High-pressure displacive phase transition of a natural Mg-rich pigeonite

A high-pressure single-crystal X-ray diffraction study has been carried out on a P21/c natural Mg-rich pigeonite sample with composition ca. Wo6En76Fs18 using a diamond anvil-cell. The unit-cell parameters were determined at 14 different pressures to 7.14 GPa. The sudden disappearance of the b-type reflections (h + k = odd) and a strong discontinuity (about 2.8%) in the unit-cell volume indicated a first-order P21/c-C2/c phase transition between 4.66 and 4.88 GPa. The P(V) data of the P21/c phase were fitted to 4.66 GPa by a third-order Birch-Murnaghan equation of state (BM3 EoS), whereas the limited number of experimental data collected within the C2/c phase between 4.88 and 7.14 GPa were fitted using the same equation of state but with K′ constrained to the value obtained for the P21/c fitting. The equation of state coefficients are V0 = 424.66(6) Å3, KT0 = 104(2) GPa and K′ = 8(1) for the P21/c phase, and V0 = 423.6(1) Å3, KT0 = 112.4(8) GPa, and K′ fixed to 8(1) for the C2/c phase. The axial moduli for a, b, and c for the P21/c phase were obtained using also a BM3-EoS, while for the C2/c phase only a linear calculation could be performed, and therefore the same approach was applied for comparison also to the P21/c phase. In general the C2/c phase exhibits axial compressibilities (βc > βa ≫ βb) lower than those of the P21/c phase (βb > βc ≈ βa; similar to those found in previous studies in clinopyroxenes and orthopyroxenes). The lower compressibility of the C2/c phase compared with that of the P21/c could be ascribed to the greater stiffness along the b direction. A previously published relationship between Pc and M2 average cation radius (i. r.) has been updated using all the literature data on P21/c clinopyroxene containing large cations at M2 site and our new data. The following weighted regression was obtained: Pc (GPa) = 26(4) - 28(5) × M2 i.r (Å), R2 = 0.97. This improved equation can be used to predict the critical pressure of natural P21/c clinopyroxene samples just knowing the composition at M2 site.