The high-temperature behaviour of riebeckite

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Marco E. Ciriotti
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The high-temperature behaviour of riebeckite

Messaggio da Marco E. Ciriotti » mer 10 gen, 2018 16:16

Prossima pubblicazione.

▪ Oberti, R., Boiocchi, M., Zema, M., Hawthorne, F.C., Redhammer, G.J., Susta, L., Della Vebtura, G. (2018): The high-temperature behaviour of riebeckite: expansivity, deprotonation, selective Fe oxidation and a novel cation disordering scheme for amphiboles. European Journal of Mineralogy, 30, (in press).


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+0.06Ti3+0.01) T(Si7.95Al0.05) O22W (OH1.9F-0:10). The onset of the deprotonation process is detected at ∼700K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623K 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 873K 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 298–698K for riebeckite (αa = 1.40(2) · 10-5K-1, αb = 0.67(1) · 10-5K-1, αc = 0.17(2) · 10-5K-1, αβ =-0.07(1) · 10-5K-1 and αV = 2.27(2) · 10-5K-1) and in the 298–1173K range for the oxo-amphibole (αa = 1.53(2) · 10-5K-1, αb = 0.77(1) · 10-5K-1, αc = 0.25(1) · 10-5K-1, αβ = 0.10(1) · 10-5K-1 and αV = 2.52 (2) · 10-5K-1).
Marco E. Ciriotti

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