Temperature-induced structural transformations in fluorophlogopite studied by in situ high-temperature Raman spectroscopy

Phlogopite (nominally ^AK^MMg₃^T(AlSi₃)O₁₀^X(OH)₂) is the magnesian trioctahedral endmember of the biotite solid-solution series. This phyllosilicate can accommodate substantial amounts of fluorine, usually up to 5 weight percent (wt%) and up to 8.7 wt% in extreme cases (Gianfagna et al., 2007), which substitutes for hydroxyl (OH) groups at the anionic site. Given that phlogopite is a commonly found hydrous silicate, acting as an important F reservoir in ultramafic and ultrapotassic upper mantle lithologies and an accessory mineral in various igneous and metamorphic rocks, it can contribute to the Earth’s volatile cycles. Therefore, the deeper understanding of the temperature-induced phlogopite breakdown into pyrope and forsterite (Trønnes, 2002) and the role of F in the crystal structure of fluorophlogopite during its structural collapse can provide valuable information in several Earth’s dynamic systems. For instance, in metasomatic processes in the peridotitic mantle wedge of subduction zones, mineral stability at upper mantle conditions, and complex volcanic systems. Additionally, the high-temperature atomic dynamics of complex hydrous silicates containing Fe²⁺, characterized by considerable structural anisotropy, as studied by Raman spectroscopy, can offer valuable information about the thermal activation of charge carriers (delocalized H⁺ and e^-), thus about lithosphere conductivity anomalies (Bernardini et al., 2023).

 

This study focuses on the temperature-induced changes in the local structure and atomic vibrations of Fe²⁺-containing fluorophlogopite. For this purpose, a fluorophlogopite sample from Cardiff, Ontario in Canada was subjected to in situ Raman spectroscopy in the temperature range of 300-1450 K. The exact chemical formula of this mineral specimen was determined by wavelength-dispersive electron microprobe analysis (EMPA) and it is ^A(K_0.93Na_0.06Ba_0.01)^M(Mg_2.81Fe²⁺_0.15Ti_0.01Mn_0.01)^T(Si_2.99Al_0.99Ti_0.02)O₁₀^X(F_1.60OH_0.40). We show that fluorophlogopite undergoes stepwise structural and chemical changes, which can be monitored by the evolution of the Raman-active phonon modes at ~93 cm^-1 (interlayer vibrations), 195, 279, and 325 cm^-1 (dominated by octahedral vibrations) as well as at 684 and 739 cm^-1 (TO₄-ring mode vibrations). Near 500-600 K an interlayer structural instability occurs, which most probably results in a rearrangement of the layer stacking sequence. This activates the mobility of K⁺ cations in the interlayer space in the temperature range between 600 and 1000 K. Two independent heating-cooling runs to 1100 and 1450 K indicate a partial loss of K⁺ above 1000 K, which was confirmed by subsequent WD-EMPA. Oxidation of ^MFe²⁺ takes place between 900 and 1300 K. A partial thermal decomposition of fluorophlogopite occurs above 1300 K, leading to the formation of a minor amount of nanosized forsterite within the phlogopite matrix, but the overall biotite structure type persists up to 1450 K. 

 

References

• Bernardini, G. Della Ventura, J. Schlüter, B. Mihailova, Geochem. 2023, 83, 125942.
• Gianfagna, F. Scoradri, S. Mazziotti-Tagliani, G. Ventruti, L. Ottolini, Am Mineral. 2007, 92, 1601.
• R.G. Trønnes, Mineral Petrol. 2002, 74, 129.