We have studied the structures of Al/Si disordered leucite and bicchulite with a quantum mechanical version of the virtual crystal approximation, VCA. In leucite, the average tetrahedron has a composition of (AlSi2)O4, while bicchulite represents an extreme case with (Al2Si)O4 tetrahedra. Both structures are well described with the VCA. In conjunction with an earlier study, where we have shown that the (AlSi)O4 tetrahedra in gehlenite and Al,Si-disordered octahedra are also well reproduced, we have now established that the VCA gives a reliable description of the averaged structure of disordered aluminosilicates over the whole compositional range. The current calculations confirm that Al/Si ordering is not driving the cubic to tetragonal phase transformation in leucite. In bicchulite, the model calculations are consistent with hydrogen on Wyckoff position 8c, in agreement with the result of a single crystal X-ray diffraction study, but in variance with results based on a neutron powder diffraction study.

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