Ammoniovoltaite, (NH4)(2)Fe52+Fe33+Al(SO4)(12)(H2O)(18), is a new voltaite-group mineral. The mineral was discovered at the Severo-Kambalny (North-Kambalny) geothermal field, Kambalny volcanic ridge, Southern Kamchatka, Russia. Anunoniovoltaite forms at similar to 100 degrees C around geothermal gas/steam vents in association with alunogen, tschermigite and pyrite. Crystals of ammoniovoltaite have euhedral habit, are up to 50 mu m in size and grow on alunogen plates. Ammoniovoltaite is black with vitreous lustre, opaque, brittle and water-soluble. Neither cleavage nor parting is found, the fracture is conchoidal. The mineral is isotropic, with the refractive index n = 1.602(2) (589 nm). Infrared spectra contain an absorption band at 1433 cm(-1) distinctive for the ammonium ion. The chemical composition is (iron content is given in accordance with Mossbauer data, H2O calculated from a crystal-structure refinement, wt.%): FeO 13.26, Fe(2)O(3)11.58, MgO 2.33, ZnO 0.04, Al2O3 2.74, SO3 47.46, K2O 0.19, CaO 0.11, (NH4)O-2 2.96, H2O 16.03, total 96.70. The empirical formula based on S = 12 atoms per formula unit is [(NH4)(1.88)K0.08Ca0.04](Sigma 2.00) (Fe3.742+Mg1.17Fe0.053+Zn0.01)(Sigma 4.97) Al-1.00(SO4)(12.00)(H2O)(18.00). The crystal structure has been refined to R-1 = 0.031 and 0.030 on the basis of 1217 and 1462 unique reflections with I>2 sigma(I) collected at 100 K and room temperature, respectively. Ammoniovoltaite is the ammonium analogue of voltaite. The mineral is cubic, Fd (3) over barc, a = 27.250(1) and V = 20234(3) (3) (at 100 K); and a = 27.322(1) and V = 20396(3) (3) (at RT), with Z = 16. The strongest lines of the powder X-ray diffraction pattern [d, (I,%)(hkl)] are: 9.67 (74) (022), 7.90 (56) (222), 5.58 (84) (422), 3.560 (100) (731), 3.418 (100) (008) and 2.8660 (37) (931). A brief review of ammonium minerals from various volcanically active geological environments is given.