The structure of mullite, 2Al₂O₃.SiO₂, and relationship with the structures of sillimanite and andalusite

Abstract

A synthetic crystal of mullre, 2A120 a . SiO 2, was studied by X-ray diffraction methods. The structure was found to be describable as the result of the following changes imposed upon the structure of sillmanre: re-distribution of silicon and aluminium of tetrahedral coordination into a random way over the same set of positions, random removal of some of the oxygen atoms shared by the above cations, replacement of silicon by aluminium and shift of some of the cations of tetrahedral coordination into open sites. In terms of the structure thus described, decomposition of sillimanite and andalusite was interpreted as due to the formation of a stable arrangement of three aluminium atoms around an oxygen atom.