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

A natural Ca-rich pigeonite (En(47)Fs(43)Wo(10)), free of augite exsolution products, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P2(1)/c (a = 9.719(7) Angstrom, b = 8.947(9) Angstrom, c = 5.251(3) Angstrom, beta = 108.49(5), V = 433.0(6) Angstrom(3)), was annealed up to 1000 degreesC to induce a phase transition from P2(1)/c to C2/c symmetry. Complete single-crystal X-ray diffraction data collections were carried out in situ at 650, 750, 850 and 950 degreesC 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 P2(1)/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.