Titanium substitution mechanisms in forsterite

Andrew J. Berry; Andrew M. Walker; Jörg Hermann; Hugh St C. O'Neill; Garry J. Foran; Julian D. Gale

doi:10.1016/j.chemgeo.2007.03.010

Titanium substitution mechanisms in forsterite

Andrew J. Berry^*, Andrew M. Walker, Jörg Hermann, Hugh St C. O'Neill, Garry J. Foran, Julian D. Gale

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

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Abstract

Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, together with atomic scale calculations, indicate that Ti occupies the Si site in anhydrous forsterite. The energy and intensity of the 1s → 3d transition in the XANES spectrum is diagnostic of Ti on a four-coordinate site. The EXAFS spectra determine a Ti-O bond length of 1.81 ± 0.01 Å. This value is in excellent agreement with that of 1.8 Å determined computationally by periodic density functional theory and an embedded cluster approach for Ti on the Si site; the calculated bond length for Ti on a Mg site is 2.0 Å. Both computational methods further find that the direct substitution of Ti for Si is energetically favoured relative to substitution of Ti for Mg, charge balanced by either a Mg vacancy or Mg on a Si site. Together the results provide unambiguous evidence for Ti occupying the Si site in anhydrous forsterite. Ti-bearing olivine synthesised at upper mantle conditions in the presence of water, however, exhibits a pre-edge feature corresponding to six-fold coordination. This is consistent with the identification of a Ti-clinohumite-like point defect in samples of olivine from upper-mantle spinel peridotite. The change in Ti site thus provides a mechanism for the incorporation of water in olivine. The total Ti content of olivine will comprise varying contributions from Mg₂ ^[4]TiO₄ and Ti-clinohumite-like Mg^[6]TiO₂(OH)₂ substitutions.

Original language	English
Pages (from-to)	176-186
Number of pages	11
Journal	Chemical Geology
Volume	242
Issue number	1-2
DOIs	https://doi.org/10.1016/j.chemgeo.2007.03.010
Publication status	Published - 30 Jul 2007

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10.1016/j.chemgeo.2007.03.010

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@article{1cef8c7d91b6486c8799ca8de8603597,

title = "Titanium substitution mechanisms in forsterite",

abstract = "Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, together with atomic scale calculations, indicate that Ti occupies the Si site in anhydrous forsterite. The energy and intensity of the 1s → 3d transition in the XANES spectrum is diagnostic of Ti on a four-coordinate site. The EXAFS spectra determine a Ti-O bond length of 1.81 ± 0.01 {\AA}. This value is in excellent agreement with that of 1.8 {\AA} determined computationally by periodic density functional theory and an embedded cluster approach for Ti on the Si site; the calculated bond length for Ti on a Mg site is 2.0 {\AA}. Both computational methods further find that the direct substitution of Ti for Si is energetically favoured relative to substitution of Ti for Mg, charge balanced by either a Mg vacancy or Mg on a Si site. Together the results provide unambiguous evidence for Ti occupying the Si site in anhydrous forsterite. Ti-bearing olivine synthesised at upper mantle conditions in the presence of water, however, exhibits a pre-edge feature corresponding to six-fold coordination. This is consistent with the identification of a Ti-clinohumite-like point defect in samples of olivine from upper-mantle spinel peridotite. The change in Ti site thus provides a mechanism for the incorporation of water in olivine. The total Ti content of olivine will comprise varying contributions from Mg2 [4]TiO4 and Ti-clinohumite-like Mg[6]TiO2(OH)2 substitutions.",

keywords = "Density functional theory, EXAFS, Forsterite, Hydrous defects, Nominally anhydrous minerals, XANES",

author = "Berry, {Andrew J.} and Walker, {Andrew M.} and J{\"o}rg Hermann and O'Neill, {Hugh St C.} and Foran, {Garry J.} and Gale, {Julian D.}",

year = "2007",

month = jul,

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doi = "10.1016/j.chemgeo.2007.03.010",

language = "English",

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journal = "Chemical Geology",

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TY - JOUR

T1 - Titanium substitution mechanisms in forsterite

AU - Berry, Andrew J.

AU - Walker, Andrew M.

AU - Hermann, Jörg

AU - O'Neill, Hugh St C.

AU - Foran, Garry J.

AU - Gale, Julian D.

PY - 2007/7/30

Y1 - 2007/7/30

N2 - Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, together with atomic scale calculations, indicate that Ti occupies the Si site in anhydrous forsterite. The energy and intensity of the 1s → 3d transition in the XANES spectrum is diagnostic of Ti on a four-coordinate site. The EXAFS spectra determine a Ti-O bond length of 1.81 ± 0.01 Å. This value is in excellent agreement with that of 1.8 Å determined computationally by periodic density functional theory and an embedded cluster approach for Ti on the Si site; the calculated bond length for Ti on a Mg site is 2.0 Å. Both computational methods further find that the direct substitution of Ti for Si is energetically favoured relative to substitution of Ti for Mg, charge balanced by either a Mg vacancy or Mg on a Si site. Together the results provide unambiguous evidence for Ti occupying the Si site in anhydrous forsterite. Ti-bearing olivine synthesised at upper mantle conditions in the presence of water, however, exhibits a pre-edge feature corresponding to six-fold coordination. This is consistent with the identification of a Ti-clinohumite-like point defect in samples of olivine from upper-mantle spinel peridotite. The change in Ti site thus provides a mechanism for the incorporation of water in olivine. The total Ti content of olivine will comprise varying contributions from Mg2 [4]TiO4 and Ti-clinohumite-like Mg[6]TiO2(OH)2 substitutions.

AB - Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, together with atomic scale calculations, indicate that Ti occupies the Si site in anhydrous forsterite. The energy and intensity of the 1s → 3d transition in the XANES spectrum is diagnostic of Ti on a four-coordinate site. The EXAFS spectra determine a Ti-O bond length of 1.81 ± 0.01 Å. This value is in excellent agreement with that of 1.8 Å determined computationally by periodic density functional theory and an embedded cluster approach for Ti on the Si site; the calculated bond length for Ti on a Mg site is 2.0 Å. Both computational methods further find that the direct substitution of Ti for Si is energetically favoured relative to substitution of Ti for Mg, charge balanced by either a Mg vacancy or Mg on a Si site. Together the results provide unambiguous evidence for Ti occupying the Si site in anhydrous forsterite. Ti-bearing olivine synthesised at upper mantle conditions in the presence of water, however, exhibits a pre-edge feature corresponding to six-fold coordination. This is consistent with the identification of a Ti-clinohumite-like point defect in samples of olivine from upper-mantle spinel peridotite. The change in Ti site thus provides a mechanism for the incorporation of water in olivine. The total Ti content of olivine will comprise varying contributions from Mg2 [4]TiO4 and Ti-clinohumite-like Mg[6]TiO2(OH)2 substitutions.

KW - Density functional theory

KW - EXAFS

KW - Forsterite

KW - Hydrous defects

KW - Nominally anhydrous minerals

KW - XANES

UR - http://www.scopus.com/inward/record.url?scp=34250735135&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2007.03.010

DO - 10.1016/j.chemgeo.2007.03.010

M3 - Article

SN - 0009-2541

VL - 242

SP - 176

EP - 186

JO - Chemical Geology

JF - Chemical Geology

IS - 1-2

ER -

Titanium substitution mechanisms in forsterite

Abstract

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