Fluorophlogopite Alters Under Heat, Raman Spectroscopy Shows

A team led by Christian Reinberg, Stylianos Aspiotis, Thomas Malcherek, Stefan T. M. Peters, and Boriana Mihailova from the University of Munich investigated how iron(II)-bearing fluorophlogopite changes when heated. Their work appeared in Eur. J. Mineral., vol. 38, pp. 461–475, 2026.¹

Fluorophlogopite (Ca₂Fe²⁺Si₆O₁₈F) is an orthopyroxene that can incorporate hydroxyl groups ((OH)⁻). By applying in situ high-temperature Raman spectroscopy to samples containing both (OH)⁻ and Fe²⁺, the authors monitored vibrational modes up to temperatures exceeding 1,300 K. The experiments revealed a phonon-driven structural instability around 600 K that triggers the mobilization of interlayer potassium ions (K⁺), which act as charge carriers during subsequent thermal events.¹

As temperature rose above 1,100 K, all hydrogen cations (H⁺) delocalized, contributing further to transport processes within the crystal lattice. Above 1,300 K, the study documented oxidation of Fe²⁺ to Fe³⁺ alongside limited dehydrogenation and dehydroxylation reactions, facilitating partial K⁺ leakage and structural decomposition.¹

The findings provide a detailed kinetic sequence for how iron-bearing fluorophlogopite responds to increasing heat: initial phonon activation at modest temperatures, followed by ion mobility and finally oxidative breakdown at extreme temperatures. This multistep pathway clarifies why fluorophlogopite is often absent in high‑temperature metamorphic assemblages despite its stability field at lower grades.¹