Ammoniovoltaite, (NH4)2Fe2+5Fe3+3Al(SO4)12(H2O)18, is a new voltaite-group mineral. The mineral was discovered at Severo-Kambalny geothermal field, Kambalny volcanic ridge, Southern Kamchatka, Russia. Ammoniovoltaite forms at ~ 100°C around geothermal gas-steam vents in association with alunogen, tschermigite and pyrite. Crystals of ammoniovoltaite have euhedral habit up to 50 µm wide and grow on alunogen plates. Ammoniovoltaite is black with vitreous luster, opaque, brittle and water-soluble. Neither cleavage nor parting is found, the fracture is concoidal. The mineral is isotropic, the refractive index n =1.602(2) (589 nm). Infrared spectra contain absorption band at 1433 cm-1 distinctive for ammonium ion. The chemical composition is (iron content is given in accordance with Mössbauer data, H2O calculated by structure refinement, wt. %): FeO 13.26, Fe2O3 11.58, MgO 2.33, ZnO 0.04, Al2O3 2.74, SO3 47.46, K2O 0.19, CaO 0.11, (NH4)2O 2.96, H2O 16.66, total 97.33. The empirical formula based on S = 12 atoms per formula unit is [(NH4)1.88K0.08Ca0.04]Σ2.00(Fe2+3.74Mg1.17Fe3+0.05Zn0.01)Σ4.97(Fe3+2.89Al0.09)Σ2.98Al1.00(SO4)12.00(H2O)18.72. The crystal structure has been reﬁned to R1 = 0.0361 on the basis of 1217 unique reﬂections with I >2σ(I). Ammoniovoltaite is cubic, Fd-3c, a = 27.250(1) Å, V = 20234(2) Å3, Z = 16. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 9.67 (74) (022), 7.90 (56) (222), 5.58 (84) (224), 3.560 (100) (137), 3.418 (100) (008), 2.8660 (37) (139). A brief review of ammonium minerals from various volcanically active geological environments is given.