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Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach

Experimentally, silica activity (aSiO(2)) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO(2) and aluminium content (the main contaminant in some recent experimental studies), and their co-eff...

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Autores principales: Muir, Joshua M. R., Jollands, Michael, Zhang, Feiwu, Walker, Andrew M.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: Springer Berlin Heidelberg 2020
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7686175/
https://www.ncbi.nlm.nih.gov/pubmed/33268914
http://dx.doi.org/10.1007/s00269-020-01123-5
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author Muir, Joshua M. R.
Jollands, Michael
Zhang, Feiwu
Walker, Andrew M.
author_facet Muir, Joshua M. R.
Jollands, Michael
Zhang, Feiwu
Walker, Andrew M.
author_sort Muir, Joshua M. R.
collection PubMed
description Experimentally, silica activity (aSiO(2)) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO(2) and aluminium content (the main contaminant in some recent experimental studies), and their co-effect, on Mg diffusion in forsterite, using thermodynamic minimisations of defect formation energies [calculated using density functional theory (DFT)] and a Monte-Carlo diffusion model. These two variables, in isolation, do not appreciably change the defect concentrations of forsterite and thus do not affect the diffusivity of Mg. However, when elevated together, they cause large increases in the Mg vacancy content and thus can increase the Mg diffusivity by one to six orders of magnitude depending on temperature, with little pressure dependence. This effect is largely independent of Al(2)O(3) concentration above ~ 1 wt. ppm, and thus, for all practical purposes, should occur wherever forsterite is in the presence of enstatite. It is also largely dependent upon configurational entropy and is thus highly sensitive to the chemistry of the crystal. A low concentration of structurally bound hydroxyl groups at low temperatures (1000 K) suppresses this effect in pure forsterite, but it is likely robust in the presence of water either when alternative water sinks (such as Ti or Fe) are present, or at high temperatures (> 1500 K). This effect is also robust in the presence of ferrous iron (or other substitutional Mg defects) at all temperatures. Fe(2)O(3) can operate like Al(2)O(3) in this reaction and should enhance its effect. These findings explain the experimentally observed dependency of Mg diffusion of aSiO(2), and elucidate how chemical activity variations in both experiments and natural settings could affect not only the diffusivity of Mg in forsterite, but of olivine-hosted cations in general. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00269-020-01123-5) contains supplementary material, which is available to authorized users.
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spelling pubmed-76861752020-11-30 Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach Muir, Joshua M. R. Jollands, Michael Zhang, Feiwu Walker, Andrew M. Phys Chem Miner Original Paper Experimentally, silica activity (aSiO(2)) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO(2) and aluminium content (the main contaminant in some recent experimental studies), and their co-effect, on Mg diffusion in forsterite, using thermodynamic minimisations of defect formation energies [calculated using density functional theory (DFT)] and a Monte-Carlo diffusion model. These two variables, in isolation, do not appreciably change the defect concentrations of forsterite and thus do not affect the diffusivity of Mg. However, when elevated together, they cause large increases in the Mg vacancy content and thus can increase the Mg diffusivity by one to six orders of magnitude depending on temperature, with little pressure dependence. This effect is largely independent of Al(2)O(3) concentration above ~ 1 wt. ppm, and thus, for all practical purposes, should occur wherever forsterite is in the presence of enstatite. It is also largely dependent upon configurational entropy and is thus highly sensitive to the chemistry of the crystal. A low concentration of structurally bound hydroxyl groups at low temperatures (1000 K) suppresses this effect in pure forsterite, but it is likely robust in the presence of water either when alternative water sinks (such as Ti or Fe) are present, or at high temperatures (> 1500 K). This effect is also robust in the presence of ferrous iron (or other substitutional Mg defects) at all temperatures. Fe(2)O(3) can operate like Al(2)O(3) in this reaction and should enhance its effect. These findings explain the experimentally observed dependency of Mg diffusion of aSiO(2), and elucidate how chemical activity variations in both experiments and natural settings could affect not only the diffusivity of Mg in forsterite, but of olivine-hosted cations in general. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00269-020-01123-5) contains supplementary material, which is available to authorized users. Springer Berlin Heidelberg 2020-11-24 2020 /pmc/articles/PMC7686175/ /pubmed/33268914 http://dx.doi.org/10.1007/s00269-020-01123-5 Text en © The Author(s) 2020 Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
spellingShingle Original Paper
Muir, Joshua M. R.
Jollands, Michael
Zhang, Feiwu
Walker, Andrew M.
Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title_full Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title_fullStr Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title_full_unstemmed Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title_short Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach
title_sort explaining the dependence of m-site diffusion in forsterite on silica activity: a density functional theory approach
topic Original Paper
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7686175/
https://www.ncbi.nlm.nih.gov/pubmed/33268914
http://dx.doi.org/10.1007/s00269-020-01123-5
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