Atomistic insights into redox processes and conductivity phenomena in Hornblende

To date, processes such as volatile-element cycling and redox reactions in subduction and the mid-crust zones are still not fully understood. Particularly, a better understanding of the redox processes in rock-forming silicate minerals is essential for developing a more accurate picture of lithospheric electrical conductivity. Amphiboles (AB₂C₅T₈O₂₂W₂) are major constituents of subduction-zone lithologies and can store substantial amounts of water in the form of W-site hydroxyl groups, making them important contributors to the global water cycling. Recent Raman scattering studies have shown that Fe²⁺-bearing hydrous amphiboles can undergo reversible temperature-induced oxidation and dehydrogenation, leading to the formation of mobile charge carriers, polarons (delocalized e⁻ coupled with polar phonons) and delocalized H⁺, and hence, to polaronic conductivity and H⁺ diffusion (Della Ventura et al., 2018; Mihailova et al., 2022; Bernardini et al., 2025).

Magnesio-hornblende (nominally ^A⧠^BCa₂^C(Mg₄Al)^T(Si₇Al)O₂₂^W(OH)₂) is of special interest in this context because it represents one of the most abundant amphibole groups, the hornblendes (^TAl-containing Ca-amphiboles), but the influence of its tetrahedrally coordinated Al on the redox processes remains largely unexplored. Thus, the goal of this study is to investigate the atomistic mechanisms of charge-carrier activation and thermal stability in magnesio-ferri-hornblende by in situ high-temperature Raman spectroscopy in the range 300–1400 K. The exact chemical composition of the studied sample was determined by wavelength-dispersive electron microprobe analysis:

^A(Na_0.06K_0.01)^B(Ca_1.94Na_0.03Mn_0.03)^C(Mg_3.54Fe²⁺_0.8Fe³⁺_0.54Mn²⁺_0.11Zn_0.02Cr_0.001)^T(Si_7.42Al_0.51Fe_0.06Ti_0.01)O₂₂^W((OH)_1.92F_0.05O_0.02Cl_0.01). Experiments were conducted under both oxidizing conditions (air) and vacuum (~ 10^-4 bar) to evaluate the role of external O₂ on the activation temperatures and reversibility of these processes.

First results obtained in air reveal that magnesio-ferri-hornblende is stable up to 1400 K. The observed temperature-induced anomalies in both framework vibrations and OH-stretching indicate the onset of oxidation of Fe²⁺ to Fe³⁺ coupled with delocalization of H⁺ next to Fe²⁺Fe²⁺Mg and Fe²⁺MgMg chemical species. These processes are expressed by the disappearance of the corresponding OH-stretching Raman peaks upon heating and characteristic Fe²⁺O₆-related Raman-active modes. At temperatures above 1150 K even H⁺ cations next to MgMgMg, but the corresponding OH-stretching peaks reappear on cooling, indicating mobile H⁺ cations in a large temperature range. Furthermore, after cooling down to room temperature, a strong direction-dependent resonance Raman scattering (RRS) is observed, demonstrating strong mutual alignment of the polaron dipoles, which is a precondition of highly anisotropic polaronic conductivity. As a next step, in situ high-temperature Raman scattering experiments under an applied external electric field will be conducted, allowing for the simultaneous monitoring of temperature-induced electron–phonon coupling, H⁺ delocalization, and the evolution of electrical conductivity.

References:

• Della Ventura, G., Mihailova, B., Susha, U., Guidi, M. C., Marcelli, A., Schlüter, J., Oberti, R. (2018): Am. Mineral., 103, 1103 -1111, https://doi.org/10.2138/am-2018-6382
• Mihailova, B., Della Ventura, G., Waeselmann, N., Bernardini, S., Xu Wei, Marcelli, A. (2022): Condens. Matter, 7, 68, https://www.mdpi.com/2410- 3896/7/4/68
• Bernardini, S., Della Ventura, G., Hawthorne, F.C., Marcelli, A., Salvini, F., Mihailova, B., (2025): Sci. Rep., 15, 14244, https://doi.org/10.1038/s41598-025-98025-9