High-pressure P2(1)/c-C2/c phase transitions in clinopyroxenes: Influence of cation size and electronic structure

The high-pressure behavior of the clinopyroxenes kanoite Mn0.9Mg1.1Si2O6, M nSiO3, and CrMgSi2O6 (all space group P2(1)/c) was studied by single-crysta l X-ray diffraction in a diamond-anvil cell at room temperature. Phase tran sitions from P2(1)/c to C2/c clinopyroxene were found and reversed at 5.06 +/- 0.14 GPa for kanoite, 2.905 +/- 0.005 GPa in MnSiO3, and 3.60 +/- 0.03 GPa in CrMgSi2O6. The phase transitions are first-order in character and ar e accompanied by a volume decrease of approximately 2.3%. The structure of high-pressure C2/c kanoite was determined from single-crystal X-ray intensi ty data collected at 5.20 GPa, and is very similar to that of high-pressure (HP) C2/c-ferrosilite. Although the space group C2/c is the same as for th e high-temperature (HT) kanoite form, the two phases have significantly dif ferent structures. The silicate chains are extremely kinked in HP kanoite w hile they are almost straight in HT kanoite. Compared with the transition p ressure of clinoenstatite-clinofenosilite, the new data suggest that the ef fective ionic radii of M1 and M2 cations do not exclusively control the tra nsition pressure and that the HP C2/c clinopyroxenes with Cr2+ and Fe2+ gai n additional stabilization energy from crystal field effects.