Structural states of Mg cordierite. IV. Raman spectroscopy and local order parameter behaviour

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Abstract
For pt.III see Phys. Chem. Minerals, vol.16, p.365 (1988). The time evolution of Al-Si ordering and ferroelastic distortion in isothermally annealed Mg cordierite (Mg2Al4Si5O18) is studied using Raman spectroscopy. The transition from the initial hexagonal to the modulated structure is found to be discontinuous, with the quenched modulated structure being locally orthorhombic. The bilinear coupling between Al-Si ordering and lattice distortion proposed previously to account for observed behaviour in the macroscopically orthorhombic phase obtained after prolonged annealing is shown to be also valid on a local scale in the modulated phase. Modes of the hexagonal phase, which split on passing into the modulated phase, are found to be inhomogeneously broadened. This is related quantitatively to sub-unit cell distortions brought about by Al-Si ordering. The same ordering is found to be related linearly to the decrease in widths of modes of other symmetries with annealing.