Study of Minerals by Molecular Spectroscopy

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. Full article

During grinding with forged steel media, sulphides such as pyrite undergo surface changes due to the occurrence of oxidation–reduction reactions, which affect its depression during the concentration process. For this reason, in this work, the surface modification of pyrite during grinding was studied; FTIR, ICP-OES, XRD and SEM-EDS were used for the materials’ characterisation. It was found that the pyrite obtained during grinding showed magnetic susceptibility due to the absorption and superficial formation of magnetite Fe₃O₄, Fe–O bonds identified by FTIR at 598 cm⁻¹, and of other species, such as oxy-hydroxy-sulphates at 696 cm⁻¹ and goethite α-FeOOH at 875 cm⁻¹. This caused the reversal of the zeta potential magnitude (ζ) from positive to negative at pH 8.3 and 30 min of grinding. The ζ of the pyrite throughout the studied pH ranges was, overall, positive, i.e., +5 mV. However, at pH 10.5 and 15 min of grinding, the ζ turned negative. This was associated with the formation of Fe–CO₃ (−2) bonds in the siderite, which were identified with the absorption bands corresponding to 756, 1448 and 1493 cm⁻¹. Full article

Between 2004 and 2018, NASA’s rover Opportunity found huge numbers of small, hematite-rich spherules (commonly called blueberries) on the Meridiani Planum of Mars. The standard oxide composition distributions of blueberries have remained poorly constrained, with previous published analyses leaving hematite content somewhere in the broad range of 24–100 wt%. A searching mass-balance analysis is introduced and applied to constrain possible standard oxide composition distributions of blueberries consistent with the non-detection of silicates in blueberries by Opportunity’s instruments. This analysis found three groups of complete solution sets among the mass-balance ions consistent with the non-detection of silicates; although, a simple extension of the analysis indicates that one larger space of solutions incorporates all three groups of solutions. Enforcing consistency with the non-detection of silicates in blueberries constrains the hematite content in most of blueberry samples to between 79.5 and 99.85 wt%. A feature of the largest group of complete solution sets is that five oxides/elements, MgO, P₂O₅, Na₂O, SO₃, and Cl, collectively have a summed weight percentage that averages close to 6 wt%, while the weight percentage of nickel is close to 0.3 wt% in all solutions. Searches over multidimensional spaces of filtering composition distributions of basaltic and dusty soils were a methodological advance. Full article

Natural minerals and earths with coloring properties have been widely used as artistic pigments since prehistoric times. Despite being extensively studied, the complex chemistry of earth pigments is still unsatisfactory described with respect to their mineralogical and structural variability and origin. In this study, a large group of earth pigments from various geographical locations was investigated using easily accessible spectroscopic techniques and multivariate analysis with the aim to identify distinctive mineralogical and chemical characteristics of natural pigment sources. Portable X-ray fluorescence (p-XRF), Fourier transform infrared spectroscopy (FTIR) and fiberoptic Raman spectroscopy were used for the elemental, molecular and structural characterization of the investigated pigments. Diagnostic spectral features and chemical patterns (fingerprints) were identified and discussed with respect to their geological sources. Due to the occurrence of similar accompanying minerals, it was observed that the differentiation of red and yellow ochers is more challenging compared to green, brown and black pigments. However, for some of the investigated pigments, the presence of certain accessory minerals and/or of certain chemical patterns can have diagnostic value. Principal component analysis (PCA) of the FTIR and XRF data matrices showed promising results in terms of geological attribution, highlighting a promising tool for provenance research. The results of the study demonstrate the potential benefits of this rapid and nondestructive approach for the characterization and differentiation of earth pigments with similar hues coming from different geological sources. Full article

Fulgurites are naturally occurring structures that are formed when lightning discharges reach the ground. In this investigation, the mineralogical compositions of core and shell compartments of a rare, iron-rich fulgurite from the Mongolian Gobi Desert were investigated by X-ray diffraction and micro-Raman spectroscopy. The interpretation of the Raman data was helped by chemometric analysis, using both multivariate curve resolution (MCR) and principal component analysis (PCA), which allowed for the fast identification of the minerals present in each region of the fulgurite. In the core of the fulgurite, quartz, microcline, albite, hematite, and barite were first identified based on the Raman spectroscopy and chemometrics analyses. In contrast, in the shell compartment of the fulgurite, the detected minerals were quartz, a mixture of the K-feldspars orthoclase and microcline, albite, hematite, and goethite. The Raman spectroscopy results were confirmed by X-ray diffraction analysis of powdered samples of the two fulgurite regions, and are consistent with infrared spectroscopy data, being also in agreement with the petrographic analysis of the fulgurite, including scanning electron microscopy with backscattering electrons (SEM-BSE) and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) data. The observed differences in the mineralogical composition of the core and shell regions of the studied fulgurite can be explained by taking into account the effects of both the diffusion of the melted material to the periphery of the fulgurite following the lightning and the faster cooling at the external shell region, together with the differential properties of the various minerals. The heavier materials diffused slower, leading to the concentration in the core of the fulgurite of the iron and barium containing minerals, hematite, and barite. They first underwent subsequent partial transformation into goethite due to meteoric water within the shell of the fulgurite. The faster cooling of the shell region kinetically trapped orthoclase, while the slower cooling in the core area allowed for the extensive formation of microcline, a lower temperature polymorph of orthoclase, thus justifying the prevalence of microcline in the core and a mixture of the two polymorphs in the shell. The total amount of the K-feldspars decreases only slightly in the shell, while quartz and albite appeared in somewhat larger amounts in this compartment of the fulgurite. On the other hand, at the surface of the fulgurite, barite could not be stabilized due to sulfate lost (in the form of SO₂ plus O₂ gaseous products). The conjugation of the performed Raman spectroscopy experiments with the chemometrics analysis (PCA and, in particular, MCR analyses) was shown to allow for the fast identification of the minerals present in the two compartments (shell and core) of the sample. This way, the XRD experiments could be done while knowing in advance the minerals that were present in the samples, strongly facilitating the data analysis, which for compositionally complex samples, such as that studied in the present investigation, would have been very much challenging, if possible. Full article

With the unique advantages of mineral TCMs gradually emerging in clinical treatment, health care, and precaution, it has played an important role in the international medical market. Commonly, mineral TCMs with similar appearance and different processing methods have different effects, but they are easy to be confused in preparation, storage, transportation, and other links, which affects the use and causes related problems. In this paper, six kinds of easily confused mineral TCMs, including coral skeleton, ophicalcitum, calamine, matrii sulfas exsiccatus, gypsum, and alumen, are rapidly characterized using Raman spectroscopy, which can be distinguished with different Raman peaks at 0–300 cm⁻¹ due to the different lattice structure. The THz spectra of these mineral TCMs show that different mineral TCMs have different THz absorption coefficients at 0.3–2.0 THz. Furthermore, compared with the ineffectiveness of the Raman spectrum for differentiating mineral TCMs prepared with disparate processing methods, the terahertz absorption spectrum plays an active role in making up the limitation of low-wavenumber Raman spectroscopy. The combination of low-wavenumber Raman and THz spectroscopy provides a simple and feasible scheme for the identification of mineral TCMs, which could play an important role in the quality control of mineral TCMs. Full article

In this first exploration of europium-doped carbonated apatites the location of carbonate was determined using the environment model for the analysis of IR and NMR spectra. Europium-doped carbonated apatites, containing Eu/(Eu + Ca) mole ratios of about 10%, were prepared by aqueous one-step and addition syntheses. The IR and NMR spectra of the carbonate in the samples are described using the environment model: A-type carbonate is assigned to channels containing only calcium ions (A = Ca6) or to channels containing one Na⁺ or a vacancy (A’ = Ca5Na or Ca5). The presence of the channel Eu³⁺ and the use of triammonium phosphate in the synthesis produce considerable A-type carbonate. For the apatites reported here, the carbonate is distributed in approximately a 60 to 40 ratio for channel occupancy versus replacement of phosphate. The europium is assumed to have replaced calcium ions in the Ca(II) (channel) location and the stoichiometry of the products is used to propose that, contrary to much of the Eu(III) substitution literature, the charge-balance mechanism is likely to involve the substitution of two europium ions for three calcium ion with the concomitant formation of a calcium vacancy. The environment model is also used in the correlation of the a-axial lattice parameter with the percent A-type carbonate. Full article