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)O22W(OH1:9F0:10). 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 (αa = 1.40(2) · 10−5 K−1, αb = 0.67(1) · 10−5 K−1, αc = 0.17(2) · 10−5 K−1, αβ = −0.07(1) · 10−5 K−1and αV = 2.27(2) · 10−5 K−1) and in the 298–1173 K range for the oxo-amphibole (αa = 1.53(2) · 10−5 K−1, αb = 0.77(1) · 10−5 K−1, αc = 0.25(1) · 10−5 K−1, αβ = 0.10(1) · 10−5 K−1and αV = 2.52(2) · 10−5 K−1).