In situ Synchrotron XRD Investigation of Phase Stability and Structural Transitions in Kalsilite, (KAlSiO4) at High P and T

Kalsilite (KAlSiO ) is a diagnostic mineral essential for understanding ultrapotassic magmatism and mantle metasomatism (Lustrino et al., 2020; Innocenzi et al., 2024). Despite its geological significance, the stability limits of its polymorphs under simultaneous high-pressure and high-temperature conditions remain poorly constrained. In this study, we present new in situ angle-dispersive X-ray powder diffraction data collected at the ID06 beamline (ESRF) using a large volume press (LVP). Two experimental runs were conducted up to approximately 6 GPa and 1000 K. In both experiments, a structural transition was observed from the initial trigonal framework (P3 c) to a high-pressure/high- temperature (HP-HT) phase. This HP-HT phase is characterized by a nepheline-type metric, showing a doubling of the c-axis parameter compared to the starting kalsilite. This transition was identified at intermediate P-T conditions (ca. 700 K and 4 GPa), providing the first in situ evidence of such a transformation under combined non-ambient conditions. The post-transition behavior exhibited significant differences between the two runs. In the first run, the transformation proved to be reversible upon cooling; however, the thermal discrepancy between the forward and backward transitions suggests the presence of marked hysteresis. In the second run, the HP-HT polymorph with a nepheline-type cell was metastably preserved down to ambient conditions, indicating an irreversible transformation path possibly depending on the specific P-T trajectory followed. These results allow for a refinement of the kalsilite phase diagram, providing new constraints for thermodynamic models of metasomatized mantle environments and the characterization of alkali reservoirs in the Earth's interior. Innocenzi F. et al. (2024) - On the occurrence of kalsilite in melilite-bearing ultrapotassic lavas from the Roman Province (Vulsini Mts., central Italy). Lithos. 482-483, 107704. https://doi.org/10.1016/j.lithos.2024.107704 (https://doi.org/10.1016/j.lithos.2024.107704) Lustrino M. et al. (2020) - Strongly SiO -undersaturated, CaO-rich kamafugitic Pleistocene magmatism in Central Italy (San Venanzo volcanic complex) and the role of shallow depth limestone assimilation. Earth Sci. Rev. 208, 103256. https://doi.org/10.1016/j.earscirev.2020.103256 (https://doi.org/10.1016/j.earscirev.2020.103256)