Aluminium silicate
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Through Mineral and Metamorphic rock
Main representatives
Andalusite, kyanite, and sillimanite are the principal aluminium silicate minerals. The triple point of the three polymorphs is located at a temperature of 500 °C (932 °F) and a pressure of 0.4 GPa (58,000 psi). 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 450 °C (842 °F).
- Al6Si2O13, (3Al2O3·2SiO2), the mineral mullite, the only thermodynamically stable intermediate phase in the Al2O3-SiO2 system at atmospheric pressure. 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).
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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.