Transition temperature and thermodynamic behaviour vs. crystal-chemical composition in clinopyroxenes

Many experimental studies are available which are aimed to characterise phase transitions in clinopyroxenes. Regarding the HT displacive P21/c ? C2/c transition, an inverse dependence has been found between the transition temperature (Tc) and either the aggregate ionic radius at the M1 site in M2Li-clinopyroxenes or the aggregate ionic radius at the M2 site in M2R2+-clinopyroxenes. However, no information is available for more complex compositions, such as those with heterovalent substitutions at both sites. We used X-ray single-crystal diffraction to monitor the above transition in a synthetic clinopyroxene with composition M2(Li0.85Mg0.09Fe2+0.06) M1(Fe3+0.85Mg0.15)Si2O6. The presence of 0.15 apfu of divalent cations at both sites makes the M1 site larger and the M2 site smaller than in ferrispodumene, LiFe3+Si2O6, for which Tc is -40 °C. The evolution of the structure has been studied during controlled "in-situ" annealing up to 500 °C, and subsequently during controlled cooling down to room temperature. Unit-cell parameters and selected pairs of adjacent reflections with h+k = 2n and h+k = 2n+1 were measured at each step (by 5 °C from 25 to 140 °C and by 10-20 °C at higher T). Collections of diffracted intensities in the ? range 2-27° were done at selected temperatures (25, 70, 95, 125, 150 and 200°C). The HT displacive P21/c ? C2/c transition occurs at 105 °C, and no hysteresis was observed in the reversal. On the contrary, some residual spontaneous strain is observed after Tc, which is probably related to the presence of microstructures. Intensity data were fitted with a 246 Landau potential, and the variation of Q with temperature was calculated. Our data provide the first example of a tricritical behaviour for this transition in clinopyroxenes, whereas previous data showed either discontinuous or continuous thermodynamic behaviour depending on composition. The observed Tc is ~150 °C higher than in ferrispodumene, whereas the changes in the aggregate ionic radii are very small (?<riM1> = 0.011 Å; ?<riM2> = -0.002 Å). Attempts were done to model the variation of Tc as a function of the aggregate ionic radius of both the M1 and M2 sites, but were pretty unsuccessful, also because many of the available experimental studies do not report individual site populations. Better results were however obtained when taking into account also the aggregate ionic charges at both sites.