The new eudialyte-group mineral siudaite, ideally Na₈(Mn²⁺ ₂Na)Ca₆Fe³⁺ ₃Zr₃NbSi₂₅O₇₄(OH)₂Cl·5H₂O, was discovered in a peralkaline pegmatite situated at the Eveslogchorr Mt., Khibiny alkaline massif, Kola Peninsula, Russia. The associated minerals are aegirine, albite, microcline, nepheline, astrophyllite, and loparite-(Ce). Siudaite forms yellow to brownish-yellow equant anhedral grains up to 1.5 cm across. Its lustre is vitreous, and the streak is white. Cleavage is none observed. The Mohs’ hardness is 4½. Density measured by hydrostatic weighing is 2.96(1) g/cm³. Density calculated using the empirical formula is equal to 2.973 g/cm³. Siudaite is nonpleochroic, optically uniaxial, negative, with ω = 1.635(1) and ε = 1.626(1) (λ = 589 nm). The IR spectrum is given. The chemical composition of siudaite is (wt%; electron microprobe, H₂O determined by HCN analysis): Na₂O 8.40, K₂O 0.62, CaO 9.81, La₂O₃ 1.03, Ce₂O₃ 1.62, Pr₂O₃ 0.21, Nd₂O₃ 0.29, MnO 6.45, Fe₂O₃ 4.51. TiO₂ 0.54, ZrO₂ 11.67, HfO₂ 0.29, Nb₂O₅ 2.76, SiO₂ 47.20, Cl 0.54, H₂O 3.5, –O = Cl − 0.12, total 99.32. According to Mössbauer spectroscopy data, all iron is trivalent. The empirical formula (based on 24.5 Si atoms pfu, in accordance with structural data) is [Na_7.57(H₂O)_1.43]_Σ9(Mn_1.11Na_0.88Ce_0.31La_0.20Nd_0.05Pr_0.04K_0.41)_Σ3(H₂O)_1.8(Ca_5.46Mn_0.54)_Σ6(Fe³⁺ _1.76Mn²⁺ _1.19)_Σ2.95Nb_0.65(Ti_0.20Si_0.50)_Σ0.71(Zr_2.95Hf_0.04Ti_0.01)_Σ3Si_24.00Cl_0.47O₇₀(OH)₂Cl_0.47·1.82H₂O. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3m, with a = 14.1885(26) Å, c = 29.831(7) Å, V = 5200.8(23) ų and Z = 3. Siudaite is chemically related to georgbarsanovite and is its analogue with Fe³⁺-dominant M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.38 (60) (–114), 4.29 (55) (–225), 3.389 (47) (131), 3.191 (63) (–228). 2.963 (100) (4–15), 2.843 (99) (–444), 2.577 (49) (3–39). Siudaite is named after the Polish mineralogist and geochemist Rafał Siuda (b. 1975).