Solubility behavior of δ-AlOOH and ε-FeOOH at high pressures

Abstract

Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth's surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ε-FeOOH with CaCl2-type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH–FeOOH binary system at 15–25 GPa and 700–1200 °C. X-ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ε-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl2-type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.

Elastic Properties of the Pyrite‐Type FeOOH‐AlOOH System From First‐Principles Calculations

“…This supposition is supported by the recent synthesis of pyrite‐type Fe 0.75 Al 0.25 OOH at 160 GP by Nishi et al. (2020) and the experimentally confirmed solid solution of CaCl 2 ‐type δ ‐(Al,Fe)OOH across a broad compositional range (Hsieh et al., 2019; Kawazoe et al., 2017; Nishi et al., 2019; Ohira et al., 2019; Su et al., 2020; Xu et al., 2019; H. Yuan et al., 2019). Importantly, aluminum substitution enhances the P ‐ T stability of similar hydrous phases, including the lower‐pressure CaCl 2 ‐type polymorph of this system (Ohira et al., 2014; Pamato et al., 2015; Sano et al., 2008).…”

“…These findings are presented alongside those of pyrite-type AlO 2 H, with which pyrite-type FeO 2 H likely forms a solid solution. This supposition is supported by the recent synthesis of pyrite-type Fe 0.75 Al 0.25 OOH at 160 GP by Nishi et al (2020) and the experimentally confirmed solid solution of Ca-Cl 2 -type δ-(Al,Fe)OOH across a broad compositional range (Hsieh et al, 2019;Kawazoe et al, 2017;Nishi et al, 2019;Ohira et al, 2019;Su et al, 2020;Xu et al, 2019;H. Yuan et al, 2019).…”

Partial Deoxygenation and Dehydration of Ferric Oxyhydroxide in Earth's Subducting Slabs

“…Recent high P-T experiments showed that FeOOH could form solid solutions with the CaCl 2 -type phases (e.g., δ-AlOOH and MgSiO 4 H 2 ) and/or α-PbO 2 structure phases (e.g., SiO 2 and TiO 2 ) over a wide composition range and then remain stable at higher temperatures (e.g., X. Liu et al, 2018;Nishi et al, 2019;Nishihara & Matsukage, 2016;Ohira et al, 2014;Panero & Caracas, 2017;Xu et al, 2019;Yuan et al, 2019). Therefore, as FeOOH is present in hydrous sediments, peridotite, and mid-ocean ridge basalt compositions, it may be thermodynamically stable in some very cold subducting slabs and likely transport water into the deep lower mantle.…”

“…; E. C. Thompson et al, 2020;Xu et al, 2019). FeOOH is one of the important hydrous phases in the subducting slabs of the Earth's mantle (e.g., Hu et al, 2016;Nishi et al, 2017), containing nominally 10.13 wt.% water.…”

High-pressure synthesis of ε-FeOOH from β-FeOOH and its application to the water oxidation catalyst

“…Among the three FeOOH polymorphs, b-FeOOH is signicantly less compressible as a function of pressure (at room temperature), owing to its high bulk modulus of 284(1) GPa. 42 The HP/HT phase of b-FeOOH is currently unknown, although Xu et al 43 reported that 3-FeOOH is a HP phase of b-FeOOH without any experimental evidence.…”