X-ray single-crystal structural characterization of Na2MgSiO4 with cristobalite-type structure synthesised at 22 GPa and 1800 °C

Na2MgSiO4 is a mineralogical phase known in the literature as N2MS and reported in several high-pressure experiments (13.6<P<22GPa) to coexist with wadsleyite, garnet and pyroxene. Although the crystal structure of room-PT Na2MgSiO4 is known, a structural studyof thephase recovered in high-pressure experiments has not been done yet.Herewepresent a chemical and structural study of a crystal of Na2MgSiO4 synthesised in the model system Na2MgSiO4–Mg2SiO4 at 22GPa and 1800 °C. The compound was found to crystallize with the orthorhombic cristobalite-type structure previously described at room conditions, space group Pna21, with lattice parameters a = 10.8193(8), b = 5.2702(3), c = 7.0598(4)A,V = 402.55(4)A3, andZ = 4.The structurewas refined toR1 = 0.0184 using 644 observed reflections [Fo>4s(Fo)] and 73 parameters. The unit-cell of the phase studied here is nearly identical to that of Na2MgSiO4 synthesised at room conditions, thus indicating that it likely relaxed during the pressure release occurring during quenching.