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

We have studied quenched high pressure phases in the CaSiO₃ system by x‐ray diffraction (XRD) and ²⁹Si MAS NMR, XRD study of the previously reported “ε‐CaSiO₃ phase” synthesized at 12 GPa and 1500 °C reveals that it is actually a mixture of β‐Ca₂SiO₄ (larnite) and a previously unknown CaSi₂O₅ phase. This result is supported by the ²⁹Si NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi₂O₅ phase may have a titanite (CaTiSiO₅) 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 CaSiO₃‐perovskite was identified by both XRD and NMR. The ²⁹si NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.