Unveiling the Crystal Structure of Sanidine: A Polarized Raman and Ab Initio Simulation Exploration

Sanidine, the high-temperature polymorph of potassic feldspar, plays a crucial role in the formation of volcanic rocks. Despite extensive research, the complexity of K-feldspar polymorphs remains poorly understood. This study, by combining polarized Raman spectroscopy with ab initio calculations, aims to definitively assign the vibrational modes predicted by theory to those observed experimentally. A sanidine megacryst from the rhyo-dacite sub-intrusive body of Terlano (Ora mega-caldera complex, Southern Alps, Italy) was used for this purpose. Polarized Raman spectra were collected in six scattering geometries, allowing for precise mode selection within the 40-1200 cm-1 range. Our vibrational analysis, based on density functional theory, provides wave-numbers and normal modes having trends that largely align with experimental data. The combination of experimental data and theoretical calculations also indicated that the models in which aluminum is located at the T1 tetrahedral site are more stable than those with aluminum at the T2 site. This finding provides further insight into the order-disorder phase transition in K-feldspar, a phenomenon that is difficult to replicate under laboratory conditions. These results further improve our understanding of the relationship between theoretical models and the actual spectral characteristics of this disordered mineral. The full assignment of normal modes presented here lays the foundation for future studies of phase transitions in K-feldspars using Raman spectroscopy.