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Elastic properties of the pyrite-type FeOOH\textendash AlOOH system from first principles calculations
  • Elizabeth Colette Thompson,
  • Andrew J. Campbell,
  • Jun Tsuchiya
Elizabeth Colette Thompson
Sewanee: The University of the South, Sewanee: The University of the South

Corresponding Author:ecthomps@sewanee.edu

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Andrew J. Campbell
University of Chicago, University of Chicago
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Jun Tsuchiya
Ehime University, Ehime University
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The stability, structure, and elastic properties of pyrite-type (FeS\textsubscript{2} structured) FeO\textsubscript{2}H were determined using density functional theory based computations with a self-consistent Coulombic self-interaction term (\textit{U\textsubscript{eff}}). The properties of pyrite-type FeO\textsubscript{2}H are compared to that of pyrite-type AlO\textsubscript{2}H with which it likely forms a solid solution at high temperature, as well as the respective lower pressure CaCl\textsubscript{2}-type polymorphs of both endmembers: $\epsilon$-FeOOH and $\delta$-AlOOH. Due to substantial differences in the CaCl\textsubscript{2}-type$\rightarrow$pyrite-type structural transition pressures of these endmembers, the stabilities of the (Al,Fe)O\textsubscript{2}H solid solution polymorphs are anticipated to be compositionally driven at lower mantle pressures. As the geophysical properties of (Al,Fe)OOH are structurally dependant, interpretations regarding the contribution of pyrite-type FeO\textsubscript{2}H to seismically observed features must take into account the importance of this broad phase loop. With this in mind, Fe-rich pyrite-type (Al,Fe)OOH may coexist with Al-dominant CaCl\textsubscript{2}-type $\delta$-(Al,Fe)OOH in the deep Earth. Furthermore, pyrite-type (Al\textsubscript{0.5-0.6},Fe\textsubscript{0.5-0.4})O\textsubscript{2}H can reproduce the reduced compressional and shear velocities characteristic of seismically observed Ultra Low Velocity Zones (ULVZs) in the Earth’s lowermost mantle while Al-dominant but Fe-bearing CaCl\textsubscript{2}-type $\delta$-(Al,Fe)OOH may contribute to Large Low Shear Velocity Provinces (LLSPs).