Non-convergent ordering and displacive phase transition in pigeonite : in situ HT XRD study

A natural Ca-rich pigeonite (En47Fs43Wo10), free of augite exsolution products, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c (a = 9.719(7) Å, b = 8.947(9) Å, c = 5.251(3) Å, β = 108.49(5), V = 433.0(6) Å3), was annealed up to 1000°C to induce a phase transition from P21/c to C2/c symmetry. Complete single-crystal X-ray diffraction data collections were carried out in situ at 650, 750, 850 and 950°C after the crystal had reached equilibrium for the Fe-Mg intracrystalline exchange reaction at each temperature. The variation, with increasing temperature, of lattice parameters, of intensity of hkl reflections with h + k = 2n + 1 (which vanish at high temperature) and of some geometrical parameters from structure refinement, showed that the displacive phase transition P21/c ↔ C2/c was continuous in character. This contrasts with the first-order character for the HT phase transition in pigeonite containing significantly less calcium.