Micro-Raman spectra were obtained on an unpolished sample of suenoite in nearly backscattered geometry with a Jobin-Yvon Horiba XploRA apparatus, equipped with a motorised x–y stage and an Olympus BX41 microscope with a 10× objective (Dipartimento di Scienze della Terra, University of Pisa, Pisa, Italy). The 532 nm line of a solid-state laser was used. The minimum lateral and depth resolution was set to a few µm. The system was calibrated using the 520.6 cm⁻¹ Raman band of silicon before each experimental session. Spectra were collected through multiple acquisitions with single counting times of 60 s. Backscattered radiation was analysed with a 1200 grooves mm⁻¹ grating monochromator.

Figure 2 shows the Raman spectrum of suenoite. The region between 1200 and 30 cm⁻¹ is shown in Fig. 2a. According to Waeselmann et al. (2019), the Mg–Fe–Mn amphiboles have pronounced Raman scattering below 100 cm⁻¹. These authors also proposed that the Raman peak occurs at ca. 80–85 and 60–70 cm⁻¹ in monoclinic and orthorhombic amphiboles, respectively. However, in suenoite, a sharp Raman band occurs at 85 cm⁻¹. The presence of a Raman signal was observed with the c axis parallel and perpendicular to the polarisation direction of the incident laser, in agreement with other orthorhombic amphiboles. An additional band occurs at 50 cm⁻¹. The strongest Raman feature is a band at 665 cm⁻¹, related to the stretching modes of T–O–T bonds. The slight shift of the band position at wavenumbers lower than 670 cm⁻¹ can be attributed to the occurrence of ^CFe²⁺ and minor ^TAl (Waeselmann et al., 2019). The band at 1036 cm⁻¹ is able to give some information about the ^CFe²⁺ content and its distribution between the M1 and M3 sites. Using the relations proposed by Waeselmann et al. (2019) for ^CMn-free amphiboles, a total amount of ^CFe²⁺ of 1.32(8) atoms per formula unit (apfu) can be predicted, with ^M1,M3Fe2+=0.93 apfu. Consequently, ^M2Fe²⁺ may be 0.39(5) apfu. These data have to be compared with the results of Mössbauer spectroscopy and crystal-structure refinement (see below). In the region of the O–H stretching modes (Fig. 2b), four strong bands occur due to different local atomic arrangements, in line with those observed through infrared spectroscopy.

Polarised FTIR absorption spectra (Fig. 3) were measured on holotype material (36 µm thick prismatic grain) using a Bruker Vertex spectrometer equipped with a Hyperion II microscope, a Globar source, a KBr beam splitter, and an MCT detector (Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden). Data were acquired during 128 scans in the wavenumber range 6000–600 cm⁻¹ with a resolution of 2 cm⁻¹.

Bands occur at 3674 (shoulder), 3669, 3657, 3640, and 3620 cm⁻¹. These bands can be assigned to different local atomic arrangements around OH3A and OH3B. In agreement with Hawthorne and Della Ventura (2007), let us consider the site configuration symbol M1M1M3–O3–A:T1T1–M2M2M3. This symbol was proposed for monoclinic amphiboles but can be modified and applied to orthorhombic amphiboles. In suenoite, A and T sites can be considered occupied by □ and Si only, respectively, neglecting minor Na, Ca, and Al. In addition, very minor F replacing OH can be neglected. Bands at 3674 and 3669 cm⁻¹ are likely related to tremolite-like arrangements, i.e. MgMgMg. The slight shift of the band at 3669 cm⁻¹ may be related to the replacement of Ca²⁺ by (Mn,Fe)²⁺ at M4. The band at 3657 cm⁻¹ may be related to the atomic arrangement MgMgM²⁺, where M2+= Fe, Mn, neglecting minor Zn. Finally, the bands at 3640 and 3620 cm⁻¹ can be due to the arrangements MgM²⁺M²⁺ and M²⁺M²⁺M²⁺, respectively.

The Mössbauer spectrum of suenoite (Fig. 4) was collected at room temperature in transmission mode using a ⁵⁷Co point source in the Rh matrix with a nominal activity of 10 mCi (Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden). The Mössbauer spectrum was acquired during 16 d over the velocity range ±4 mm s⁻¹ and was calibrated against α-Fe foil. The Mössbauer absorber consisted of a few mg of carefully selected amphibole grains, which were axially pressed onto a ∼1.5 mm² area of a mylar window. This absorber preparation resulted in a preferred orientation of the fibrous to long prismatic mineral grains and caused intensity asymmetry of the recorded Mössbauer quadrupole doublets. The spectrum could be adequately fitted with two quadrupole doublets using the programme MossA (Prescher et al., 2012), resulting in the hyperfine parameters given in Table 1. In agreement with previous assignments of quadrupole doublets observed in Mössbauer spectra of amphibole-supergroup species (e.g. Hawthorne, 1988), the outer doublet with a quadrupole splitting of 2.77 mm s⁻¹ is assigned to Fe²⁺ at the M1–M3 sites, and the inner doublet with a quadrupole splitting of 1.66 mm s⁻¹ is assigned to Fe²⁺ at the M4 site. No absorption related to Fe³⁺ was observed in the recorded spectrum. Due to the limited amount of pure suenoite crystal fragments available, the absorption of the present spectrum is merely 0.4 %, and for this reason the signal-to-noise ratio is of medium quality even after more than 2 weeks of data acquisition. Consequently, small amounts (less than 5 % of the total iron content of the sample) of Fe³⁺ cannot be entirely ruled out. However, as amphiboles with mixed iron valences are in general strongly green to blue in colour due to Fe²⁺–Fe³⁺ intervalence charge transfer absorption that occurs in the red portion of the visible spectrum, the colourless nature of suenoite strongly suggests that it is indeed virtually free from Fe³⁺.

The general features of the crystal structure of suenoite are the same as those of the orthorhombic Pnma species belonging to the amphibole supergroup (e.g. Hawthorne and Oberti, 2007) and are not detailed here. Its structural formula can be written as A□M4(Mn_1.64Fe_0.36)_Σ2.00 ^M1−M3(Mg_3.55Fe_0.85Mn_0.60)_Σ5.00Si₈O₂₂ (OH)₂.

Electron microprobe data indicate the occurrence of minor Ca and Na at the A site, i.e. Ca_0.07Na_0.02. However, the single-crystal X-ray diffraction study does not provide any indication of the partial occupancy at this position, and the maximum residual in the difference-Fourier map is less than 1e Å⁻³, to be compared with an expected site-scattering value of ca. 1.6e. This minor discrepancy may be interpreted as the possible unobserved intergrowth of domains of exsolved (Ca,Na)-amphiboles in the studied crystal of suenoite, as described for instance in cummingtonite-(P2₁/m) by Hawthorne et al. (2025). On the other hand, the measured Ca may also occur as B cation, as its full attribution as A cation in the formula calculation is solely driven by the excess of (B+C) cations. This interpretation is supported by the absence of significant residual electron density (>1e Å⁻³) at the A site position in the difference-Fourier map, suggesting no unaccounted for cation occupancy.

The octahedral strip, formed by the M1, M2, and M3 polyhedra, hosts Mg, Mn²⁺, and Fe²⁺. The site population at the M1, M2, and M3 sites was proposed on the basis of the refined site scattering and the spectroscopic data. Raman spectroscopy data suggest a total amount of 1.32 ^CFe²⁺ apfu, with the sum of Fe²⁺ at M1 and M3 of 0.93 apfu and, consequently, 0.39 Fe²⁺ at the M2 site. These values were derived using a relation proposed by Waeselmann et al. (2019) for ^CMn-free amphiboles. However, even if the total amount of Fe²⁺, measured through electron microprobe data, 1.27 apfu, is in agreement with the predicted value, Mössbauer data suggest that 28 % of Fe²⁺ is at M4. Consequently, the values obtained using the relation of Waeselmann et al. (2019) were interpreted as giving the total sum of ^C(Fe,Mn). Then, in agreement with Oberti et al. (2018), Mn was distributed considering the site preference M1 > M2 ≫M3. Indeed, the larger amount of Mn²⁺ at the M1 site is consistent with the relatively larger < M1–O> distance with respect to the other two octahedra. The proposed site populations at the M1–M3 sites result in calculated site-scattering values which closely match the refined site-scattering values obtained by free refinement of the occupancy (Table 7). Overall, the total site-scattering value at M1+M2+M3 is 79.6 e (from the refined site occupancies) vs. 79.7 e (from the proposed site populations). Bond-valence sums at the M1–M3 sites range between 2.10 and 2.18 valence units (v.u.) (Table 8).

The M4 site, at the junction between the strips of octahedra and the double chains of tetrahedra, has five bond distances shorter than 2.50 Å and two additional longer ones (up to 2.97 Å). The eighth O atom is at a very long distance of 3.417 Å. The site population was based on both refined site scattering (Table 7) and the results of Mössbauer spectroscopy, indicating the partial replacement of Mn²⁺ by Fe²⁺. The bond-valence sum at the M4 site is 1.83 v.u., in accordance with the occurrence of divalent cations.

The T sites are mainly occupied by Si; in accordance with chemical data, only minor Al occur. Average <T–O> distances range from 1.622 to 1.632 Å, whereas the ≪T–O≫ distance is 1.6265 Å. Following the relationship between the ≪T–O≫ and the content of ^[4]Al (Hawthorne and Oberti, 2007), this latter value should be around 0.3 Al apfu, larger than the value found in electron microprobe analysis, i.e. 0.06 Al apfu. However, bond-valence sums at the T sites vary between 3.92 and 4.03 v.u., in accordance with an almost full occupancy by Si.

Finally, bond-valence sums of O atoms are in the range 1.92–2.07 v.u., the only exception being represented by the atoms at the OH3A and OH3B that have bond-valence sums of 1.10 and 1.11 v.u., respectively, in agreement with their occupancy by (OH) groups.

Suenoite is a new addition to the Pnma amphiboles (Table 9). The 10 species currently known belong to different amphibole subgroups and occur in different geological environments.

Suenoite belongs to the subgroup of magnesium–iron–manganese amphiboles (Hawthorne et al., 2012), along with anthophyllite, ferro-anthophyllite, gedrite, ferro-gedrite, ferro-papikeite, and papikeite. These latter minerals usually occur in medium- to high-grade metamorphic rocks (e.g. Rabbitt, 1948; Klein, 1966; Ford and Skippen, 1997; Elliot-Meadows et al., 2000; Peck and Valley, 2000; Hinchey and Carr, 2007; Hawthorne et al., 2022; Kihle et al., 2023). Suenoite occurs in a Mn ore deposit metamorphosed under greenschist-facies conditions (e.g. Molli et al., 2002; Fellin et al., 2007). The monoclinic dimorph clino-suenoite has been found in a similar geological setting, i.e. a metamorphosed Mn mineralisation (Oberti et al., 2018); a monoclinic amphibole, whose chemical composition based on energy-dispersive spectroscopy data is the same as that of clino-suenoite, has also been found at the Scortico–Ravazzone ore deposit and is currently under investigation.

The remaining three Pnma amphiboles belong to the subgroups of lithium amphiboles. These amphiboles typically crystallise owing to the interaction between Li-rich fluids and country rocks. Holmquistite is a grandfathered member of the amphibole supergroup, having been described at the beginning of the 20th century (Osann, 1913), whereas the other two Fe-bearing species, ferro-holmquistite and ferro-ferri-holmquistite, have been described since the beginning of the 21st century (Cámara and Oberti, 2005; Nagashima et al., 2022).

As stated above, the root name suenoite was first adopted for the mineral proto-ferro-suenoite, the third known mineral of the amphibole supergroup denoted by the prefix “proto-”, i.e. having Pnmn symmetry, after proto-ferro-anthophyllite (Sueno et al., 1988) and proto-anthophyllite (Konishi et al., 2003).