The P2₁/m ↔ C2/m phase transition in amphiboles: new data on synthetic Na(NaMg)Mg₅Si₈O₂₂F₂ and the role of differential polyhedral expansion

The P2₁/m ↔ C2/m displacive phase transition has been studied on a synthetic amphibole with unit formula ^ANa_0.90^B(Na_0.92Mg_1.08)^C(Mg_4.98V_0.02)^TSi₈O₂₂^O3F₂. The evolution of the unit-cell parameters and of the intensities of a set of super-lattice reflections was monitored in the T range 100–944 K. Polynomial fitting of a 24 Landau potential to the evolution of the order parameter with T yielded a critical temperature (T_c) of 395 ± 5 K (398 ± 6 when considering low-T saturation of the order parameter), and Landau coefficients compatible with second-order transition. The values of T_c and the coefficients for the phase transition obtained in this work are compared to those previously obtained for synthetic ^ANa_0.83^B(Na_0.83Mg_1.17)^CMg₅^TSi₈O₂₂^O3(OH)₂ and to other data available in the literature. The substitution of OH by F at the O(3) site does not change the character of the transition, but decreases the T_c by at least 132 K, showing that the size of the M(4) polyhedron is not the only significant factor, and that the size of the octahedra and/or their different thermal expansion also play a role. Single-crystal structure refinements done at 100, 160, 220, 280, 340, 406, 514, 621, 729, 836 and 944 K allowed monitoring the changes in the structure and in the shape of electron density, which occur approaching the transition and during further heating. Mean thermal expansion coefficients for the different polyhedra in the structure were calculated for the high-TC2/m phase, and are compared with those of other amphiboles with 5 Mg atoms per formula unit at the C-sites. The presence of F significantly affects both cation ordering within the A cavity and the thermal expansion along the b and c edges.