Unit-cell parameters as a function of isothermal MSDC-0160 (Table 1 and Figs. 4a, b and 5) indicate the presence of a first-order phase transition at a critical transition pressure Pc between 5.21 and 5.25 GPa at room temperature. The transition is accompanied by a pronounced discontinuity in the volume and β angle, which decrease by 3.7% and 1.0%, respectively. In particular the volume discontinuity at Pc (≈5.23±0.02 GPa) is extremely large relative to the entire compression of −ΔV/V0=−4.1% of the P21/n structure up to the transition point. Equivalent discontinuities with pronounced steps in the relative length of lattice vectors are displayed for the a, b and c parameters, which correspond to relative length changes by −1.3, −2.7 and −5.6%, respectively. The structural transformation at the above reported Pc was achieved under compression, i.e. increasing the pressure from one to the subsequent pressure point. Nevertheless, pressure release on decompression for the sample loadings prepared for structural investigations yield the high-P phase pertained to pressures as low as 4.51 GPa and show a hysteresis between compression and decompression within a pressure interval of at least 0.70 GPa ( Fig. 5). No evidences of coexistence of both the low-pressure and high-pressure phases were found as instead rarely reported for previous clinopyroxene compounds, i.e. LiGaSi2O6 .