Temperature-activated charge carriers in Fe2+-bearing glaucophane revealed by Raman spectroscopy

Anomalous high-conductivity layers (HCL) are a defining feature of subduction zones and play a critical role in global processes as water cycling, seismicity, and arc magmatism. Hydrous minerals, especially amphiboles (general formula AB₂C₅T₈O₂₂W₂), are considered to be major contributors to these conductivity anomalies in the Earth crust, but the atomic-scale links between mineral oxidation, charge transport, and thermal stability are still not fully understood. Given their crustal abundance, the oxidation of Fe²⁺-bearing hydrous amphiboles has therefore been extensively investigated in recent years. Electrical conductivity measurements on several amphibole species have suggested the formation of polarons (conduction electrons coupled with longitudinal optical phonons) at high temperatures (HT). However, only recently direct evidence for the existence of polarons in Fe²⁺-bearing amphiboles has been provided by Raman spectroscopy [1,2,3]. In addition, reversible delocalisation of H⁺ cations has been detected [1,3,4].

Glaucophane (□Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂) is a common amphibole in blueschist facies rocks; therefore, its HT behaviour should play an important role in geological processes occurring in subduction zones. The aim of our study is to elucidate the role of ^CAl in the formation of charge carriers in glaucophane at HT. We have analysed Fe²⁺-bearing glaucophane [(□_0.91Na_0.08K_0.01)(Na_0.87Fe_0.07Ca_0.06)₂(Mg_0.54Al_0.34Fe_0.12)₅(Si_0.99Al_0.01)₈O₂₂((OH)_0.98F_0.02)₂] from Pollone (Piedmont, Italy) by in situ HT Raman spectroscopy in air [5] and under vacuum. The results indicate that, similar to ^CAl-free amphiboles, glaucophane undergoes a multi-step process controlled by structural anisotropy and cation site occupancy. The first Fe²⁺ oxidation stage occurs between ~600 and 650 K. This stage is accompanied by shifts in Raman peaks linked to TO₄ ring vibrations, indicating electron delocalization and topological reorganization of the tetrahedral framework, together with changes in the anisotropic local structure. A second oxidation stage develops between ~800 and 1000 K, where Fe²⁺ oxidation is localised in M(1) polyhedra. This stage is marked by strong intensity reductions in low-frequency lattice modes and further rearrangement of the TO₄ rings. The final stage, preceding decomposition, occurs between ~950 and 1150 K and involves progressive H⁺ cations delocalization from OH groups, with incomplete recovery upon cooling, evidencing irreversible structural changes.

Under vacuum (~10^-4 bar), both the oxidation stages and H⁺ delocalization occur at temperatures approximately 100 K higher than in air, indicating that the absence of oxygen raises the temperature required for these processes to take place. Hence, octahedrally coordinated Al does not suppressed the temperature-induced electron and H⁺ delocalization in either presence or absence of external O₂.

Results from ongoing electrical-conductivity experiments combined with in situ Raman spectroscopy at different temperatures will also be discussed.

 

References

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

[2] Mihailova et al. (2022) Condens Matter 7, 68

[3] Bernardini et al. (2025) Sci Rep 15, 14244

[4] Della Ventura et al. (2017) Am Min 102(1), 117-125

[5] Kei Yin Ngan (2025) Master thesis: Oxidation and thermal decomposition of Fe-containing glaucophane. University of Hamburg, Germany