Acoustic velocity measurement of Na-contained stishovite according to the post-stishovite phase transition

Stishovite is considered to be an important constituent of subducted oceanic basalts and sediments in the Earth's deep interior. It has the tetragonal rutile structure and transforms to an orthorhombic CaCl2 structure at around 60 GPa in the pure SiO2 system. According to theoretical models, the phase transition is trigged by the lattice instability of a soft transverse acoustic mode associated with the shear elastic constant and it has been suggested that this elastic softening could relate to several distinctive seismic structures in the Earth's lower mantle (e.g., [1], [2]). This possibility was denied by the phase study of pure SiO2 at high pressure and high temperature [3], however, natural stishovite in subducted basalts and sediments may contain several impurities, such as Al, Mg, and Na. Therefore, the effect of impurities on the phase transition and elastic properties of stishovite should be clarified. In this study, we investigated elastic properties of sodium contained stishovite by simultaneous measurements of acoustic velocity and X-ray diffraction across the post-stishovite phase transition at room temperature and a pressure range of 0-70 GPa.The phase transition from rutile-structure to CaCl2-structure was observed at around 25GPa with X-ray diffraction and a dipping of transversal velocity was also observed at the transition pressure. [1]Karki et al. (1997) GRL 24, 3269-3272. [2]Kaneshima & Helfrich (1999) Science 283, 1888-1891 [3] Ono et al. (2002) EPSL 197, 187-192.