Single-crystal diffraction and Raman spectroscopy of hedenbergite up to 33 GPa

Pyroxenes are important minerals in Earth’s upper mantle and subducting plate. Here, we report results of high-pressure single-crystal X-ray diffraction and Raman spectroscopy experiments conducted on natural Ca, Fe pyroxene hedenbergite up to ~33 GPa in diamond anvil cell. Unit cell parameters a, b, c, β and V, as well as bond lengths of hedenbergite are reported within the studied pressure range. Cell parameters exhibit continuous decrease on compression. Axial compressibilities of a, b and c are calculated to be 1.7(2), 4.9(5) and 2.13(9) × 10−3 GPa−1, respectively. Bulk modulus and its pressure derivative are determined to be 131(4) GPa and 3.8(3) by fitting third-order Birch–Murnaghan equation of state. Compression mechanism is dominated by polyhedral and bond compression trends typical of clinopyroxenes. In general, shorter bonds show lower compressibility, and SiO4, the smallest polyhedron, shows the lowest compressibility. Angle and elongation distortions are reported for the three types of polyhedra at high pressure. Thirteen vibrational modes are observed with Raman spectroscopy up to ~33 GPa. All observed mode frequencies increase as pressure increases.