In situ high-temperature Raman spectroscopy for the thermal decomposition of Mn2+- and Fe2+-bearing talc

Talc (nominally Mg₃Si₄O₁₀OH₂) is a Mg-dominant trioctahedral layered silicate with empty interlayer space that can accommodate minor quantities of Fe²⁺ and Mn²⁺ at the octahedral sites, replacing Mg. Given that talc is a common hydrous silicate mineral, containing up to 5.5 weight percent of H₂O and commonly occurring under a variety of geological conditions, it can contribute to the volatile cycling in the vicinity of subduction zones. Therefore, the deeper understanding of the temperature-induced talc breakdown into Mg-rich pyroxene and the role of Fe and Mn in the crystal structure of talc during its structural collapse can provide valuable information in several fields of Geosciences such as metamorphic petrology, geophysics, and environmental sciences. Moreover, the information from the dehydroxylation and thermal decomposition of talc can be applied beyond Geosciences; in particular, to understand the weathering processes and firing conditions of soapstone-based cultural-heritage objects. Moreover, the high-temperature atomic dynamics of Fe²⁺-bearing hydrous silicates with strong structural anisotropy, as probed by Raman spectroscopy,  can shed light on the thermal activation of charge carriers (delocalized H⁺ and e^-) and hence, on conductivity anomalies in the lithosphere (Bernardini et al., 2023).

In this study, we present the results of the thermally-induced changes in Fe²⁺- and Mn²⁺-containing talc in the temperature range of 100-1400 K by considering the framework (15-1215 cm^-1) and OH-stretching vibrations (3400-3800 cm^-1). For this purpose, two samples from Tyrol in Austria were examined, whose exact chemical formulas, particularly (Mg_2.93Fe²⁺_0.08Ni_0.01Si_4.04OH_1.81O_0.19) and (Mg_2.95Mn²⁺_0.07Fe²⁺_0.01Si_4.06OH_1.69O_0.31), were determined by wavelength-dispersive electron microprobe analysis (Aspiotis et al., 2023). We show that structural transformations at an atomic-scale level occur at different temperatures for the Fe²⁺- and Mn²⁺-containing talc samples, as the Raman peak position of the TO₄ stretching vibration at ~1050 cm^-1 starts deviating from the linear dependence with elevated temperatures at 1250 and 1150 K, respectively. In addition, the in situ Raman heating/cooling experiments demonstrate that the OH-stretching mode at ~ 3660 cm^-1 associated with a MgMgFe²⁺ or MgMgMn²⁺ local configuration vanishes at 1250K for both talc crystals but recovers when cooling down to room temperature, whereas the OH-stretching vibration at 3675 cm^-1 related to a MgMgMg triplet can still be identified at least at 1350 K. This reveals that reversible oxidation processes are related to Mn²⁺-bearing hydrous minerals besides those containing Fe²⁺. The complete irreversible thermal decomposition of talc occurs at 1400 K, where clinoenstatite is formed.

 

References

S. Aspiotis, J. Schlüter, F. Hildebrandt, B. Mihailova, J. Raman Spectrosc. 2023; 54, 1502.

S. Bernardini, G. Della Ventura, J. Schlüter, B. Mihailova, Geochem. 2023, 83, 125942.