Atomistic insight into lithospheric conductivity revealed by phonon-electron excitations in hydrous iron-bearing silicates

Amphiboles are essential components of the continental crust and subduction zones showinganomalous anisotropic conductivity. Rock properties depend on the physical properties oftheir constituent minerals, which in turn depend on the crystal phonon and electron density ofstates. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, weapplied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon andelectron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling,reveals a direction-dependent formation of mobile polarons associated with coupled FeO6phonons and electron transitions. Hence, temperature-activated electron-phonon excitationsin hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropiclithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similartemperatures even in a reducing atmosphere. The occurrence of either type of charge carriersdoes not require initial mixed-valence state of iron or high oxygen fugacity in the system.