Paradox of Peroxy Defects and Positive Holes in Rocks - Part I: Effect of Temperature

Abstract

Though ubiquitous in minerals of igneous and high-grade metamorphic rocks, peroxy defects have been widely overlooked in the past. The charge carriers of interest are positive holes, chemically equivalent to O$^-$ in a matrix of O$^{2-}$, physically defect electrons in the O$^{2-}$ sublattice, highly mobile, able to propagate fast and far. O$^-$ are oxidized relative to O$^{2-}$. As such O$^-$ are not supposed to exist in minerals and rocks that come from deep within the Earth crust or upper mantle, where the environments are overwhelmingly reduced. In order to understand how peroxy defects are introduced, we look at peroxy defects in a crystallographically and compositionally well characterized model system: single crystals of nominally high-purity MgO, grown from the melt under highly reducing conditions. During crystallization the MgO crystals incorporate OH$^-$ through dissolution of traces of H$_2$O into the MgO matrix, leading to a solid solution (ss) Mg$_{1-\delta}$(OH)$_{2\delta}$O$_{1-2\delta}$, where $\delta$ <1. During cooling, ss turns into a metastable supersaturated solid solution (sss). During further cooling, OH$^-$ pairs at Mg$^{2+}$ vacancy sites rearrange their electrons, undergoing a redox conversion, which leads to peroxy anions, O$_2^{2-}$, plus molecular H$_2$. Being diffusively mobile, the H$_2$ can leave their Mg$^{2+}$ vacancy sites, leaving behind cation-deficient Mg$_{1-\delta}$O. During reheating O$_2^{2-}$ break up, releasing positive hole charge carriers, which affect the electrical conductivity behavior. In rocks, similar changes in the electrical conductivity are observed in the temperature window, where peroxy defects of the type O$_3$Si-OO-SiO$_3$ break up. They release positive holes, which control the electrical conductivity response along the geotherm.