Mullite

Alternate Names: Mulite

Description: Calcined Mullite

Oxide	Analysis	Formula
K2O	0.20%	-
Fe2O3	1.00%	0.01
Na2O	0.20%	0.01
TiO2	0.60%	0.01
Al2O3	57.00%	1.00
SiO2	41.00%	1.22
Oxide Weight		178.95
Formula Weight		178.95

Notes

Mullite is found rarely as a mineral in nature; it is named after a deposit on the Isle of Mull in Scotland. Mullite sands, grogs and powders are man-made materials. The crystal shape has little in common with the mullite needle-crystals that grow in kaolin-containing bodies when they vitrify (see below).

It also has a low thermal conductivity and is very refractory. The theoretical formula of 71.8% alumina and 28.2% silica bears little resemblance to a real-world material (we have provided a typical non-theoretical analysis). This is because the conversion process, from kaolin, yields both mullite and silica.

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 crystals can also be formed within special-purpose low-feldspar porcelains by incorporating similar minerals into the recipe and firing to the necessary degree to decompose them into mullite. These include andalusite (cone 13), kyanite (cone 12), and sillimanite (cone 20). The resulting bodies display low thermal expansion and are useful in spark plugs, laboratory ware, etc. and in thermal shock-resistant refractories.

The high temperature firing of ordinary kaolin-containing stoneware and porcelain bodies provides the necessary temperature to produce mullite crystals from the decomposition of kaolinite. The resulting lattice 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, SiO₂.

Related Information

Links

URLs	http://www.azom.com/details.asp?ArticleID=925 Overview of Mullite at Azom.com
URLs	https://en.wikipedia.org/wiki/Mullite Mullite at Wikipedia
Materials	M70 Mullite
Materials	Kyanite Kyanite is a granular material used in the manufacture of ceramics and abrasives. It is notable for low thermal expansion and one-way expansion on heating.
Materials	Pyrophyllite A refractory aluminum silicate mineral often used in clay body recipes to lower thermal expansion, control fired maturity, mullite development catalyst, etc.
Materials	Virginia Kyanite
Materials	Calcined Alumina
Materials	Mulcoa 70 Mullite
Materials	Silica Sand
Materials	Sillimanite
Typecodes	Generic Material Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes	Aluminum Silicate Materials not classifiable as commonly known aluminum silicates. For example, kaolin is a common aluminum silicate.
Typecodes	Refractory Materials that melt at high temperatures. These are normally used for kiln bricks, furniture, etc. or for ceramics that must withstand high temperatures during service.
Glossary	Mineralogy Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
Minerals	Mullite Mullite (3Al₂O₃·2SiO₂) is a naturally occurring aluminum silicate crystalline mineral. It forms in h
Temperatures	Metakaolin converts to spinel phase (950-1050)
Temperatures	Spinel phase meta kaolin converts to mullite (1050-1200+)

Data

Hardness (Moh)	6-7
Frit Softening Point	1810C

Mechanisms

Body Thermal Expansion	This material has a very low thermal expansion and high melting temperature.

By Tony Hansen
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