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.