Characterization of quenched high pressure phases in CaSiO₃ system by XRD and ²⁹Si NMR

Abstract

We have studied quenched high pressure phases in the CaSiO 3 system by x‐ray diffraction (XRD) and 29 Si MAS NMR, XRD study of the previously reported “ε‐CaSiO 3 phase” synthesized at 12 GPa and 1500 °C reveals that it is actually a mixture of β‐Ca 2 SiO 4 (larnite) and a previously unknown CaSi 2 O 5 phase. This result is supported by the 29 Si NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi 2 O 5 phase may have a titanite (CaTiSiO 5 ) structure in which Ti is replaced by an octahedral Si. Samples quenched from 15 GPa and 1500°C consist mostly of an amorphous phase, but a small amount of CaSiO 3 ‐perovskite was identified by both XRD and NMR. The 29 si NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.