  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.
