The crystal structure of larnite, a natural analog of synthetic β-Ca₂SiO₄, has been determined: a = 5.5051(3) Å, b = 6.7551(3) Å, c = 9.3108(5) Å, β = 94.513(4) ^o , sp. gr. P2₁/n, Z = 4, and R ₁ = 0.0532 for 1071 reflections with I > 2σ (I). Larnite was found in skarn xenoliths (Lakargi, Kabardino-Balkaria, Russia). The mineral structure is based on a heteropolyhedral glaserite-like framework of interconnected Ca polyhedra and isolated [SiO₄] tetrahedra. Based on an analysis of the layer-by-layer packing of atoms in the structures of larnite and other Ca₂SiO₄ polymorphs, the structural features and mechanisms of transitions from high-temperature (α, α′ _L , and α′ _H ) to low-temperature (β and γ) Ca₂SiO₄ modifications, as well as their relationship with natural glaserite-like orthosilicates (merwinite Ca₃Mg[SiO₄]₂ and bredigite Ca₇Mg[SiO₄]₄), have been considered. The most likely atomic arrangement in hypothetical Ca₂SiO₄ models has been calculated by the method of atomistic potentials.

A new sodium manganese-nickel phosphate of alluaudite supergroup with the general formula NaMnNi₂(H_2/3PO₄)₃ was synthesized by a hydrothermal method. The synthesis was carried out in the temperature range from 540 to 660 K and at the general pressure of 80 atm from the oxides mixture in the molar ratio MnCl₂: 2NiCl₂: 2Na₃PO₄: H₃BO₃: 10H₂O. The crystal structure was studied by a single-crystal X-ray diffraction analysis: space group C2/c (No. 15), a = 16.8913(4), b = 5.6406(1), c = 8.3591(3) Å, β = 93.919(3), V = 794.57(4) ų. The compound belongs to the alluaudite structure type based upon a mixed hetero-polyhedral framework formed by MX₆-octahedra and TX₄-tetrahedra. The characteristic feature of the title compound is the absence of cations or H₂O molecules in channel II, while the negative charge of the framework is balanced by the partial protonation of PO₄ tetrahedra. The presence of the transition metals at the A-type sites results in the changes of stoichiometry and the local topological features. Topological analysis of the hetero-polyhedral alluaudite-type frameworks and its derivatives (johillerite-, KCd₄(VO₄)₃-, and keyite-type) and quantitative characterization of their differences was performed by means of natural tilings.

The crystal structures of compounds with the general formula Cs{^[6]Al₂[^[4]TP₆O₂₀]} (where T = Al, B) display order-disorder (OD) character and can be described using the same OD groupoid family. Their structures are built up by two kinds of nonpolar layers, with the layer symmetries Pc(n)2 (L_2n+1-type) and Pc(a)m (L_2n-type) (category IV). Layers of both types (L_2n and L_2n+1) alternate along the b direction and have common translation vectors a and c (a ~ 10.0 Å, c ~ 12.0 Å). All ordered polytypes as well as disordered structures can be obtained using the following partial symmetry operators that may be active in the L_2n type layer: the 2₁ screw axis parallel to c [– – 2₁] or inversion centers and the 2₁ screw axis parallel to a [2₁ – –]. Different sequences of operators active in the L_2n type layer ([– – 2₁] screw axes or inversion centers and [2₁ – –] screw axes) define the formation of multilayered structures with the increased b parameter, which are considered as non-MDO polytypes. The microporous heteropolyhedral MT-frameworks are suitable for the migration of small cations such as Li⁺, Na⁺ Ag⁺. Compounds with the general formula Rb{^[6]M³⁺[^[4]T³⁺P₆O₂₀]} (M = Al, Ga; T = Al, Ga) are based on heteropolyhedral MT-frameworks with the same stoichiometry as in Cs{^[6]Al₂[^[4]TP₆O₂₀]} (where T = Al, B). It was found that all the frameworks have common natural tilings, which indicate the close relationships of the two families of compounds. The conclusions are supported by the DFT calculation data.

A novel sodium and chromium borophosphate, obtained by hydrothermal synthesis in the Na2O–P2O5–B2O3–Cr2O3–Н2О system, has been investigated using X-ray diffraction and IR spectroscopy. The...

The crystal structure of a new calcium pentaborate, Ca₂[B₅O₈(OH)]₂ B(OH)₃ · H₂O, synthesized by the hydrothermal method, has been determined (a = 6.6620(4) Å, b = 6.5990(4) Å, c = 10.5830(6) Å, α = 103.372(5)°, β = 78.296(5)°, γ = 120.931(7)°, space group P1, Z = 1; R ₁ = 0.0712 for 4263 reflections with I > 2σ (I)). This structure is built up of three-layer stacks isolated from each other and consisting of Ca (Sr, Ba) polyhedra joined into columns with boron-oxygen networks located on both sides. Building blocks of the network are the pentaborate groups [B ₂ ^t B ₃ ^Δ O₈(OH)]²⁻ formed by two boron tetrahedra and three boron triangles. A comparative crystal chemical analysis has been performed for divalent metal pentaborates that have similar compositions and structures. A common fundamental building block {M[B₅O₈(OH)]}₂ · {[B(OH)₃],H₂O}₂ (M = Ca, Sr, Ba) is determined. The diversity of mineral species in the subgroup of pentaborates under consideration is associated both with the variations in the composition and symmetry of individual blocks and with the types of their joining in the structure.