A study of the mean-square displacement amplitudes of T and O atoms in framework silicates and aluminosilicates: evidence for rigid TO bonds, order, disorder, twinning and stacking faults in crystals

The mean-square displacement amplitudes (MSDA) of the tetrahedral cations Si and Al are compared to the MSDA of their coordinated O atoms in framework silica polymorphs and aluminosilicate structures. Criteria are established which indicate order, structural disorder or substitutional disorder in a framework crystal.