Electron Exchange and Conduction in Nontronite from First-Principles

Abstract

Fe-bearing clay minerals serve as an important source and sink for electrons in redox reactions in various subsurface geochemical environments. We apply first-principles calculations using a small polaron hopping approach and Marcus electron transfer theory to examine electron exchange mobilities in an Fe-rich smectite, nontronite Fe2Si4O10(OH)2. GGA+U calculations provide rates of electron hopping that agree very well with values deduced from variable-temperature Mossbauer data (Schaefer et al. Environ. Sci. Technol.2011, 45, 540), indicating a surprisingly fast electron mobility at room temperature. Evaluation of the electron transfer (ET) rates within the Hartree–Fock cluster framework for the Fe2+/Fe3+ electron hopping in tetrahedral (TS) and octahedral sheets (OS), as well as across the sheets (TS–OS), shows that the dominant contribution to the bulk electronic conductivity should come from the ET within the OS. Deprotonation of structural OH groups mediating ET between the Fe ions in the OS is found...