Pressure-induced phase transitions in rhodonite and related chain silicates

Abstract Rhodonite (MnSiO3) is a pyroxenoid with a single-chain silicate framework structurally related to pyroxenes. In this study, we investigated the structural evolution of synthetic rhodonite up to 35.0(1) GPa using in-situ synchrotron single-crystal X-ray diffraction. The results indicate that rhodonite retains its triclinic structure (P1¯) throughout the pressure range; however, it undergoes an isosymmetric second-order phase transition at 10.9(1) GPa, driven primarily by an increase in Mn coordination number. Contrary to earlier findings on amorphization, rhodonite retains its crystallinity up to 35.0(1) GPa. Moreover, this study also reviews the high-pressure phase transitions in orthopyroxenes, clinopyroxenes, and pyroxenoids, with a particular focus on mechanisms and characteristics of the structural phase transitions. Through comparative analysis, we have identified both commonalities and differences among these chain silicates. Specifically, we have further highlighted the crucial roles played by the rotations of tetrahedra, increases in coordination numbers, and the interplay between these factors in governing their phase transitions. These findings not only enhance our understanding of pyroxenoid structural evolution at high pressure, but also provide valuable insights into chain silicate stability in deep Earth.