By applying in situ high-temperature Raman spectroscopy to (OH)^-- and Fe²⁺-bearing fluorophlogopite we show that a phonon-driven structural instability near 600 K triggers the mobilization of interlayer K⁺ cations, which can act as charge carriers. Above 1100 K all H⁺ cations delocalize and can also contribute to transport processes. Above 1300 K oxidation of Fe takes place, along with limited dehydrogenation and dehydroxylation, facilitating partial K⁺ leakage and structural decomposition.

This study explains how scientists determine whether a newly found solid phase is truly a natural mineral. We refine existing guidelines by introducing updated recommendations that improve the credibility of proposals for minerals with uncertain origins. Using three examples, we show how geological and textural evidence can confirm or challenge natural authenticity. Our new checklist helps reduce misinterpretation and ensures more reliable mineral recognition.

The thermal reaction Fe²⁺ + OH^- → Fe³⁺ + O^2- + e^- + H⁺ in hydrous silicates has tremendous implications in planetary-scale phenomena like electrical anomalies, water cycling, seismicity, volcanism, and ore generation. We annealed riebeckite crystals up to 750 °C and P = 0.7 GPa and show that the amphibole stability is extended without any Fe oxidation, implying that thermally activated electron hopping contributes to the electrical conductivity of lithospheric rocks during subduction.

We provide a tool for fast preparation-free estimation of the Fe³⁺ content in Al–Fe³⁺ series epidotes by Raman spectroscopy. The peaks near 570, 600, and 1090 cm⁻¹, originating from Si₂O₇ vibrations, strongly correlated with Fe content, and all three signals are well resolved in a random orientation. Among them, the 570 cm⁻¹ peak is the sharpest and easily recognized. Hence, the linear trend, ω₅₇₀ = 577.1(3) − 12.7(4)x, gives highly reliable Fe content, x, with accuracy ± 0.04 Fe³⁺ apfu.

Combined Raman-scattering and wavelength-dispersive electron microprobe (WD-EMP) analyses of natural biotites expanding over the whole biotite solid-solution series demonstrate that the chemical composition of the MO₆ octahedra, TO₄ tetrahedra, and interlayer space can be non-destructively determined by Raman spectroscopy with relative uncertainties below 8 %. The content of critical minor elements such as Ti at the octahedral site can be quantified as well with a relative error of ~ 20 %.