AN IN SITU STUDY OF THE I2/A TO IMAB PHASE TRANSITION IN SIO2 MOGANITE USING HARD MODE RAMAN SPECTROSCOPY

The silica polymorph moganite is commonly intergrown with quartz in microcrystalline silica varieties that are less than ~100 Ma in age, and it thereby serves as a widespread and significant indicator mineral. Concentrations of moganite in excess of 20 wt% in cherts are strong signs of formation in evaporitic environments (Heaney 1995), and moganite appears to control the activity of silica in river waters associated with basaltic bedrock due to the higher solubility of moganite relative to quartz (Gislason et al. 1997). Heaney and Post (2001) presented synchrotron X-ray diffraction evidence for a displacive phase transition when moganite is heated above ~570 K, with an increase in symmetry from I2/a to Imab.

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.