X-ray single-crystal and Raman study of (NaMg)(MgTi)SiO, a new pyroxene synthesized at 7 GPa and 1700 °C

A new pyroxene with formula (NaMg)(MgTi)SiO, synthesized in a high-pressure toroidal ‘anvil-with-hole’ apparatus at  = 7 GPa and  = 1700 °C, was characterized by X-ray single-crystal diffraction and Raman spectroscopy. The compound was found to be monoclinic (1 = 2.56 %), space group 2/, with lattice parameters  = 9.687(2),  = 8.814(1),  = 5.290(1) Å, β = 107.853(2)°,  = 430.08(1) Å. The coexistence of Mg and Ti at the M1 site does not induce strong modifications either to the M1 site or to the adjacent M2 site. The Raman spectrum of synthetic Na–Ti-pyroxene was obtained for the first time and compared with that of MgSiO (with very low concentrations of Na and Ti). The structural characterization of the Na–Ti–Mg-pyroxene is important, because the study of its thermodynamic constants provides new constraints on thermobarometry of the upper mantle assemblages.