The transition from crystal to superionic phases for the pyrite-type(Fe,Mg)O2H
The dense hydrous minerals in Earth's deep interiors could exist in the form of superionic phase.Their physicochemical properties should be thoroughly studied.Previous works suggest that when the transition from ordered crystal to superionic solid states of pyrite-type FeO2H was occurring,H ions became mobile because of the break of conventional hydroxyl bond and moved freely in the solid lattice of FeO2.However,FeO2H is minor existence in Earth's deep interiors and could not represent the composition of lower mantle.Herein,we have extended the mineral system to the pyrite-type(Fe,Mg)O2H which was formed through the reaction between ferropericlase and water in Earth's deep lower mantle.By using first-principles calculation,We found that the superionic phase transition of(Fe,Mg)O2H occurred at above 80 GPa and 1750 K.Such transition pressure of(Fe,Mg)O2H is similar to those of FeO2H.However,comparing to the FeO2H,the(Fe,Mg)O2H has relatively higher proton diffusion coefficients at lower temperatures,and thus serves as a more probable candidate inducing the electric conductivity anomaly in Earth's deep interiors.The(Fe,Mg)O2H should be more widely distributed than the FeO2H in the lower mantle.Its physical and chemical properties will be useful to trace the water cycling in the deep earth.
deep part of the lower mantlehydrous mineralssuperionic stateelectrical conductivity anomaly
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