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 Mössbauer spectroscopy. The crystal-chemical composition of the crystal studied is: A(K0.05Na0.04) B(Na1.86Ca0.09Fe2+0.05) C(Fe2+2.94Mg2+0.22Mn2+0.02Zn2+0.01Fe3+1.74Al3+0.06Ti4+0.01) T(Si7.95Al0.05) O22 W(OH1.9F-0.10). The onset of the deprotonation process is detected at ?700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mössbauer 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 (?a = 1.40(2)·10-5 K-1, ?b = 0.67(1)·10-5 K-1, ?c = 0.17(2)·10-5 K-1, ?? = -0.07(1)·10-5 K-1and ?V = 2.27(2)·10-5 K-1) and in the 298-1173K range for the oxo-amphibole (?a = 1.53(2)·10-5 K-1, ?b = 0.77(1)·10-5 K-1, ?c = 0.25(1)·10-5 K-1, ?? = 0.10(1)·10-5 K-1and ?V = 2.52(2)·10-5 K-1). object Object]