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

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The H T 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 (K0.05 Na0.04)B( Na1.86 Ca0.09 Fe2+ 0.05)C(Fe2+ 2.94Mg2+ 0.22Mn2+ 0.02Zn2+ 0.01Fe3+ 1.74Al3+ 0.06Ti4+ 0.01)T (Si7.95 Al0.05) O22 W(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,α b = 0.67(1) · 10–5K–1, α c = 0.17(2) · 10–5K–1, αβ = –0.07(1) · 10–5 K–1 and α V = 2.27(2) · 10–5K–1) and in the 298–1173 K range for the oxo-amphibole (α a = 1.53(2) · 10–5K–1, α b = 0.77(1) · 10–5K–1, α c = 0.25(1) · 10–5K–1, αβ = 0.10(1) · 10–5K–1 and α V = 2.52(2) · 10-5K-1).
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Keywords: AMPHIBOLE; CATION ORDER; CRYSTAL CHEMISTRY; CRYSTAL-STRUCTURE; DEPROTONATION; EXPANSIVITY; H T BEHAVIOUR; INFRARED SPECTROSCOPY; MÖSSBAUER; RIEBECKITE; SPECTROSCOPY

Document Type: Research Article

Publication date: June 1, 2018

This article was made available online on June 22, 2018 as a Fast Track article with title: "The high-temperature behaviour of riebeckite: expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles".

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  • The European Journal of Mineralogy publishes original papers, review articles and letters dealing with the mineralogical sciences s.l. These include primarily mineralogy, petrology, geochemistry, crystallography and ore deposits, as well as environmental, applied and technical mineralogy. Nevertheless, papers in any related field, including cultural heritage, will be considered.
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