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.

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.