An X-ray Study of Natural and Artificial Mullites

JSGT_1939_V23_T338_T346

X-ray studies indicate that there are three varieties of mullite distinguishable by the differences in axial ratios of the unit cell. They are: α-mullite made from pure materials corresponding to the formula 3Al₂O₃,2SiO₂ and containing 28.2% SiO₂, 71.8 Al₂O₃; β-mullite containing excess alumina in solution; and γ-mullite containing small proportions of ferric oxide and/or titania in solution. All three varieties have been prepared artificially, but so far only the β- and γ- varieties have been found as minerals. β-mullite can contain as high as 78% of alumina, although there is a progressive change in the lattice from the α- to the β-variety as the alumina content rises above 72%. γ-mullite is obtained when comparatively small proportions of iron or titanium oxides are present; with larger proportions a general expansion of the lattice occurs. {3-mullite containing 78% of alumina corresponds to the formula 2Al₂O₃,SiO_2. It is clear that the replacement of silicon by aluminium is not limited to the change from sillimanite to α-mullite, but can be carried a stage further until the composition of β-mullite is reached.

H. P. Rooksby, B.Sc., F.Inst.P., and J. H. Partridge, Ph.D