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Ab initio crystal structure and elasticity of tuite, γ-Ca3(PO4)2, with implications for trace element partitioning in the lower mantle

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Abstract

Tuite forms by the breakdown of apatite at high pressure and is thus expected to play a role in extending the phosphorus cycle beyond the stability field of apatite and into the lower mantle. With its large, high-coordination cation sites, tuite is thought to be able to dissolve large quantities of incompatible elements such as rare earth elements, Sr, Th, and U, and is potentially an important mantle reservoir for these elements. In this paper, ab initio calculations of the structure and elasticity of tuite to lower mantle pressure are presented and used to probe trace element incorporation. The calculated zero-pressure volumes of the M1 and M2 cation sites were 50.23 and 36.61 Å3, while the corresponding bulk moduli K 0 are 116.1 and 94.2 GPa, significantly lower than the 234.1 GPa calculated for the M site of CaSiO3 perovskite (cpv), another likely host for incompatible elements in the mantle. The partitioning of impurities between tuite and cpv is investigated using a lattice strain model, parameterized by the ab initio calculations, to calculate isovalent substitution energies across a range of pressures and impurity sizes. Additionally, energies of strontium and barium defects in tuite are compared with those of equivalent defects in cpv, and it is found that both elements will partition strongly from cpv into tuite.

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Acknowledgements

Calculations were performed on the Terrawulf cluster, a computational facility supported through the AuScope initiative. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme. Ian Jackson, Mark Ghiorso, and an anonymous reviewer are thanked for their helpful comments. AMW is grateful for support from the UK Natural Environment Research Council (NE/K008803/1 and NE/M000044/1). RS is supported by an Australian Government Research Training Program (RTP) Scholarship.

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Correspondence to Richard Skelton.

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Communicated by Mark S Ghiorso.

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Skelton, R., Walker, A.M. Ab initio crystal structure and elasticity of tuite, γ-Ca3(PO4)2, with implications for trace element partitioning in the lower mantle. Contrib Mineral Petrol 172, 87 (2017). https://doi.org/10.1007/s00410-017-1406-5

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  • DOI: https://doi.org/10.1007/s00410-017-1406-5

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