals or synthetic. Their chemical formulae are often expressed as xAl₂O₃·ySiO₂·zH₂O. It is known as E number E559.

Main representatives

Andalusite, kyanite, and sillimanite are the principal aluminium silicate minerals.^{[1] [2] [3]} The triple point of the three polymorphs is located at a temperature of and a pressure of 0.4GPa. These three minerals are commonly used as index minerals in metamorphic rocks.

• Al₂SiO₅, (Al₂O₃·SiO₂), which occurs naturally as the minerals andalusite, kyanite and sillimanite which have distinct crystal structures.
• Al₂Si₂O₇, (Al₂O₃·2SiO₂), called metakaolinite, formed from kaolin by heating at .
• Al₆Si₂O₁₃, (3Al₂O₃·2SiO₂), the mineral mullite, the only thermodynamically stable intermediate phase in the Al₂O₃-SiO₂ system at atmospheric pressure.^[4] This also called '3:2 mullite' to distinguish it from 2Al₂O₃·SiO₂, Al₄SiO₈ '2:1 mullite'.
• 2Al₂O₃·SiO₂, Al₄SiO₈ '2:1 mullite'.

The above list mentions ternary materials (Si-Al-O). Kaolinite is a quaternary material (Si-Al-O-H). Also called aluminium silicate dihydrate, kaolinite occurs naturally as a mineral. Its formula is Al₂Si₂O₅(OH)₄, (Al₂O₃·2SiO₂·2H₂O).^[5]

Aluminium silicate composite materials, fibres

Aluminium silicate is a type of fibrous material made of aluminium oxide and silicon dioxide, (such materials are also called aluminosilicate fibres). These are glassy solid solutions rather than chemical compounds. The compositions are often described in terms of % weight of alumina, Al₂O₃ and silica, SiO₂. Temperature resistance increases as the % alumina increases. These fibrous materials can be encountered as loose wool, blanket, felt, paper or boards.^[6]

External links

• Aluminum silicates at the University of Waterloo, Canada

Notes and References

1. Web site: Andalusite, Handbook of Mineralogy. live. https://web.archive.org/web/20120516011115/http://rruff.geo.arizona.edu/doclib/hom/andalusite.pdf. 2012-05-16.
2. Web site: Kyanite . Handbook of Mineralogy. live. https://web.archive.org/web/20120224015145/http://rruff.geo.arizona.edu/doclib/hom/kyanite.pdf. 2012-02-24.
3. Web site: Sillimanite . Handbook of Mineralogy . live. https://web.archive.org/web/20111119233945/http://rruff.geo.arizona.edu/doclib/hom/sillimanite.pdf. 2011-11-19.
4. Ceramic and Glass Materials: Structure, Properties and Processing, James F. Shackelford, R. H. Doremus, Springer, 2008,
5. Handbook of Inorganic Compounds, Dale L. Perry, Taylor & Francis, 2011,
6. Concise Encyclopedia of Composite Materials, ed. Anthony Kelly, MIT Press, 1989,