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Mullite
Calcined Mullite
Chemistry
Chemistry available on new site, see link above
Links to Other Materials
- Kyanite - Related
- Pyrophyllite - Related
- Mulcoa 70 Mullite - Related
- Calcined Alumina - Related
- Cache
Miscellaneous
- Family: AlumSilicate
- Region: North America
- Mined At: Unspecified
- Raw Mineral: No
- Generic: Yes
Notes
Theoretical formula: 3Al2O3 2SiO2.
Mullite is a mineral of long interlocking needle-like crystal structure that is very resistant to thermal shock failure (has a low thermal expansion). It is also has a low thermal conductivity and is very refractory thus the theoretical formula bears little resemblance to the real world material of 71.8% alumina and 28.2% silica (we have provided a typical analysis). It is found rarely in nature, it is named after a deposit on the Isle of Mull in Scotland. However, it can be synthesized by calcining kyanite, bauxite or alumina/kaolin mixtures of proper Al2O3:SiO2 ratio.
On a scale of lowest to highest thermal expansions at 2000F where fused silica is almost zero and quartz is 1.5%, mullite is about one third of the way. It has a lower expansion than fused alumina (0.9%) and stabilized zircon (0.8%).
Mullite can also be formed within special purpose porcelains by incorporating similar minerals into the recipe and firing to the necessary temperature and heating curve to decompose them into mullite. These include andalusite (cone 13), kyanite (cone 12), sillimanite (cone 20). The resulting bodies display low thermal expansion and a useful in spark plugs, laboratory ware, etc. and in thermal shock resistant refractories.
Firing of ordinary stoneware bodies provides the necessary temperature and adequate kaolin to produce mullite crystals from the decomposition of kaolinite (kaolinite looses some silica and the remaining higher alumina reorients itself to a higher melting compound). The resulting lattice of crystals is potentially much stronger than the simple glass-weld bonds of low-fire ceramics.
The chemistry of mullite depends on the parent material. Impurities tend to be TiO2, Fe2O3, Na2O, K2O.
Properties
Body Thermal Expansion -
This material has a very low thermal expansion and high melting temperature.
Data
- Melting Point (MP): 1810C
- Hardness (Moh): 3.1
- Hardness (Moh): 3.1
- Melting Point (MP): 1810C
URLs
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
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