A comparison between ab initio calculated and measured vibrational frequencies of the triclinic albite

A COMPARISON BETWEEN AB INITIO CALCULATED AND MEASURED VIBRATIONAL FREQUENCIES OF THE TRICLINIC ALBITE Aliatis I. 1, Lambruschi E. 1, Mantovani L. 1, Bersani D. 1, Andò S. 3, Gatta G. D. 2, Gentile P. 3, Salvioli-Mariani E. 1, Prencipe M. 4, Tribaudino M. 1, Lottici P.P. 1; 1. Università di Parma, Dipartimento di Fisica e Scienze della Terra, 2. Università di Milano, Dipartimento di Scienze della Terra, 3. Università di Milano Bicocca, Dipartimento di Scienze dell'Ambiente e del Territorio e di Scienze della Terra, 4. Università di Torino, Dipartimento di Scienze della Terra Feldspar-group minerals are among the most common rock-forming minerals of planetary crusts. On Earth, they occur in igneous, metamorphic and sedimentary rocks. On the Moon, plagioclase is the most abundant mineral and feldspars are one of the main minerals in Martian meteorites. Feldspars show subtle variations in crystal structure that allow petrologists to gain information about the chemical and physical conditions prevailing during rock formation. The whole spectrum of their possible structural states can be found in rocks with different cooling histories, providing an opportunity to study the way a structure responds to a slowly changing geological environment. Albite, NaAlSi3O8, is the sodic end-member of high temperature plagioclase and alkali feldspar solid solutions. The analysis of its vibrational frequencies and their assignments to the fundamental normal modes are the aim of this study. The Raman spectrum of albite has been calculated by using the CRYSTAL14 program (Dovesi et al., 2014). Calculations were made at the hybrid Hartree-Fock/Density Functional level (HF/DFT): the WC1LYP Hamiltonian, which provides excellent agreement between calculated and experimentally measured vibrational frequencies (see e.g. Prencipe et al., 2012), was employed. The comparison between the frequencies measured by Raman spectroscopy on an ordered natural albite, var. Cleavelandite from Minas Gerais (Brasil), with those calculated by HF/DFT is discussed. All the expected 39 Ag modes have been identified in the Raman spectra and their frequencies well correspond to the calculated ones (the average absolute discrepancy (<|Δ|>) is ~ 3 cm-1; the maximum discrepancy (|Δ|max) is ~ 10 cm-1). The good quality of the WC1LYP results allowed for reliable assignments to specific patterns of atomic vibrational motion (normal modes). Dovesi R., Saunders V. R., Roetti C., Orlando R., Zicovich-Wilson C. M., Pascale F., Civalleri B., Doll K., Harrison N. M., Bush I. J., D’Arco P., Llunell M., Causà M. & Noël Y. 2014. CRYSTAL14 User's Manual. University of Torino. Prencipe M., Mantovani L., Tribaudino M., Bersani D. & Lottici P. P. 2012. The Raman spectrum of diopside: a comparison between ab initio calculated and experimentally measured frequencies. Eur. J. Mineral., 24, 457-464. Keywords: Ab initio calculation, Raman Spectroscopy, albite