Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on synthetic clinopyroxenes at room temperature to a maximum pressure of 40 GPa. We studied two crystals with different compositions. A Na-Ti-pyroxene with formula (Na0.86Mg0.14)(Mg0.57Ti0.43)Si2O6 synthesized at P = 7 GPa and T = 1700 °C, and a Na-pyroxene with composition (Na0.886Mg0.085Fe0.029) (Si0.442Mg0.390Fe0.168)Si2O6 synthesized at P = 15 GPa and T = 1500 °C. These phases were found to be monoclinic with the space group C2/c and exhibit KTo of 106.8(2), 121.8(4) GPa, respectively. Na-Tipyroxene is more compressible than Fe-bearing Na-Mg-Si-pyroxene, likely due to the fact that the FeO6 octahedron is significantly more rigid than MgO6 at high pressure. The formation of Na-rich pyroxenes in the deep mantle is related to crystallization of low-degree alkaline carbonate-silicate melts formed when the crust and mantle interact during the slab descent and its stagnation in the transition zone.

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