AN IN SITU STUDY OF THE I2/A TO IMAB PHASE TRANSITION IN SIO2 MOGANITE USING HARD MODE RAMAN SPECTROSCOPY
In this study, we employed hard mode Raman spectroscopy in order to confirm the existence of the alpha-beta moganite transition and to offer complementary insight into the transition mechanism. Powdered samples from Gran Canaria were heated and cooled in a Pt crucible between 300 and 873 K, and parallel-polarized Raman spectra were collected in back-scattered geometry with an Ar+ laser (at a wavelength of 4579 ). An analysis of the displacement of the main symmetric stretching-bending vibration (A1 mode) of moganite at 501 cm-1 with changing temperature clearly revealed the occurrence of the transition. On heating, the peak position decreased linearly to ~593 K, at which point the peak remained fixed at 496 cm-1 to ~723 K. Above this temperature, the peak position continued to move towards smaller wavenumbers, but with a shallower slope relative to the behavior at low temperature. This trend was repeated on cooling, but with a hysteresis of over 100 K. When these Raman data are considered in conjunction with subtle anomalies in the variation of lattice parameters with temperature as revealed by X-ray diffraction, two scenarios seem possible: 1) As in quartz and tridymite, moganite may exhibit an intermediate phase that presumably is incommensurate, as there is no intermediate space group symmetry in this Brillouin zone center transition; or 2) As in the alpha-beta cristobalite transition, the moganite transformation may be martensitic and involve phase co-existence over a large temperature interval.