Exploring the Structural Complexity of Feldspathoids at Non-Ambient Conditions: The Case of Leucite and Kalsilite

Leucite (KAlSi O ) is a key feldspathoid for modeling alkaline, potassium-rich magmatic systems. While its polymorphic behavior is well-documented at high temperature or high pressure alone (Lange et al., 1986; Hatch et al., 1990; Palmer et al., 1997; Gatta et al., 2008), its structural response to simultaneous high pressure (P) and high temperature (T) remains less explored. In this study, we investigated the P-T behavior of leucite through in situ single-crystal and powder synchrotron X-ray diffraction. Initial single-crystal experiments (P02.2 at PETRA-III and ID15B at ESRF) revealed an unexpected transition from the tetragonal I4 /a polymorph toward a trigonal form (R-3) at pressures above 2.2-2.5 GPa. This trigonal phase, never reported before, is characterized by a significantly higher structural complexity, with two independent K sites and four tetrahedral sites. Further experiments conducted at the ID06 beamline (ESRF) using a Large Volume Press (LVP) confirmed this phase transition. The LVP data demonstrate that the transformation is reproducible and have allowed for the first time the determination of a clear phase stability field under combined P-T conditions. This trend toward structural complexity appears to be a common feature among feldspathoids. Parallel investigations on kalsilite (KAlSiO ) have similarly identified new high-pressure phases. Notably, preliminary single-crystal XRD data on kalsilite at high-P and ambient-T show a polymorph characterized by a complex supercell, whereas XRD data at combined P-Tshow a transition toward a polymorph metrically analogous to nepheline (c’ = 2c). Together, these findings underscore the necessity of high-resolution in situ crystallography to understand the sophisticated structural rearrangements that govern the stability of alkali-bearing framework silicates in the Earth's interior. Gatta G.D. et al. (2008) - Leucite at high pressure: Elastic behavior, phase stability, and petrological implications. Am. Mineral., 93, 1588-1596, https://doi.org/10.2138/am.2008.2932. Hatch D.M. et al. (1990) - Phase transitions in leucite, KAISi O . Phys. Chem. Minerals, 17, 220-227, https://doi.org/10.1007/BF00201453. Lange R.A. et al. (1986) - Phase transitions in leucite (KAlSi O ), orthorhombic KAlSiO , and their iron analogues (KFeSi O , KFeSiO ). Am. Mineral., 71, 937-945. 2 6 1 4 2 6 2 6 4 2 6 4 12/06/26, 09:47 BECong Admin https://becong.it/BECong/view_abstract_auth.php?id=11960 1/2 Palmer D.C. et al. (1997) - Structural behavior, crystal chemistry, and phase transitions in substituted leucite: High-resolution neutron powder diffraction studies. Am. Mineral., 82, 16-29, https://doi.org/10.2138/am-1997-1- 203. leucite, kalsilite, phase transition.