Infrared and Raman Spectroscopy of Ammoniovoltaite, (NH 4 ) 2 Fe 2 + 5 Fe 3 + 3 Al(SO 4 ) 12 (H 2 O) 18

: Ammoniovoltaite, (NH 4 ) 2 Fe 2 + 5 Fe 3 + 3 Al(SO 4 ) 12 (H 2 O) 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 NH 4 group is its ν 4 mode observed at 1432 cm − 1 with a shoulder at 1510 cm − 1 appearing due to NH 4 disorder. The Raman spectrum of ammoniovoltaite is basically di ﬀ erent from that of voltaite by the appearance of an intensive band centered at 3194 cm − 1 and attributed to the ν 3 mode of NH 4 . The latter can serve as a distinctive feature of ammonium in voltaite-group minerals in resemblance to recently reported results for another NH 4 -mineral—tschermigite, where ν 3 of NH 4 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 Fe 2 + X 6 ( 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(H 2 O) 6 or NH 4 .

Infrared (IR) spectra were obtained using the KBr pellet method and a Bruker Vertex 70 FTIR spectrometer at room temperature in the 4000 cm −1 to 400 cm −1 range of wavenumbers and 4 cm −1 resolution.
The Raman spectra were obtained with a spectrometer Horiba Jobin-Yvon LabRam HR 800 in the range of 4000 cm −1 to 70 cm −1 and the 2 cm −1 to 3 cm −1 resolution. The excitation source was an Ar + laser with a wavelength of 514 nm and a maximum power of 50 mW, the power at the sample~8 mW. The spectra were recorded at a room temperature.
Band component analysis was undertaken using the OriginPro 7.0 software package that enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using Gaussian function. The positions of the bands and their variance were refined by the steepest descent method, by minimizing the sum of the squares of the deviations of the experimental curve and the theoretical one (the sum of Gaussians), using the algorithms implemented in the OriginPro 7.0 program.

Crystal Structure
All voltaite-group minerals are cubic and crystallize in Fd-3c space group [3] with the exception of pertlikite [9] that is tetragonal, space group I4 1 /acd. The crystal structures of voltaite-type minerals ( Figure 1a) consists of 3D polymerized kröhnkite-type chains composed of corner-shared iron-centered octahedra with sulphate tetrahedra (Figure 1b) where Fe 2+ X 6 (X = O, OH) and Fe 3+ O 6 octahedra are alternating in the chain. The Al(H 2 O) 6 complexes and ammonium ions are located in cavities ( Figure 1b). . Infrared (IR) spectra were obtained using the KBr pellet method and a Bruker Vertex 70 FTIR spectrometer at room temperature in the 4000 cm −1 to 400 cm −1 range of wavenumbers and 4 cm -1 resolution.
The Raman spectra were obtained with a spectrometer Horiba Jobin-Yvon LabRam HR 800 in the range of 4000 cm −1 to 70 cm −1 and the 2 cm −1 to 3 cm −1 resolution. The excitation source was an Ar + laser with a wavelength of 514 nm and a maximum power of 50 mW, the power at the sample ~8 mW. The spectra were recorded at a room temperature.
Band component analysis was undertaken using the OriginPro 7.0 software package that enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using Gaussian function. The positions of the bands and their variance were refined by the steepest descent method, by minimizing the sum of the squares of the deviations of the experimental curve and the theoretical one (the sum of Gaussians), using the algorithms implemented in the OriginPro 7.0 program.

Crystal Structure
All voltaite-group minerals are cubic and crystallize in Fd-3c space group [3] with the exception of pertlikite [9] that is tetragonal, space group I41/acd. The crystal structures of voltaite-type minerals ( Figure 1a) consists of 3D polymerized kröhnkite-type chains composed of corner-shared ironcentered octahedra with sulphate tetrahedra (Figure 1b

Local Symmetry; Infrared and Raman Band Activiation
The sites, their occupancy and symmetry in the crystal structure of ammoniovoltaite are given in Table 1. The crystal structure of ammoniovoltaite can be considered as consisting of the following fragments for interpretation of vibrational spectra: NH4, FeX6, FeO6, Al(H2O)6 and SO4 (Table 1). Table  1 also shows the symmetry transformations of the infrared and Raman vibrations of structural fragments in accordance with the local symmetry and lattice symmetry.
It is worth noting that ammonium cation represents tetrahedra with symmetry Td, whereas site symmetry of the N atom (in NH4 group) in the ammoniovoltaite crystal structure is lower, D3. This mismatch of the symmetry of the cation and its environment results in the disorder of the NH4 group in ammoniovoltaite. To the best of our knowledge, no information on the character of this disorder

Local Symmetry; Infrared and Raman Band Activiation
The sites, their occupancy and symmetry in the crystal structure of ammoniovoltaite are given in Table 1. The crystal structure of ammoniovoltaite can be considered as consisting of the following fragments for interpretation of vibrational spectra: NH 4 , FeX 6 , FeO 6 , Al(H 2 O) 6 and SO 4 (Table 1). Table 1 also shows the symmetry transformations of the infrared and Raman vibrations of structural fragments in accordance with the local symmetry and lattice symmetry.
It is worth noting that ammonium cation represents tetrahedra with symmetry T d , whereas site symmetry of the N atom (in NH 4 group) in the ammoniovoltaite crystal structure is lower, D 3 . This mismatch of the symmetry of the cation and its environment results in the disorder of the NH 4 group in ammoniovoltaite. To the best of our knowledge, no information on the character of this disorder (dynamic or statistic) is available. The same situation may be characteristic for other ions having their own symmetry, for example, hydroxonium, H 3 O, as observed for hydroniumjarosite and hydronium-bearing jarosites [30,31].

Results
The IR and Raman spectra of ammoniovoltaite are given in Figure 2; the details of the spectra regions 4000 cm −1 to 2000 cm −1 , 2000 cm −1 to 800 cm −1 and 800 cm −1 to 400/70 cm −1 (400 cm −1 -infrared, 70 cm −1 -Raman) are given in  In general, both infrared and Raman spectra of ammoniovoltaite are characterized by an abundance of bands ( Figure 2), which corresponds to the diversity of structural fragments and variations of their local symmetry. Table 2 lists the infrared and Raman bands with their assignment. (dynamic or statistic) is available. The same situation may be characteristic for other ions having their own symmetry, for example, hydroxonium, H3O, as observed for hydroniumjarosite and hydroniumbearing jarosites [30,31].

Results
The IR and Raman spectra of ammoniovoltaite are given in Figure 2; the details of the spectra regions 4000 cm −1 to 2000 cm −1 , 2000 cm −1 to 800 cm −1 and 800 cm −1 to 400/70 cm −1 (400 cm −1 -infrared, 70 cm −1 -Raman) are given in  In general, both infrared and Raman spectra of ammoniovoltaite are characterized by an abundance of bands ( Figure 2), which corresponds to the diversity of structural fragments and variations of their local symmetry. Table 2 lists the infrared and Raman bands with their assignment.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies.

Infrared and Raman Spectra in the Region 2000 cm −1 to 800 cm −1
The 2000 cm −1 to 800 cm −1 region of Raman and infrared spectra is shown in Figure 5. The water bending mode is registered at the IR spectrum at 1637 cm −1 . The ammonium modes are evident at the IR spectrum: ν2 (NH4) at 1694 cm -1 and ν4 (NH4) at 1510 cm −1 and 1432 cm −1 ; the ammonium bands at 1544 cm −1 and 1453 cm −1 are hardly visible at the Raman spectrum. The weak shoulder at the IR spectrum at about 1337 cm −1 is likely due to overtones of 2ν4 (SO4) and 2ν3 (AlO6).
The most intense bands in the region 2000 cm −1 to 800 cm −1 at both IR and Raman spectra correspond to sulphate modes. The Raman 1207 cm −1 and 1149 cm −1 and infrared 1166 cm −1 and 1121 cm -1 bands correspond to ν3 (SO4) vibrations. The bands assigned to ν1 (SO4) vibration are found at 1032 cm −1 , 1005 cm −1 and 986 cm −1 in the Raman spectrum and at 1059 cm −1 , 1004 cm −1 and 985 cm -1 in the IR spectrum. The weak shoulder in the IR spectrum at 875 cm −1 corresponds to the Fe 2+ -OH fragment.

Infrared and Raman Spectra in the Region 800 cm −1 to 400(70) cm −1
The 800 cm −1 to 70 cm −1 and 800 cm −1 to 400 cm −1 regions of Raman and infrared spectra, respectively, are shown in Figure 6. The infrared bands at 740 cm −1 and 725 cm −1 correspond to librational vibrations of water coordinated to Al. Hydroxyl groups in the crystal structure of ammoniovoltaite appear as a result of dynamic equilibrium: Me-H2O + SO4 = Me-OH + HSO4, which is the sum of two processes, acid dissociation of aquacomplexes and protonation of the sulfate ion.   The Raman and infrared spectra of the 4000 cm −1 to 2000 cm −1 region is shown in Figure 3. This region includes the vibrational spectrum of the stretching vibrations of hydroxyl, water and ammonium units. It is also worth noting that two types of water are present in ammoniovoltaite: hydroxyl group coordinated to Fe 2+ and water molecules coordinated to Al 3+ . The IR spectrum contains the following bands 3585 cm −1 , 3467 cm −1 with a shoulder~3400 cm −1 , 3242 cm −1 , 3190 cm −1 , 2982 cm −1 , 2483 cm −1 . The Raman spectrum contains the following bands 3525 cm −1 , 3419 cm −1 , 3235 cm −1 , 3194 cm −1 and 2954 cm −1 .
The shoulder observed at 3585 cm −1 (IR spectrum) and 3525 cm −1 (Raman spectrum) is assigned to the symmetric stretching mode of the hydroxyl units (Al-OH), (Fe-OH) and ν 3 vibration of (H 2 O) fragments. The stretching vibrations of water molecules occur at lower wavenumbers than that of the hydroxyl unit. The ν 1 vibration of (H 2 O) is found at 3467 cm −1 with a shoulder~3400 cm −1 at the IR spectrum and at 3419 cm −1 at the Raman spectrum. The overlapping modes of ν 3 (NH 4 ), ν 3 (H 2 O) and 2ν 2 (H 2 O) occur at 3242 cm −1 and 3235 cm −1 in the IR and Raman spectrum, respectively. The details of OH and HOH modes are shown in Table 3.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The d(O···H) distances calculated from the position of bands at 3585 cm −1 , 3467 cm −1 and 3400 cm −1 range from 1.99 Å to 2.25 Å, while d(O···O) distances are within 2.83 Å to 3.24 Å range. The calculated values from infrared spectra d(O···H) and d(O···O) distances agree well with those derived from structure refinement [1] for H1 and H2 atoms:  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies.

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H2O)6 octahedra or NH4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H2O)6 octahedra or NH4 tetrahedra that have not been localized in the previous studies. The modes of ammonium ion are registered at (a) 3190 cm −1 (IR spectrum) and 3194 cm −1 (Raman spectrum) assigned to ν 3 (NH 4 ) vibration and at (b) 2982 cm −1 (IR spectrum) and 2954 cm −1 (Raman spectrum) that refers to completely symmetric vibration ν 1 (NH 4 ) ( Figure 3). The band at 2483 cm −1 corresponds to the vibrations of ν 3 (Al-H 2 O) (see Table 3) and ν (HSO 4 ); the hydrosulfate in the structure appears as a result of the equilibrium Me-H 2 O + SO 4 = Me-OH + HSO 4 (where Me-metal).

Hydrogen Bonding
Previous studies of natural [1] and synthetic [3] ammoniovoltaite included structure determination and refinement based on single-crystal X-ray diffraction data with localization of H1 and H2 hydrogen atoms that belong to Fe 2+ X 6 (X = O, OH) octahedra. The hydrogen atoms that belong to Al(H 2 O) 6 octahedra or NH 4 group have not been localized previously due to significant disorder of both units. The study of Libowitzky [32] has shown that correlation between OH stretching frequencies and both the O· ·O and the H· ·O bond distances exists.
The  (Table 4). This stronger bonding may refer to H atoms of Al(H 2 O) 6 octahedra or NH 4 tetrahedra that have not been localized in the previous studies.

Infrared and Raman Spectra in the Region 2000 cm −1 to 800 cm −1
The 2000 cm −1 to 800 cm −1 region of Raman and infrared spectra is shown in Figure 5. The water bending mode is registered at the IR spectrum at 1637 cm −1 . The ammonium modes are evident at the IR spectrum: ν2 (NH4) at 1694 cm -1 and ν4 (NH4) at 1510 cm −1 and 1432 cm −1 ; the ammonium bands at 1544 cm −1 and 1453 cm −1 are hardly visible at the Raman spectrum. The weak shoulder at the IR spectrum at about 1337 cm −1 is likely due to overtones of 2ν4 (SO4) and 2ν3 (AlO6).
The most intense bands in the region 2000 cm −1 to 800 cm −1 at both IR and Raman spectra correspond to sulphate modes. The Raman 1207 cm −1 and 1149 cm −1 and infrared 1166 cm −1 and 1121 cm -1 bands correspond to ν3 (SO4) vibrations. The bands assigned to ν1 (SO4) vibration are found at 1032 cm −1 , 1005 cm −1 and 986 cm −1 in the Raman spectrum and at 1059 cm −1 , 1004 cm −1 and 985 cm -1 in the IR spectrum. The weak shoulder in the IR spectrum at 875 cm −1 corresponds to the Fe 2+ -OH fragment.

Infrared and Raman Spectra in the Region 800 cm −1 to 400(70) cm −1
The 800 cm −1 to 70 cm −1 and 800 cm −1 to 400 cm −1 regions of Raman and infrared spectra, respectively, are shown in Figure 6. The infrared bands at 740 cm −1 and 725 cm −1 correspond to librational vibrations of water coordinated to Al. Hydroxyl groups in the crystal structure of ammoniovoltaite appear as a result of dynamic equilibrium: Me-H2O + SO4 = Me-OH + HSO4, which is the sum of two processes, acid dissociation of aquacomplexes and protonation of the sulfate ion. The 2000 cm −1 to 800 cm −1 region of Raman and infrared spectra is shown in Figure 4. The water bending mode is registered at the IR spectrum at 1637 cm −1 . The ammonium modes are evident at the IR spectrum: ν 2 (NH 4 ) at 1694 cm −1 and ν 4 (NH 4 ) at 1510 cm −1 and 1432 cm −1 ; the ammonium bands at 1544 cm −1 and 1453 cm −1 are hardly visible at the Raman spectrum. The weak shoulder at the IR spectrum at about 1337 cm −1 is likely due to overtones of 2ν 4 (SO 4 ) and 2ν 3 (AlO 6 ).
The most intense bands in the region 2000 cm −1 to 800 cm −1 at both IR and Raman spectra correspond to sulphate modes. The Raman 1207 cm −1 and 1149 cm −1 and infrared 1166 cm −1 and 1121 cm −1 bands correspond to ν 3 (SO 4 ) vibrations. The bands assigned to ν 1 (SO 4 ) vibration are found at 1032 cm −1 , 1005 cm −1 and 986 cm −1 in the Raman spectrum and at 1059 cm −1 , 1004 cm −1 and 985 cm −1 in the IR spectrum. The weak shoulder in the IR spectrum at 875 cm −1 corresponds to the Fe 2+ -OH fragment. The 800 cm −1 to 70 cm −1 and 800 cm −1 to 400 cm −1 regions of Raman and infrared spectra, respectively, are shown in Figure 6. The infrared bands at 740 cm −1 and 725 cm −1 correspond to librational vibrations of water coordinated to Al. Hydroxyl groups in the crystal structure of ammoniovoltaite appear as a result of dynamic equilibrium: Me-H 2 O + SO 4 = Me-OH + HSO 4 , which is the sum of two processes, acid dissociation of aquacomplexes and protonation of the sulfate ion.  The infrared bands observed at 661 cm −1 , 625 cm −1 and 591 cm −1 and Raman bands at 659 cm −1 , 625 cm −1 and 589 cm −1 are due to the overlap of ν 4 (SO 4 ), ν 3 (AlO 6 ) and ν 3 (FeO 6 ). Theoretically, the ν 3 (AlO 6 ) vibration has one active component in both IR and Raman spectra ( Figure 6). Most likely, this vibration overlaps with ν 4 (SO 4 ) vibration and is found at 591 cm −1 (IR spectrum) and 589 cm −1 (Raman spectrum). The bands 481 cm −1 , 450 cm −1 and 437 cm −1 in the IR spectrum and 463 cm −1 , 452 cm −1 and 432 cm −1 in the Raman spectrum correspond to overlaps ν 2 (SO 4 ), ν 3 (AlO 6 ) and ν 3 (FeO 6 ).

Infrared Spectroscopy of Voltaites
The infrared spectrum of ammoniovoltaite obtained in this work is compared to that of ammoniomagnesiovoltaite, (NH 4 ) 2 (Table 5). The position of bands in the 3500 cm −1 to 3000 cm −1 region differs for voltaites. This is probably due to difference in the cation composition (compared samples differ in the composition of divalent cation) affecting the hydrogen bonding system. The presence of ammonium likely also affects the spectra shape in the region 3300 cm −1 to 2900 cm −1 , but this change is almost imperceptible since there is a stronger change related to modes of water and hydroxyl. In general, the bands at 3600 cm −1 to 3000 cm −1 are due to various O-H and N-H stretching vibrations.
The band assigned to Al-H 2 O, hydrosulphate or both as a result of dynamic equilibrium: Me-H 2 O + SO 4 = Me-OH + HSO 4 mode is weak, but distinctive at all spectra at 2501 cm −1 to 2483 cm −1 . The H-O-H bending in the H 2 O molecules is evident by two bands at 1641 cm −1 to 1630 cm −1 and 1694 cm −1 to 1686 cm −1 . As noted previously [3] the distinctive infrared band of ammonium occurs at 1432 cm −1 to 1431 cm −1 due to the asymmetric bending vibrations of NH 4 . In samples studied by us, this band has a shoulder~1510 cm −1 that we attribute to ammonium disorder.
In the region 1200 cm −1 to 980 cm −1 sulphate vibrations occur: ν 3 at 1130 cm −1 to 1121 cm −1 and 1182 cm −1 to 1143 cm −1 , while ν 1 at 1065 cm −1 to 1053 cm −1 , 1014 cm −1 to 1004 cm −1 and 985 cm −1 . The M 2+ -OH mode is present at 879 cm −1 to 854 cm −1 . The position of the band depends on the cation: for samples with Fe 2+ (ammoniovoltaite, voltaite) it is observed at 879 cm −1 to 875 cm −1 ; for Mg-and Mn-dominated samples the band occurs at lower wavenumbers: 866 cm −1 and 854 cm −1 , respectively. The band centered at 735 cm −1 to 725 cm −1 in voltaites is assigned to Al-H 2 O mode. In the region below 700 cm −1 the most intensive bands are found at 634 cm −1 to 625 cm −1 , 596 cm −1 to 591 cm −1 and 445 cm −1 to 437 cm −1 (with lower intensity shoulders listed in Table 5) and are assigned to complex vibration of the sulphate group and metal-oxygen octahedra.

Raman Spectroscopy of Voltaites
The Raman spectrum of ammoniovoltaite obtained in this work is compared to very recently published spectra of voltaite [34] and tschermigite, (NH 4 )Al(SO 4 ) 2 ·12H 2 O, the latter is an ammonium alum [35], but it is chemically related to voltaites since it is hydrated ammonium sulphate ( Table 6). The Raman spectrum of voltaite from Iron Mountain Mine Superfund Site (Redding, CA, USA) has also been reported previously [22]; however, in the cited work, the inverse problem of identifying minerals by spectra without their detailed chemical characteristics is solved. Therefore, these data are not used for comparison.
The main difference between Raman spectra of ammoniovoltaite and voltaite [34] is in the shape of the 3400 cm −1 to 2800 cm −1 region. The spectra of ammoniovoltaite has a very intensive and distinctive band centered at 3194 cm −1 , although the spectrum of voltaite has a band with similar Raman shift, 3209 cm −1 , the shape of the spectra in this region is evidently different. It should be noted that Raman spectrum of tschermigite [35] contains the bands at 3163 cm −1 and 3124 cm −1 that were absent in the spectrum of its K-analogue and thus assigned to ν 3 (NH 4 ). On that basis, we assign the bands in the Raman spectrum of ammoniovoltaite as the following (Table 6): 3525 cm −1 and 3419 cm −1 to O-H stretching, 3235 cm −1 to overlap of O-H and N-H stretching and 3194 cm −1 exclusively to N-H stretching. The very weak bands at 1544 cm −1 and 1453 cm −1 refer to ν 4 of NH 4 , the band splitting is due to NH 4 disorder similar to that observed for the infrared spectrum. Sulphate vibrations are manifested in the 1210 cm −1 to 980 cm −1 region: ν 3 mode at 1207 cm -1 and 1143 cm −1 , while ν 1 mode at 1032 cm −1 , 1005 cm −1 and 986 cm −1 . In the region below 980 cm −1 and above 300 cm −1 the complex overlapping vibrations of different modes of SO 4 tetrahedra and metal-oxygen octahedra are detected. The bands below 300 cm −1 are assigned to lattice modes involving MeO 6 octahedra, SO 4 and NH 4 tetrahedra.  5. The N-H bending vibrations are evident as very weak Raman bands centered at 1544 cm −1 and 1453 cm −1 . The sulphate vibrations in the Raman spectrum are detected at 1207 cm −1 , 1149 cm −1 , 1032 cm −1 , 1005 cm −1 and 986 cm −1 .