HR: 16:45h
AN: MR44A-04    [Abstracts]
TI: First Principles Simulation of the Energetics, Structure and Effect of the 'Titanium-Clinohumite' Defect in Forsterite
AU: * Walker, A M
EM: andrew.m.walker@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Berry, A J
MR44A-04 AF: Department of Earth Science and Engineering, South Kensington Campus, Imperial College London, London, SW7 2AZ United Kingdom
AU: Hermann, J
MR44A-04 AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: O'Neill, H S
MR44A-04 AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AB: Infrared spectroscopy (IR) of hydrogen bearing forsterite and olivine synthesised under differing chemical conditions show that the O-H stretching modes are controlled by the point defects present in the sample. Experiments where the silica activity was varied yield distinct infrared fingerprints that clearly distinguish when forsterite is buffered by MgO to form, in the simplest case, hydrated silicon vacancies from when forsterite is buffered by pyroxene to form hydrated magnesium vacancies[1,2]. This poses a problem for the interpretation of the spectra of natural mantle-derived samples as their most common fingerprint is more similar to that found in the MgO buffered experiments, despite the presence of pyroxene in the Earth's upper mantle. Recent experiments have shown that it is necessary to introduce trace amounts of titanium to reproduce exactly the infrared fingerprint of mantle olivines, and that this titanium fingerprint is present whether the sample is buffered by MgO or pyroxene[3]. In this work, we use first principles calculations to predict the structure of this titanium-bearing hydrous defect and assess its likely impact on the physical properties of mantle olivine. Calculations utilising the SIESTA[4] methodology for the implementation of density functional theory allow the energies and structures of titanium and hydrous defects in forsterite to be probed. This approach, which makes use of a numerical basis of a linear combination of atomic orbitals to describe the valence electrons, and pseudopotentials to describe the core electrons and nuclei, allows the large number of possible defect configurations to be examined efficiently. The calculations indicate that: (i) The most stable configuration for titanium in anhydrous forsterite is a direct substitution for silicon; this is supported by XANES and EXAFS data. (ii) The most stable configuration for cations and hydrogen in titanium-free forsterite coexisting with pyroxene is the formation of hydrated magnesium vacancies on the smaller of the two magnesium sites. (iii) The reaction of titanium and the hydrated cation vacancy is thermodynamically favoured, and results in the formation of a cluster of point defects with six-coordinated titanium on a magnesium site adjacent to a vacant silicon site containing two hydrogen ions. The existence of this configuration is supported by XANES and IR data. The calculations provide an atomic scale view of the naturally occurring hydrogen defect in olivine, which permits conclusions about the effect of hydrogen on, for example, electrical conductivity and cation mobility, to be assessed. References: [1] Matveev, O'Neill, Ballhaus, Taylor and Green (2001), J. Petrol., 42 721-729; [2] Lemaire, Kohn and Brooker (2004), Contrib. Miner. Petrol., 147 48-57; [3] Berry, Hermann, O'Neill and Foran (in press), Geology; [4] Soler, Artacho, Gale, Garcí-a, Junquera, Ordejón and Sánchez-Portal (2002), J. Phys. Conden. Mat., 14 2745-2776.
DE: 3904 Defects
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3939 Physical thermodynamics
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005