Polarons in amphiboles: evidence from resonance Raman scattering and DFT calculations

The properties of rocks are determined from the physical properties of minerals composing the rock, which in turn ultimately depend on the crystal phonon modes and electron density of states. Amphiboles (AB2C5T8O22W2, C5 = M12M22M3) are among the major hydrous-silicate mineral constituents of the lithosphere, whose structure consists of strips of 6-membered TO4-rings attached to MO6 octahedra. To elucidate the atomistic origin of the rock conductivity observed in peculiar geological settings, we have studied single crystals of MFe-containing amphiboles from different subgroups by in situ high-temperature polarized Raman spectroscopy and density-functional-theory calculations of the electron band structure of grunerite (A¨Fe2+2Fe2+5Si8O22(OH)2) as a model pure-MFe2+ amphibole [1]. Above a certain temperature resonance Raman scattering occurs, due to the formation of small polarons arising from the coupling between polar optical phonons and electron transitions within Fe2+O6 octahedra, the mobility of which through the mineral bulk is strongly anisotropic. The FeO6-related polarons coexist with delocalized H+, that is, at elevated temperatures typical of lithosphere Fe-bearing amphiboles are conductive and exhibiting two types of charge carriers: electronic polarons and H+ cations, regardless of the oxygen fugacity. However, the presence of external oxygen favours the ejection of H+ and electrons from the crystal. Therefore, at lithospheric temperatures Fe2+-bearing amphiboles can be in a unique metastable state with an anisotropic surface-to-bulk gradient in the chemical potential. Same phenomena are expected for other rock-forming minerals sharing a similar structural feature with amphiboles: linked TO4-rings and MO6-octahedra.

[1] Mihailova et al. Commun. Mater. 2, 57/1-10 (2021)