Ammoniovoltaite, (NH₄)₂Fe²⁺₅Fe³⁺₃Al(SO₄)₁₂(H₂O)₁₈, is a complex hydrated sulphate of the voltaite group that has been recently discovered on the surface of the Severo-Kambalny geothermal field (Kamchatka, Russia). Vibrational spectroscopy has been applied for characterization of the mineral. Both infrared and Raman spectra of ammoniovoltaite are characterized by an abundance of bands, which corresponds to the diversity of structural fragments and variations of their local symmetry. The infrared spectrum of ammoniovoltaite is similar to that of other voltaite-related compounds. The specific feature related to the dominance of the NH₄ group is its ν₄ mode observed at 1432 cm⁻¹ with a shoulder at 1510 cm⁻¹ appearing due to NH₄ disorder. The Raman spectrum of ammoniovoltaite is basically different from that of voltaite by the appearance of an intensive band centered at 3194 cm⁻¹ and attributed to the ν₃ mode of NH₄. The latter can serve as a distinctive feature of ammonium in voltaite-group minerals in resemblance to recently reported results for another NH₄-mineral—tschermigite, where ν₃ of NH₄ occurs at 3163 cm⁻¹. The values calculated from wavenumbers of infrared bands at 3585 cm⁻¹, 3467 cm⁻¹ and 3400 cm⁻¹ for hydrogen bond distances: d(O···H) and d(O···O) correspond to bonding involving H1 and H2 atoms of Fe²⁺X₆ (X = O, OH) octahedra. The infrared bands observed at 3242 cm⁻¹ and 2483 cm⁻¹ are due to stronger hydrogen bonding, that may refer to non-localized H atoms of Al(H₂O)₆ or NH₄.

In this study, sulphate efflorescent minerals covering the surface of the Donnoe and Dachnoe fields of the Mutnovsky volcano are described. The minerals were precipitated on the argillic facies as the result of water–rock interaction and fumarole emission. A chemical composition of Ca, Ba, (NH₄)⁺, Na-Fe³⁺, (NH₄)⁺-Al, (NH₄)⁺-Fe³⁺, Na-Al, K-Al, and K-Fe³⁺ sulphates was reported. Elements such as Sr, Mg, Co, Ni, Ti and P were found as isomorphic impurities. Ammonia species were concentrated around fumaroles. The mineral assemblage described herein is unique in relation to other geological settings and reflects the process of low-temperature mineral formation associated with volcanism. The thermal water contains cations such as H, Na, K, NH₄, Ca, Mg, Fe²⁺, Fe³⁺, and Al in different proportions with pH ranging from 2.4 to 6.5 and the dominance of acidic waters. The gas condensate bears such cations as (NH₄)⁺, Ca, and Mg and has a pH of ~5. Thus, the rest of the main cations are derived from the leaching of the host rocks. Among the identified phases, the alunite-supergroup minerals are more prone to isomorphism. The Ti, Co, and Ni impurities mark the unique geochemistry of thermal water at the Mutnovsky volcano. We postulate that the chemical composition of alunite-supergroup minerals reflects the types of hydrothermal occurrences and contains important information on the geochemistry of the hydrothermal process.

Ammoniovoltaite, (NH₄)₂Fe²⁺₅Fe³⁺₃Al(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. Ammoniovoltaite forms at ~100°C around geothermal gas/steam vents in association with alunogen, tschermigite and pyrite. Crystals of ammoniovoltaite have euhedral habit, are up to 50 µm 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 Mössbauer data, H₂O calculated from a crystal-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 16.03, total 96.70. The empirical formula based on S = 12 atoms per formula unit is [(NH₄)_1.88K_0.08Ca_0.04]_Σ2.00(Fe²⁺_3.74Mg_1.17Fe³⁺_0.05Zn_0.01)_Σ4.97(Fe³⁺_2.89Al_0.09)_Σ2.98Al_1.00(SO₄)_12.00(H₂O)_18.00. The crystal structure has been refined to R₁ = 0.031 and 0.030 on the basis of 1217 and 1462 unique reflections with I >2σ(I) collected at 100 K and room temperature, respectively. Ammoniovoltaite is the ammonium analogue of voltaite. The mineral is cubic, Fd $\bar{3}$ c, a = 27.250(1) Å and V = 20234(3) ų (at 100 K); and a = 27.322(1) Å and V = 20396(3) ų (at RT), with 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) (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.

The crystal structure of a naturally occurring layered double hydroxide mineral—desautelsite from San Benito County, California, USA—was refined using single-crystal X-ray diffraction data in the space group R-3m, a = 3.1238(2) Å, c = 23.528(3) Å, V = 198.83(4) ų, and Z = 3/8. The Mg and Mn cations are disordered occurring in one M site with occupancy Mg_0.77Mn_0.23. According to the electron microprobe analysis supported by Raman spectroscopy, the empirical formula is Mg_6.20(Mn^III_1.78Al_0.01Fe^III_0.01)_Σ1.80(OH)₁₆(CO₃)_0.90·5.35H₂O that shows higher content of interlayer (H₂O) molecules in comparison to the ideal formula that also agrees with the structure refinement. The Raman spectroscopy of two samples indicated O–H vibrations (3650/3640 cm⁻¹, ~3500 sh cm⁻¹), symmetric C–O (1055/1057 cm⁻¹), Mg–O–Mg (533/533 cm⁻¹) and Mn–O–Mn (439/438 cm⁻¹) stretching vibrations and lattice vibrations (284/287 cm⁻¹). Summing up our data and that of the current literature, we show a correlation (R² = 0.91) between the averaged effective ionic radius (x) and a unit cell parameter (y) of hydrotalcite group minerals, $y=1.9871x+1.4455$. Desautelsite follows this correlation, being the species with one of the largest a unit cell parameters among the group. The correlation can be applied for control of cation intercalation during synthesis.