INCOMMENSURATE PHASES OF TRIDYMITE. H.Graetsch, Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany
The third room temperature modification of tridymite, L3-TO(MX-1), displays satellite reflections indicating an incommensurate modulation with q 2/3a*-1/2c* (Löns, J., Hoffmann, W., Z. Kristallogr., 1987, 178, 141-143). The modulation is caused by tilting of SiO4/2-tetrahedra which form rows of corner-sharing tetrahedra running parallel to the a-axis. It can be described as a combination of transverse and longitudinal displacements of the atoms without severe distortion of the tetrahedra. The modulation vector does not markedly change with increasing temperature.
Upon heating, L3-TO(MX-1) undergoes a sequence of at least 4 phase transitions before finally transforming to hexagonal high-tridymite H1-TO(HP) at 400deg.C. Three of the five high temperature modifications are incommensurate phases. At 65deg.C monoclinic L3-TO(MX-1) transforms to a phase with orthorhombic metric (Hoffmann, W. et al., Fortschr. Mineral., 1983, 61, 96-98). A gap in the thermal expansion of the lattice parameters, discontinuous disappearance of the satellite reflections and a large hysteresis indicate a first order transition. The average structure of the new phase (H5-TO) strongly resembles that of orthorhombic high tridymite H2-TO(OC) which is known to exist between 220deg.C and 400deg.C (Dollase, W. A., Acta Cryst., 1967, 23, 617-623). However, satellites surrounding the main reflections indicate an incommensurate modulation with q 1/3a*. Further satellite reflections indicate an additional incommensurate modulation with a wavelength of nearly 5co. At 105deg.C the wavelength of the modulation along the a-axis increases and several higher orders of satellite reflections appear. The already known non-integral tridymite phase H3-TO(OS) is formed at 155deg.C (Nukui, A. et al., AIP Conf. Proc., 1978, 33, 327-329).