Antigorite is the high-temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in subducting oceanic lithosphere at the fore-arc. It has a wavy crystal structure along the a-axis and several polysomes with different $m$-values (m=13-24) have been identified in nature. The number (m) is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit mainly depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of temperature and pressure. However, it is not well understood which m-values of antigorite are stable at different temperature and pressure conditions. To investigate the pressure dependence of the stability of different m-values in antigorite, we performed first-principles calculations for several polysomes (m=14-19) at high pressure (0-14 GPa) and compared their enthalpies (T: static 0 K). We found that although the energy differences between polysomes are small, polysomes with larger m-values are more stable at ambient pressure, while polysomes with smaller m-values are more stable at elevated pressures. This suggests that the structure of antigorite in oceanic lithosphere that has subducted into the deep Earth may gradually evolve into a different polysome structure than antigorite samples observed at ambient or near-pressure conditions. This structural change may be related to the formation of the lower plane of the double seismic zone, as changes in polysome m-values are accompanied by a minor dehydration reaction.

The stability, structure, and elastic properties of pyrite-type (FeS2 structured) FeO2H were determined using density functional theory-based computations with a self-consistent Coulombic self-interaction term (Ueff). The properties of pyrite-type FeO2H are compared to that of pyrite-type AlO2H with which it likely forms a solid solution at high temperature, as well as the respective lower pressure CaCl2-type polymorphs of both endmembers: e-FeOOH and d-AlOOH. Due to substantial differences in the CaCl2-type to pyrite-type structural transition pressures of these endmembers, the stabilities of the (Al,Fe)O2H 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 FeO2H 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 CaCl2-type d-(Al,Fe)OOH in the deep Earth. Furthermore, pyrite-type (Al0.5-0.6,Fe0.4-0.5)O2H 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 CaCl2-type d-(Al,Fe)OOH may contribute to Large Low Shear Velocity Provinces (LLSPs).