Compressibility of two Na-rich clinopyroxenes; a synchrotron single-crystal X-ray diffraction study

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 (Na (sub 0.86) Mg (sub 0.14) )(Mg (sub 0.57) Ti (sub 0.43) )Si (sub 2) O (sub 6) synthesized at P = 7 GPa and T = 1700 degrees C, and a Na-pyroxene with composition (Na (sub 0.886) Mg (sub 0.085) Fe (sub 0.029) ) (Si (sub 0.442) Mg (sub 0.390) Fe (sub 0.168) )Si (sub 2) O (sub 6) synthesized at P = 15 GPa and T = 1500 degrees 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 FeO (sub 6) octahedron is significantly more rigid than MgO (sub 6) 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.