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 < 22 GPa) to coexist with wadsleyite, garnet and pyroxene. Although the crystal structure of room-PT Na₂MgSiO₄ is known, a structural study of the phase recovered in high-pressure experiments has not been done yet. Here we present a chemical and structural study of a crystal of Na₂MgSiO₄ synthesised in the model system Na₂MgSiO₄–Mg₂SiO₄ at 22 GPa 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) Å, V = 402.55(4) ų, and Z = 4. The structure was refined to R₁ = 0.0184 using 644 observed reflections [F_o > 4σ(F_o)] 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.