Atomistic insights into redox processes and conductivity phenomena in Fe-bearing hydrous silicates

• Applicant: Professorin Dr. Boriana Mihailova
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 553088160

To understand key phenomena near the subduction zones, including water cycling and redox processes, we need to understand the properties of rocks and therefore, the properties of the constituent minerals under non-ambient conditions. In fact, the redox processes consist of electron cycling and hence, they can significantly contribute to the well-known and yet poorly understood lithospheric conductivity. Among various rock-forming minerals, iron-bearing hydrous silicates are of particular importance because they can oxidize and dehydrogenize without a structural collapse, that is, they can contribute to both water and electron cycling, delivering both H+ and e- in the lithospheric rocks. Among the complex hydrous silicates, amphiboles, biotite and chlorite deserve special attention, as they are chief constituents of the continental crust in the subduction zones and important sources of water in metamorphic processes. However, the research is commonly focused on the final oxidation and/or dehydrogenation state, when electrons and hydrogen are ejected from the mineral, and the preceding processes of delocalization and mobilization of e- and H+ inside the mineral bulk are ignored. The role of atomic dynamics to trigger the delocalization and subsequent ejection of e- and H+ is also neglected. Moreover, polaron conductivity has been suggested to exist in amphiboles and phyllosilicates but never directly evidenced. Only recently, by using in situ high-temperatures polarized Raman spectroscopy, a method sensitive to both electron and phonon excitations, we have provided a direct proof for the formation of thermally activated small polarons in Al-free amphiboles, arising from the coupling between polar optical phonons and electron transitions within Fe2+O6 octahedra, as well as for the formation of mobile, delocalized H+ inside the crystal bulk. The aim of this project is to further explore the redox and transport processes in hydrous silicates by (i) studying the atomistic mechanism of formation of thermally activated polarons and mobile H+ cations in presence and absence of external O2 for Fe2+-bearing hornblende, biotite and chlorite as well as the atomic dynamics triggering the thermal decomposition of these minerals and (ii) analyzing the contribution of different types of charge carriers to the anisotropic conductivity of representative Fe2+-bearing amphibole, biotite, and chlorite mineral species.

• DFG Programme: Research Grants