Cell Dimensions, Solid Solution, Polymorphism, and Identification of Mullite and Sillimanite

Abstract

X‐ray, optical, and chemical data are presented for sillimanites and for a wide range of natural and synthetic mullites. Single‐crystal X‐ray studies have revealed a new type of mullite (called S mullite) characterized by subsidiary sharp reflections in distinction to the extra diffuse reflections of common mullite (called D mullite). Atomic substitution of aluminum in D mullite leads to expansion of a and c but not of b, whereas substitution of iron and titanium in natural mullites gives a slight initial decrease in a and b followed by a subsequent increase; c increases uniformly for all concentrations. The data are consistent with solid solution ranging between the extremes 3Al 2 O 3 .2SiO 2 , 2Al 2 O 3 .‐SiO 2 , and 3(Al 0.9 Fe 0.1 ) 2 O 3 .2SiO 2 . Cell dimensions of sillimanite vary only slightly. Consideration of the various methods of distinguishing mullite and sillimanite shows that the measurement of cell dimensions is the most reliable and for mullite has the added advantage that the S and D polymorphs may be distinguished and an estimate of the composition obtained for D mullite.