Structural response of leucite and kalsilite under non-ambient conditions: new insights from in situ synchrotron XRD

Leucite (KAlSi2O6) and kalsilite (KAlSIO4) are key framework silicates critical for modeling alkaline, ultrapotassic magmatic systems and mantle metasomatism [1-3]. Although their polymorphic behaviors under independent high-pressure (P) or high-temperature (T) regimes have been extensively documented [4-6], their structural responses and stability limits under simultaneous high P-T conditions remain poorly constrained. To address this gap, this study investigates the joint P-T behavior of both feldspathoids through in situ synchrotron X-ray diffraction, combining single-crystal and powder techniques with Large Volume Press (LVP) experiments. Synchrotron-based X-ray diffraction experiments under extreme environments have revealed that a trend toward high structural complexity is a shared response among alkali feldspathoids. In the case of leucite, initial single-crystal data collected at the P02.2 (PETRA-III) and ID15B (ESRF) beamlines uncovered an unprecedented phase transformation above 2.2–2.5 GPa, where the tetragonal (pseudo-cubic) I41/a structure evolves into a highly complex trigonal symmetry (R-3), featuring four distinct tetrahedral positions and two independent K sites. The reproducibility of this transformation and the definitive mapping of its stability field, under simultaneous high P-T conditions, were subsequently achieved through Large Volume Press (LVP) setups at the ID06 beamline (ESRF). Mimicking this behavior, parallel high-pressure investigations on kalsilite have also brought to light new polymorphic configurations. Preliminary single-crystal XRD at ambient temperature and high pressure indicates a transition toward a complex supercell architecture, whereas combined P-T environments induce a rearrangement into a phase metrically analogous to nepheline (c' = 2c). Collectively, these experimental observations emphasize how high-resolution, in situ crystallographic techniques are indispensable for decoding the sophisticated atomic rearrangements that control framework silicate stability deep within the Earth.