The new sodalite-group mineral species slyudyankaite, ideally Na28Ca4(Si24Al24O96) (SO4)6(S6)1/3(CO2)·2H2O, was discovered in altered lazurite-bearing metasomatic rock at the Malo-Bystrinskoe gem lazurite deposit, Baikal Lake area, eastern Siberia, Russia. The associated minerals are diopside, calcite, fluorapatite, phlogopite, lazurite, and pyrite. Slyudyankaite forms green to pale blue isolated anhedral equant grains up to 0.5 cm across and their aggregates. The streak is white and the luster is vitreous. Slyudyankaite is brittle, with a Mohs hardness of 5½. Cleavage and parting are not observed. Density measured by flotation in heavy liquids is equal to 2.46(2) g·cm–3. Density, calculated using the empirical formula and unit-cell volume refined from single-crystal XRD data, is 2.454 g·cm–3. Slyudyankaite was characterized using the IR, Raman, ESR, near infrared (NIR), visible (Vis), and ultraviolet (UV) absorption, XPS and photoluminescence spectroscopy methods. The chemical composition is (wt%, electron microprobe, H2O and CO2 determined by selective sorption of ignition products, CO2 confirmed by quantitative IR spectroscopic method, sulfate sulfur determined by wet chemical analysis): Na2O 19.28, K2O 0.12, CaO 5.13, Al2O3 27.01, SiO2 33.25, SO3 10.94, S 1.75, Cl 0.10, CO2 1.42, H2O 0.90, –O≡(Cl,HS) –0.03, total 99.87. The empirical formula is Na27.57Ca4.05 K0.11(Si24.52Al23.48O96)(SO4)6.06Cl0.12(CO2)1.43·2.21H2O where is the total sulfide sulfur, mainly occurring as neutral S6 and subordinate S4 molecules, according to the structural data. XPS spectroscopy confirms the presence of sulfide sulfur in neutral form. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0428. Slyudyankaite is triclinic, space group P1, a = 9.0523(4) Å, b = 12.8806(6) Å, c = 25.681(1) Å, α = 89.988(2)°, β = 90.052(1)°, γ = 90.221(1)°, V = 2994.4(2) Å3, Z = 1. Slyudyankaite contains two kinds of sodalite cages occurring in the structure in a ratio of 3:1. Cages of the first kind are completely occupied by anions and extra-framework cations, whereas cages of the second type contain only neutral molecules (S6, CO2, H2O, and minor S4). The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.45 (11) (004, 112, 020), 3.716 (100) (204, 220, 116, 132), 2.878 (12) (136, 028, 044), 2.625 (23) (208, 240), 2.431 (6) (209), 2.275 (6) (048), 2.143 (12) (0.0.12, 336), 1.784 (7) (444, 1.1.14, 356, 172).