Nine samples of carbonate-free sodalite-group minerals, including those with abnormally high contents of polysulfide groups, fluoride anion and carbon dioxide molecules as well as synthetic fluoraluminate sodalite-type compound Na₈(Si₇Al₅O₂₄)(AlF₆)^3–·5H₂O, have been studied by means of electron microprobe analyses, infrared and Raman spectroscopy; the CO₂ content was determined using the selective sorption of gaseous ignition products. This article describes a semi-quantitative method for estimating the content of carbon dioxide molecules in these minerals, based on IR spectroscopy data. The data obtained demonstrate the existence of a sulfide sodalite-group mineral with the idealized formula Na₇(Si₆Al₆O₂₄)(S₃⁻)·H₂O, which differs significantly from the formula Na₆Ca₂(Si₆Al₆O₂₄)S^2–₂ accepted for lazurite. According to single-crystal X-ray structural analysis, in the F-rich sodalite-group mineral from the Eifel paleovolcanic region, Germany with the idealized formula Na₇(Si₆Al₆O₂₄)F⁻·nH₂O fluorine occurs as an isolated F⁻ anion, unlike synthetic F-rich sodalite-type compounds.

Three new polymorphs of aluminosilicate paracelsian, BaAl₂Si₂O₈, have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al³⁺ and Si⁴⁺ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO₄ and SiO₄ tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P2₁/c) to orthorhombic (Pna2₁). The structure of paracelsian-III consists of SiO₆ octahedra, AlO₆ octahedra and distorted AlO₄ tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si⁴⁺ and Al³⁺ coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna2₁ to Pn. The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O²⁻ and Ba²⁺ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB.

Comparison of the natural and synthetic phases allows an overview to be made and even an understanding of the crystal growth processes and mechanisms of the particular crystal structure formation. Thus, in this work, we review the crystal chemistry of the family of uranyl selenite compounds, paying special attention to the pathways of synthesis and topological analysis of the known crystal structures. Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl selenite mineral formation requires heating, which most likely can be attributed to the radioactive decay. Structural complexity studies revealed that the majority of synthetic compounds have the topological symmetry of uranyl selenite building blocks equal to the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved regardless of the interstitial filling of the structures. Whereas the real symmetry of U-Se complexes in the structures of minerals is lower than their topological symmetry, which means that interstitial cations and H₂O molecules significantly affect the structural architecture of natural compounds. At the same time, structural complexity parameters for the whole structure are usually higher for the minerals than those for the synthetic compounds of a similar or close organization, which probably indicates the preferred existence of such natural-born architectures. In addition, the reexamination of the crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite is reported. As a result of the single crystal X-ray diffraction analysis of demesmaekerite, Pb₂Cu₅[(UO₂)₂(SeO₃)₆(OH)₆](H₂O)₂, the H atoms positions belonging to the interstitial H₂O molecules were assigned. The refinement of the guilleminite crystal structure allowed the determination of an additional site arranged within the void of the interlayer space and occupied by an H₂O molecule, which suggests the formula of guilleminite to be written as Ba[(UO₂)₃(SeO₃)₂O₂](H₂O)₄ instead of Ba[(UO₂)₃(SeO₃)₂O₂](H₂O)₃.

The isomorphism of S-bearing feldspathoids belonging to the cancrinite, sodalite, tugtupite, vladimirivanovite, bystrite, marinellite and scapolite structure types has been investigated using a multimethodical approach based on infrared, Raman and 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. Sapozhnikovite Na₈(Al₆Si₆O₂₄)(HS)₂ and sulfite and thiosulfate analogues of cancrinite are synthesized hydrothermally and characterized by means of electron microprobe analyses, powder X-ray diffraction and Raman spectroscopy. The possibility of the incorporation of significant amounts of SO₄²⁻, S₄ and SO₃²⁻ in the crystal structures of cancrisilite, sulfhydrylbystrite and marinellite, respectively, has been established for the first time. Thermal conversions of S-bearing groups in the synthetic sulfite cancrinite and sapozhnikovite analogues as well as natural vladinirivanovite and S₄-bearing haüyne under oxidizing and reducing conditions have been studied using the multimethodical approach. The SO₄²⁻ and S²⁻ anions and the S₃^•– radical anion are the most stable S-bearing species under high-temperature conditions (in the range of 700–800 °C); their ratio in the heated samples is determined by the redox conditions and charge-balance requirement. The HS⁻ and S₅²⁻ anions are stable only under highly reducing conditions.

Slawsonite’s (SrAl₂Si₂O₈) structure evolutions depending on temperature (27–1000 °C) have been studied by in situ single-crystal X-ray diffraction. The SrO₇ polyhedron expands regularly with the temperature increase. Silicon and aluminum cations are ordered in tetrahedral sites of the studied slawsonite; no significant changes in their distribution as temperature increases were observed. Slawsonite demonstrates a relatively high volume thermal expansion (α_V = 23 × 10⁻⁶ °C⁻¹) with high anisotropy, typical for framework feldspar-related minerals and synthetic compounds. It was found that, contrary to previously published data, the crystal structure of slawsonite is stable in the studied temperature range and no phase transitions occur up to 1000 °C. The role of Ca and Ba substitution for Sr and Al/Si ordering on polymorphism of natural MAl₂Si₂O₈ (M = Ca, Sr, Ba) is herein discussed.

Luboržákite, ideally Mn 2 AsSbS 5 , is a new mineral from the Vorontsovskoe gold deposit, Northern Urals, Russia. It forms long-prismatic crystals up to 70 x 20 μm and anhedral grains of the same size embedded in the matrix of Mn-bearing dolomite and Mn-bearing calcite. Associated minerals include pyrite, orpiment, realgar, stibnite, aktashite, alabandite, boscardinite, chabournéite, coloradoite, clerite, écrinsite, gold, routhierite, sphalerite, twinnite etc. Luboržákite is black, opaque with metallic lustre and a black streak. It is brittle and has an uneven fracture. No cleavage and parting have been observed. Mohs hardness is 4-4½. D calc = 4.181 g•cm-3. In reflected light, luboržákite is tin-white, weakly anisotropic with rotation tints varying from dark grey to grey. The chemical composition of luboržákite is (wt.%; electron microprobe, WDS mode): Mn 21.23, Cu 0.29, Ag 0.56, Pb 1.90, As 15.25, Sb 27.03, S 33.23, total 99.49. The empirical formula based on the sum of all atoms = 9 pfu is Mn 1.86 Pb 0.04 Ag 0.03 Cu 0.02 As 0.98 Sb 1.07 S 5.00. The new mineral is monoclinic, space group C2/m with a = 12.5077(6), b = 3.8034(2), c = 16.0517(8) Å, β = 94.190(4), V = 761.57(6) Å 3 and Z = 4. The crystal structure of luboržákite was solved from the single-crystal X-ray diffraction data to R = 0.0383 for 712 observed reflections with I > 3σ(I). Luboržákite is a new member of the heterochemical isostructural series of 'unit-cell twinned' structures, named the pavonite series.