Ion migration in nepheline: a dielectric spectroscopy and computer modelling study |
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| Authors: | A Jones D Palmer M S Islam M Mortimer |
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| Institution: | (1) Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: A.Jones2@open.ac.uk Tel.: +44 01908 655947; Fax: +44 01908 655151, GB;(2) Department of Chemistry, University of Surrey, Guildford, GU2 5XH, UK, GB;(3) Department of Chemistry, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK, GB |
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| Abstract: | Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of the
loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielectric properties of a natural Bancroft nepheline
have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric
loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation
in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 ± 0.02 eV.
Atomistic simulation techniques were used to elucidate the mechanism and energetics of cation migration. A mechanism involving
the hopping of Na+ ions between oval sites and partially occupied hexagonal (K+) sites, via a bottleneck consisting of a distorted sixfold ring of (Al,Si)O4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined activation
energy. These results confirm the nature of the process responsible for the observed dielectric behaviour. Overall, this study
demonstrates the intrinsic, microscopic control of cation diffusion processes in rock-forming minerals. Identifying specific
energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics.
Received: 8 May 2000 / Accepted: 21 July 2000 |
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| Keywords: | Nepheline Dielectric spectroscopy Computer modelling Ionic mobility |
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