Use of the Far Infrared Spectroscopy for NaCl and KCl Minerals Characterization—A Case Study of Halides from Kłodawa in Poland

Abstract

The paper presents research on chloride minerals of natural origin from Kłodawa (Poland), i.e., colorless, blue and purple halite as well as colorless sylvite. Selected samples of minerals were studied by chemical analysis (ICP-OES, ICP-MS, titration methods) and crystallographic measurements. Then, for the tested halides, research was carried out using far-infrared spectroscopy. Spectroscopic studies confirmed the simple way of distinguishing NaCl and KCl minerals using far-infrared spectroscopy, known in the literature. The novelty is that the article presents for the first time the experimental far infrared spectra of natural blue and purple halite. It was observed that the blue (178 cm−1) and purple (176 cm−1) halites have the strongest infrared band slightly shifted towards higher wavenumbers compared to colorless halite (174 cm−1). As part of the work, the infrared spectra of the crystal structure models of sodium and potassium chloride were calculated for the first time using the density functional theory (with the B3LYP functional and the 6–31G* basis set, 125-atom model). The proposed approach can be used not only as a powerful method differentiating NaCl and KCl minerals, but it can also help with understanding of different defects in crystal lattices for naturally occurring halides and crystals of other minerals.

Innovative Structural Characterization of Natural Villiaumite Crystal

“…No imaginary frequencies were determined. In our previous study on LiCl, NaCl, KCl, LiBr and LiF, calculations using a 125-atom model for very similar chemical compounds to those mentioned, gave reliable results [43,44]. The theoretical IR spectrum of the structure of sodium fluoride was derived by representing each band as a Lorentzian-shaped curve.…”

“…We observed the same relationship for a series of chlorides. The most intense bands for LiCl, NaCl and KCl, were observed at 200, 175 and 145 cm −1 , respectively [43,44].…”

Study of the Structure and Infrared Spectra of LiF, LiCl and LiBr Using Density Functional Theory (DFT)

“…The input geometries of salt crystal models (LiF, LiCl and LiBr) were taken for calculation from crystallographic data [50][51][52]. Our previous study showed that calculations for a 125-atom model for NaCl and KCl give reasonable results [41]. This 125-atom structure provides that each of the 27 atoms of the conventional unit cell of the studied lithium halides has six-fold coordination.…”

“…This 125-atom structure provides that each of the 27 atoms of the conventional unit cell of the studied lithium halides has six-fold coordination. Moreover, according to our previous study for NaCl and KCl, freezing of all bond angles to 90 • in the structure during the geometry optimization process leads to a good simulation of the lattice parameters of the crystals and their infrared spectra [41]. Thus in this work, the geometries of the 125-atom model structures of LiF, LiCl and LiBr were fully optimized with the restriction for all angles 90 • .…”

“…Regardless of their form (anhydrous solids or aqueous solutions), vibrational spectroscopy may be employed to assess their purity. Our previous study [41] showed that far-infrared spectroscopy is a useful tool to distinguish NaCl and KCl minerals, and even blue and colorless NaCl from Kłodawa. Although the structural, elastic, electronic, etc., properties of LiF, LiCl, LiBr, and LiI compounds have been studied, we have noticed that the far-infrared spectra of anhydrous lithium halides have not yet been reported in the literature.…”