A comparison between ab initio calculated and measured Raman spectrum of triclinic albite (NaAlSi3O8)

Albite is one of the most common minerals in the Earth's crust, and its polymorphs can be found in rocks with different cooling histories. The characteristic spectrum of vibration of the albite mineral reflects its structural Si/Al ordering. In this study, we report on the comparison between the Raman spectra measured on a natural and fully ordered (as deduced on the basis of single-crystal X-ray diffraction data) 'low albite', NaAlSi3O8, and those calculated at the hybrid Hartree-Fock/density functional theory level by employing the WC1LYP Hamiltonian, which has proven to give excellent agreement between calculated and experimentally measured vibrational wavenumbers in silicate minerals. All the 39 expected Ag modes are identified in the Raman spectra, and their wavenumbers and intensities, in different scattering configurations, correspond well to the calculated ones. The average absolute discrepancy |Δv¯| is ∼3.4 cm-1, being the maximum discrepancy |Δv|max ∼ 10.3 cm-1. The very good quality of the WC1LYP results allows for reliable assignments of the Raman features to specific patterns of atomic vibrational motion.