Extra-Framework Content in Sodalite-Group Minerals: Complexity and New Aspects of Its Study Using Infrared and Raman Spectroscopy

Stability of sulfur molecules and insights into sulfur allotropy

Using ab initio evolutionary algorithm USPEX, we predict structures of sulfur molecules S_n (n = 2 - 21). It is shown that for n ≥ 5 stable structures of sulfur molecules are closed helical rings, which is in agreement with the experimental data and previous calculations. We investigate the stability of molecules using the following criteria: second-order energy difference (Δ²E), fragmentation energy (E_frag) and HOMO-LUMO gaps. The S₈ molecule has the highest value of Δ²E and forms the most common allotropic form of sulfur (orthorhombic α-S), into which all other modifications convert over time at room temperature. Commonly found molecules S₁₂ and S₆ also have strongly positive Δ²E. Another well-known molecule, S₇, has negative Δ²E, but at temperatures above 900 K has positive second-order free energy difference Δ²G. Generally, Δ²E (or Δ²G at finite temperatures) is a quantitative measure of the stability allowing one to predict the ease of formation of molecules and corresponding molecular crystals. Temperature dependence of the above-mentioned measures of stability explains a wide range of facts about sulfur crystalline allotropes, molecules in the gas phase, etc.

Infrared spectroscopy as a tool for the analysis of framework topology and extra-framework components in microporous cancrinite- and sodalite-related aluminosilicates

The identification of the framework type in multilayer cancrinite- and sodalite-group minerals and synthetic compounds is predominantly based on the unit-cell dimensions determined from powder or single-crystal X-ray diffraction data, by analogy with previously characterized samples with known crystal structures. However, topological type of the framework cannot be reliably determined in this way because of the different possible ABC staking sequences and different sets of cages with the same number of layers in the repeat unit. To solve this problem, additional criteria are required. The use of infrared (IR) spectroscopy makes it possible to distinguish topologically different types with the same unit-cell parameters. The most important diagnostic range in the IR spectrum (the "finger-print region", from 510 to 760 cm^-1) corresponds to the O-T-O bending vibrations (T = Si, Al). The spectral bands at 705 ± 8, 528 ± 5, 547 ± 4, and 555 ± 3 cm^-1 indicate the presence of the sodalite, Losod, liottite, and giuseppettite cages, respectively. The band at 528 ± 5 cm^-1 shifts towards ∼518 cm^-1 in the case when Losod cage hosts carbonate group. The IR spectrum in the "finger-print region" can be also used to identify a mineral species belonging to two-layer or three-layer minerals with different extra-framework compositions. The wavenumber of the antisymmetric stretching mode of the ¹²CO₂ molecule, which is a common admixed extra-framework constituent in minerals belonging to the cancrinite and sodalite groups, depends on the kind of the host cage or channel: 2340-2343 cm^-1 for the sodalite cage, 2338 cm^-1 for the Losod cage, and 2351-2353 cm^-1 for the liottite cage and wide channel in the cancrinite-type framework.

Halide-sodalites: thermal behavior at low temperatures and local deviations from the average structure

Sodalites of the general type |Na8X2|[T1T2O4]6 with X = Cl−, Br−, I− have been synthesized for Al–Si, Ga–Si, Al–Ge and Ga–Ge as T1–T2 frameworks. The structures were examined using in-house and synchrotron X-ray diffraction, Raman spectroscopy, force-field structure optimizations and DFT based ab-initio molecular dynamics (MD) computations. Calculated phonon density of states (PDOS) of the 12 compounds show only minor differences within a framework composition with a lowering of certain phonon energies with increasing anion size. Earlier published Debye and Einstein temperatures obtained with a Debye-Einstein-anharmonicity (DEA) model approach are confirmed using the determined low-temperature lattice parameters (18 K–293 K) and show no correlation with the respective PDOS. Small-box refinements against radial pair distribution functions (PDF) allowed the determination of anisotropic displacement ellipsoids (ADP) for Na+ and O2−, indicating a strong dependency of the ADP of Na+ on the chemical composition. Significantly lower thermal displacements from MD calculations suggested an influence of structural displacements. For compounds with an aspherical ADP for sodium, structural models could be refined in which the sodium is located on two 8e or one 24i site (both partially occupied), and also temperature-dependent (100 K–300 K) for the compounds with Ga–Ge framework. 3D-plots of the bond-valence sums of Na+ further validate the structural differences. These results imply that the local structure of halide-sodalites in many cases is not best described by the known average structure and may even not be cubic.

Crystal Chemistry, Isomorphism, and Thermal Conversions of Extra-Framework Components in Sodalite-Group Minerals

Isomorphic substitutions of extra-framework components in sodalite-group aluminosilicate minerals and their thermal conversions have been investigated using infrared, Raman, electron spin resonance (ESR), as well as ultraviolet, visible and near infrared (UV–Vis–near IR) absorption spectroscopy methods and involving chemical and X-ray diffraction data. Sodalite-related minerals from gem lazurite deposits (haüyne, lazurite, and slyudyankaite) are characterized by wide variations in S-bearing extra-framework components including SO42− and various polysulfide groups (S2●−, S3●−, S4●− radical anions, and S4 and S6 neutral molecules) as well as the presence of CO2 molecules. Heating at 700 °C under reducing conditions results in the transformation of initial S-bearing groups SO42− and S3●− to a mixture of S2−, HS−, S2●−, and S4●− and transformation of CO2 to a mixture of CO32− and C2O42− or HC2O4− anionic groups. Further heating at 800 °C in air results in the decomposition of carbonate and oxalate groups, restoration of the SO42− and S3●− groups, and a sharp transformation of the framework. The HS− anion is stable only under reducing conditions, whereas the S3●− radical anion is the most stable polysulfide group. The HS−-dominant sodalite-group mineral sapozhnikovite forms a wide solid-solution series with sodalite. The conditions required for the formation of HS−- and CO20-bearing sodalite-group minerals are discussed.

Nature and Isomorphism of Extra-Framework Components in Cancrinite- and Sodalite-Related Minerals: New Data

New data on the isomorphism of extra-framework components (including chromophores) in two- and multilayer minerals belonging to the cancrinite and sodalite groups, are obtained using chemical and single-crystal X-ray diffraction data as well as infrared, Raman, ESR, UV–Vis–near IR absorption and photoluminescence spectroscopy methods. It is shown that the blue color of these minerals may be due to CO3•− or S3•− radical anions, whereas yellow and pink shadings are typically due to the presence of S2•− radical anions and S4•− or S4, respectively. Two kyanoxalite varieties are distinguished: (1) with predominantly acid oxalate groups and (2) predominantly neutral oxalate groups. Zundel cation H5O2+ and CO2 molecules are shown to be typical impurities in nosean. The Zundel cation is also detected in kyanoxalite and in the 12-layer, cancrinite-related mineral marinellite. Wide isomorphic series involving substitutions of SO4− for SO32− and CO32−, as well as OH− for H2O and F−, are common for eight-layer, cancrinite-group minerals with an afghanite-type framework.

Multisensor hyperspectral imaging approach for the microchemical analysis of ultramarine blue pigments

This work presents a multisensor hyperspectral approach for the characterization of ultramarine blue, a valuable historical pigment, at the microscopic scale combining the information of four analytical techniques at the elemental and molecular levels. The hyperspectral images collected were combined in a single hypercube, where the pixels of the various spectral components are aligned on top of each other. Selected spectral descriptors have been defined to reduce data dimensionality before applying unsupervised chemometric data analysis approaches. Lazurite, responsible for the blue color of the pigment, was detected as the major mineral phase present in synthetic and good quality pigments. Impurities like pyrite were detected in lower quality samples, although the clear identification of other mineral phases with silicate basis was more difficult. There is no correlation between the spatial distribution of the bands arising in the Raman spectra of natural samples in the region 1200-1850 cm-1 and any of the transition metals or rare earth elements (REE). With this information, the previous hypothesis (based on bulk analysis) attributing these bands to luminescence emissions due to impurities of these elements must be revised. We propose the consideration of CO2 molecules trapped in the cages of the aluminosilicate structure of sodalite-type. Additionally, correlation between certain Raman features and the combined presence of Ca, P, and REE, in particular Nd, was detected for the lowest quality pigment. Our results highlight the usefulness of fusing chemical images obtained via different imaging techniques to obtain relevant information on chemical structure and properties.

Spectroscopic and Crystal-Chemical Features of Sodalite-Group Minerals from Gem Lazurite Deposits

Five samples of differently colored sodalite-group minerals from gem lazurite deposits were studied by means of electron microprobe and wet chemical analyses, infrared, Raman, electron spin resonance (ESR) and UV-Visible spectroscopy, and X-ray diffraction. Various extra-framework components (SO42−, S2− and Cl− anions, S3•−, S2•− and SO3•− radical anions, H2O, CO2, COS, cis- as well as trans- or gauche-S4 neutral molecules have been identified. It is shown that S3•− and S4 are the main blue and purple chromophores, respectively, whereas the S2•− yellow chromophore and SO3•− blue chromophore play a subordinate role. X-ray diffraction patterns of all samples of sodalite-group minerals from lazurite deposits studied in this work contain superstructure reflections which indicate different kinds of incommensurate modulation of the structures.

Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

We investigated the cryptic red chromophore, accompanying the blue S3˙- radical in ultramarine pigments, which usually was tentatively assigned to an unspecified isomer of either neutral S4 or ionic S4˙- species. To reveal its identity, we performed the first systematic density functional studies on periodic and large cluster models of red ultramarines, considering several S4/S4˙- isomers embedded in aluminosilicate cages. For both neutral and charged tetrasulfides the most stable confined isomer is the planar C2v one. The only plausible candidate for the red chromophore among the tetrasulfur species is the planar C2v isomer of the neutral S4 molecule, which, apart from being thermodynamically preferable, strongly absorbs green light and its vibrational modes match very well with the available Raman data. The C2v-S4˙- radical, if present at all in red ultramarines, could be identified by strong absorption in the near infrared region and possibly by the slightly larger isotropic value of the g tensor than that of the S3˙- radical.