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

Abstract

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 |trueboldΔboldv¯| 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. Copyright © 2015 John Wiley & Sons, Ltd.

Structural insights of hierarchically engineered feldspars by confocal Raman microscopy

“…509 cm −1 ( ν a ) for the albite crystal, which indicates that the Si–O–Si and Si–O–Al bond angles are distributed in approximately two groups that give rise to these two Raman modes . ν b is mainly assigned to tetrahedral ring compression–expansion within the ab plane, and ν a is assigned to a compression–expansion of four‐membered ring along c , also contributing the translation of Na along ab plane and a ‐axis, respectively, according to the system depicted in Figure . Peak position of the strongest Raman mode has an inverse correlation with the size of the four‐membered ring of tetrahedra.…”

“…In general, Raman modes assignment in feldspars (included in Figure c) might be separated in five groups as follows: (a) Raman modes below 200 cm −1 are cage‐shear modes; (b) Raman modes between 200 and 400 cm −1 are rotation–translation modes of the rings; (c) ring‐breathing modes between 400 and 520 cm −1 ; (d) deformation modes between 600 and 900 cm −1 ; and (e) vibrational stretching modes between 900 and 1200 cm ‐1 . Raman spectrum of feldspars is easily recognized by the presence of two or three peaks located between 450 and 515 cm −1 .…”

“…The Raman modes description included in Table explains briefly the main contributions to the atomic displacements based on attributions made in previous works . Direction of motions follows the system depicted in Figure .…”

“…Then, analbite is topochemically monoclinic ( T 1 (0) = T 1 (m) ≥ T 2 (0) = T 2 (m)), but still topologically triclinic and undergoes a reversible phase transformation to monoalbite at 980°C (at 0.0001 GPa). Finally, monoalbite , which is the monoclinic and completely disordered polymorph that undergoes a symmetry change, but no change in composition . In contrast, anorthite has a Si:Al ratio of 1:1 and an essentially Al–Si ordered structure (with space group I ) except above 2000°C, at which disorder occurs (C symmetry) .…”

“…Regarding the Cartesian system XYZ, X‐axis forms an angle of about 2° with the crystallographic axis, Y‐axis approximately coincides with the b crystallographic axis, and Z‐axis is almost perpendicular to the ab plane at ~26° from the C‐axis. Adapted from the studies of Senderov and Meneghinello, Alberti, and Cruciani . [Colour figure can be viewed at wileyonlinelibrary.com]…”