The new eudialyte-group mineral sergevanite, ideally Na ₁₅(Ca ₃Mn ₃)(Na ₂Fe)Zr ₃Si ₂₆O ₇₂(OH) ₃·H ₂O, was discovered in highly agpaitic foyaite from the Karnasurt Mountain, Lovozero alkaline massif, Kola Peninsula, Russia. The associated minerals are microcline, albite, nepheline, arfvedsonite, aegirine, lamprophyllite, fluorapatite, steenstrupine-(Ce), ilmenite, and sphalerite. Sergevanite forms yellow to orange-yellow anhedral grains up to 1.5 mm across and the outer zones of some grains of associated eudialyte. Its luster is vitreous, and the streak is white. No cleavage is observed. The Mohs’ hardness is 5. Density measured by equilibration in heavy liquids is 2.90(1) g/cm ³. Calculated density is equal to 2.906 g/cm ³. Sergevanite is nonpleochroic, optically uniaxial, positive, with x ¼ 1.604(2) and e ¼ 1.607(2) (k ¼ 589 nm). The infrared spectrum is given. The chemical composition of sergevanite is (wt.%; electron microprobe, H ₂O determined by HCN analysis): Na ₂O 13.69, K ₂O 1.40, CaO 7.66, La ₂O ₃ 0.90, Ce ₂O ₃ 1.41, Pr ₂O ₃ 0.33, Nd ₂O ₃ 0.64, Sm ₂O ₃ 0.14, MnO 4.15, FeO 1.34, TiO ₂ 1.19, ZrO ₂ 10.67, HfO ₂ 0.29, Nb ₂O ₅ 1.63, SiO ₂ 49.61, SO ₃ 0.77, Cl 0.23, H ₂O 4.22, –O¼Cl –0.05, total 100.22. The empirical formula (based on 25.5 Si atoms pfu, in accordance with structural data) is H _14.46Na _13.64K _0.92Ca _4.22Ce _0.27La _0.17Nd _0.12Pr _0.06Sm _0.02Mn _1.81 Fe ^2þ _0.58Ti _0.46Zr _2.67Hf _0.04Nb _0.38Si _25.5S _0.30Cl _0.20O _81.35. The crystal structure was determined using single-crystal X-ray diffraction data. The new mineral is trigonal, space group R3, with a ¼ 14.2179(1) Å, c ¼ 30.3492(3) Å, V ¼ 5313.11(7) Å ³, and Z ¼ 3. In the structure of sergevanite, Ca and Mn are ordered in the six-membered ring of octahedra (at the sites M1 ₁ and M1 ₂), and Na dominates over Fe ^2þ at the M2 site. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 7.12 (70) (110), 5.711 (43) (202), 4.321 (72) (205), 3.806 (39) (033), 3.551 (39) (220, 027), 3.398 (39) (313), 2.978 (95) (345), 2.855 (100) (404). Sergevanite is named after the Sergevan’ River, which is near the discovery locality.