HT-XRD study of synthetic ferrian magnesian spodumene:: the effect of site dimension on the P21/c→C2/c phase transition

Ferrian magnesian spodumene was synthesized in the MLFSH system at P = 0.4 GPa, T = 700 degreesC, fO(2) = NNO + 2.3. The space group at room T is P2(1)/c [a = 9.638(3) Angstrom, b = 8.709(2) Angstrom, c = 5.258 (2) Angstrom, beta = 109.83 (3)degrees, V = 415.2 Angstrom(3)]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi2O6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding (M2)(Li0.85Mg0.09Fe0.062+) (M1)(Fe0.853+Mg0.15)Si2O6. Li is ordered at the M2 site and Fe3+ is ordered at the M1 site, whereas Mg (and Fe2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 degreesC) allowed characterization of a reversible displacive P2(1)/c --> C2/c transition at 106 degreesC. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is -44 degreesC. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P2(1)/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes.