黄铁矿型(Fe,Mg)O_(2)H晶体到超离子态相变

The transition from crystal to superionic phases for the pyrite-type(Fe,Mg)O_(2)H

地球深部的致密含水矿物可能以超离子态形式存在,其物理化学性质须重点考量。前人研究的黄铁矿型FeO_(2)H从有序晶体向超离子态转变时,H离子突破羟基束缚,在固态晶格中自由移动。然而,FeO_(2)H仅在地球深部少量存在,不能代表下地幔的组成。本文的研究体系为包含Mg在内的黄铁矿型(Fe,Mg)O_(2)H,它在下地幔深部的温压条件下通过铁方镁石和水的反应生成。通过第一性原理计算发现,它在高于80 GPa和1750 K时发生超离子态相变。超离子态(Mg,Fe)O_(2)H的转变条件与FeO_(2)H类似,但在低温区具有更高的质子扩散系数,因此更可能导致深部电导率异常。(Fe,Mg)O_(2)H比FeO_(2)H在下地幔的分布更为广泛,其物理、化学特征有助于示踪地球深部的水循环。

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 FeO_(2)H was occurring,H ions became mobile because of the break of conventional hydroxyl bond and moved freely in the solid lattice of FeO_(2).However,FeO_(2)H 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)O_(2)H 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)O_(2)H occurred at above 80 GPa and 1750 K.Such transition pressure of(Fe,Mg)O_(2)H is similar to those of FeO_(2)H.However,comparing to the FeO_(2)H,the(Fe,Mg)O_(2)H 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)O_(2)H should be more widely distributed than the FeO_(2)H in the lower mantle.Its physical and chemical properties will be useful to trace the water cycling in the deep earth.