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Rapid intracrystalline exchange of octahedrally-coordinated divalent cations in amphiboles: an in situ high-temperature neutron diffraction study of synthetic potassic richterite AKB(NaCa)C(Mg2.5Ni2.5)Si8O22(OH)2

Published online by Cambridge University Press:  05 July 2018

M. D. Welch ,
J. J. Reece  and
S. A. T. Redfern
M. D. Welch
Affiliation:
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK
J. J. Reece
Affiliation:
Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK
S. A. T. Redfern
Affiliation:
Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK
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Abstract

The distribution of Mg and Ni over the octahedrally-coordinated sites M(l,2,3) in Ni-substituted potassic richterite solid solution, AKB(NaCa)c(Mg2.5Ni2.5)Si8O22(OH)2, synthesized at 0.1 GPa/750°C (25 days), has been studied in situ to 700°C by neutron powder diffraction with Rietveld structure refinement. Using a 2.7 g sample it was possible to make short data collections (3 h) at each temperature at intervals of 50° from 50 to700°C and locate the onset of cation exchange. Above 700°C the amphibole decomposes rapidly and so only an on-heating dataset was collected. Unit-cell parameters increase smoothly upon heating, with no discontinuities evident. Site occupancies of M(l), M(2) and M(3) sites were refined from site-scattering values. The initial XNi values of M(l), M(2) and M(3) sites, corresponding to the synthesis temperature of 750°C, are 0.57(1), 0.34(1) and 0.68(2), respectively. Above 400°C, there is initial ordering whereby thermal annealing allows the disordered Mg-Ni distribution of the quenched synthesis product to adjust to be closer to those appropriate for lower temperatures of the heating sequence. This ordering involves the exchange of Mg and Ni between M(l) and M(2), with no significant change in M(3) occupancy. At 700°C, XNi values of the M(l), M(2) and M(3) sites are 0.64(1), 0.27(1) and 0.69(2), respectively. No disordering was observed, due to the short duration of the high-temperature data collections and decomposition at 750°C. The results of this study indicate that divalent cations exchange between octahedral sites in amphiboles in a matter of hours.

Keywords

amphibolesneutron diffractionrichteriteRietveld refinement
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 4 , August 2008 , pp. 877 - 886
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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