The high-temperature behaviour of riebeckite: expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles

The HT behaviour of a well-characterized sample of riebeckite has been examined by combining X-ray diffraction, FTIR and Mossbauer spectroscopy. The crystal-chemical composition of the crystal studied is: (A)(K0.05Na0.04) (B)(Na1.86Ca0.09Fe0.052+) (C)(Fe2.942+Mg0.222+Mn0.022+Zn0.012+Fe1.743+Al0.063+Ti0.014+) (T)(Si7.95Al0.05)O-22 (W)(OH1.9F0.10). The onset of the deprotonation process is detected at similar to 700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mossbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). FTIR analysis shows that a completely deprotonated oxo-amphibole is obtained after annealing at 893 K. Room-T single-crystal structure refinements of the deprotonated phase shows a very peculiar cation disorder, which has never been observed in amphiboles until now: there is significant depletion of B and C cations coupled with an increase in A cations, which implies the presence of vacancies at the M(3) and the M(4) sites in double-chain silicates. FTIR data collected at 873 K confirm both this conclusion and the onset of the cation exchange before completion of deprotonation. Axial and volume thermal expansion coefficients were determined in the T range 298-698 K for riebeckite (alpha(a) = 1.40(2).10(-5 )K(-1), alpha(b) = 0.67(1) . 10(-5) K-1, alpha(c) = 0.17(2) . 10(-5 )K(-1), alpha(beta) = -0.07(1) . 10(-5) K-1 and alpha(nu) = 2.27(2) . 10(-5) K-1) and in the 298-1173 K range for the oxo-amphibole (alpha(a) = 15.3(2) . 10(-5) K-1, alpha(b) = 0.77(1) . 10(-5 )K(-1), alpha(c) = 0.25(1) . 10(-5) K-1, alpha(beta) = 0.10(1) . 10(-5) K-1 and alpha(nu) = 2.52(2). 10(-5) K-1).