The question of polymorphism in barium disilicate has recently been discussed in this journal by R. M. Douglass (1958). Although Rogers (1932) postulated dimorphism of BaSi2O5 in order to reconcile his optical data with those of Bowen (1918) and Escola (1922), Douglass found no evidence of polymorphism in his own experiments. Douglass compared his indexed x-ray diffraction powder pattern of the mineral sanbornite with the pattern published by Austin (1947) for BaO·2SiO2. He correlated the (112/004) doublet of sanbornite occurring at 3.422 Å with the 3.55 Å peak in Austin's pattern. However, if a calculation of Austin's pattern is made, it can be seen that the 3.55 Å peak is well beyond the limit of error of any d spacing which can belong to the sanbornite structure.

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