A contribution to the mineralogy of Sicily, Italy – Kintoreite from the Tripi mine, Peloritani Mountains: occurrence and crystal structure

Abstract A new occurrence of kintoreite, ideally PbFe3(PO4)(PO3OH)(OH)6, is described from the Tripi mine, Alì, Peloritani Mountains, northeastern Sicily, Italy. Kintoreite occurs as yellow equant crystals, up to 0.05 mm in size, on quartz, associated with another alunite-supergroup mineral currently under study. The studied grains of kintoreite are chemically zoned, showing the occurrence of two distinct domains. Electron microprobe analyses gave (in wt.%): SO3 5.47(27), P2O5 17.55(14), Al2O3 7.81(35), Fe2O3 23.81(44), PbO 35.11(41), H2Ocalc 9.93, total 99.68 (darker domains) and SO3 6.03(5), P2O5 16.66(23), Al2O3 4.22(2), Fe2O3 27.31(6), PbO 33.78(22), H2Ocalc 10.25, total 98.25 (brighter domains). On the basis of 14 O atoms per formula unit (apfu) and (P+S) = 2 apfu, the darker and brighter domains have chemical formula Pb1.00(Fe3+1.89Al0.97)Σ2.86[P1.57S0.43O7.43(OH)0.57][(OH)5.58(H2O)0.42]Σ6.00 and Pb0.98(Fe3+2.21Al0.53)Σ2.74[P1.51S0.49O7.49(OH)0.51][(OH)5.18(H2O)0.82]Σ6.00, respectively. Single-crystal X-ray diffraction was performed on a grain extracted from the same sample. Unit-cell parameters are a = 7.2191(10), c = 16.834(3) Å, V = 759.8(3) Å3 and Z = 3; space group R$\bar{3}$m. The crystal structure was refined on the basis of 226 unique reflections with Fo > 4σ(Fo) and 31 least-square parameters to R1 = 0.0415. Kintoreite is isotypic with other alunite-supergroup minerals. Micro-Raman spectroscopy suggests the possible occurrence of H2O groups, supporting the possible protonation of (OH) groups as a charge-balance mechanism in this mineral. Kintoreite may play an environmental role in the Alì area, where previous authors detected high concentrations of potentially toxic elements in soils.

De Vivo et al. (1993) pointed out the presence of anomalies of As, Hg, Sb, Zn and Pb in stream sediments from the Peloritani Mountains. Cosenza et al. (2015) investigated the soil geochemistry in this area, reporting high contents of PTE such as As, Zn, Sb and Pb. One of the two anomalous areas found in this region is located between Fiumedinisi, Budali and Alì, and Pb isotopes indicate a geogenic contribution to this contamination, related to the weathering of ore deposits and mine wastes in the area studied.
In the framework of a survey of the mineralogy of the Peloritani Mountains, a sample of galena-bearing quartz vein was sampled from the adit of one of the tunnels belonging to the abandoned Tripi mine, Alì, Messina Province. The observation of this sample under the binocular microscope revealed the occurrence of alunitesupergroup minerals encrusting thin fissures in the quartz vein. One of them formed yellow equant grains, whose chemical composition indicated its classification as kintoreite, a relatively uncommon phase first described by Pring et al. (1995) from Broken Hill, New South Wales, Australia.
There are several reasons for characterising this new occurrence of kintoreite. From an environmental perspective, this phase, whose origin is related to the weathering of primary ores, could be a potential host for the Pb anomaly observed in soils in the Alì area (Cosenza et al., 2015), in agreement with previous studies showing the presence of nanoparticles of kintoreite in soils (e.g. Schindler and Hochella, 2017). Moreover, the finding of kintoreite from the Peloritani Mountains is the third Italian occurrence of this mineral, following the identification reported by Albertini (2014) from the Monte Falò mine, Piedmont, and by Guastoni et al. (2018) from the nearby locality of Chiotta Strivera, Piedmont. Only few data are reported for these occurrences and no crystal-chemical characterisation was performed. Finally, kintoreite from the Tripi mine is enriched in Al and S and consequently its full description can provide interesting insights on the crystal chemistry of alunite-supergroup minerals.

Geological background
The Peloritani Mountains represent the southernmost portion of the Calabria-Peloritani Arc, an arc-like structure that connects the Apennines with the Maghrebian Chain. It is delimited by the Pollino line, in the north, and the Taormina line, in the south (Messina et al., 2004, and references therein). This mountain belt is formed by the stacking of several tectonic units. Ore bodies are hosted mainly in the Mandanici Unit, a tectonic slice formed by low-to medium-grade metamorphic rocks affected by the Variscan orogeny; the Alpine event gave rise to cataclastic-to-mylonitic processes (Ferla and Meli, 2007;Carbone et al., 2007, and references therein). The actual genesis and evolution of the ore deposits of the Peloritani Mountains have been debated by several authors (e.g. Ferla, 1982Ferla, /1983Ferla and Omenetto, 2000;Saccà et al., 2003).

Specimen studied
Kintoreite occurs as yellow equant crystals, up to 0.05 mm in size, on quartz, associated with another alunite-supergroup mineral currently under study (Fig. 1a). The sample studied is deposited in the mineralogical collection of the Museo di Storia Naturale of the Università di Pisa, under the catalogue number 19928.

Chemical analysis
Quantitative chemical analyses were perfomed at the 'Eugen F. Stumpfl' laboratory, University of Leoben (Austria) using a Superprobe JEOL JXA 8200 electron microprobe (WDS mode, accelerating voltage = 20 kV, beam current = 10 nA and beam size = 1 μm). The following standards (element, emission line) were used: crocoite (PbKα), magnetite (FeKα), apatite (PKα), corundum (AlKα) and anhydrite (SKα). Other elements (K, Sr, Ba and As) were sought but were below the detection limit. The ZAF routine was applied for the correction of the recorded raw data. Counting times were 15 s for peak and 5 s for backgrounds. Table 1 gives chemical data.

Micro-Raman spectroscopy
Micro-Raman spectra were collected using a Horiba Jobin-Yvon XploRA Plus apparatus, equipped with a motorised x-y stage and an Olympus BX41 microscope with a 50× objective (Dipartimento di Scienze della Terra, Università di Pisa). Raman spectra were excited using the 532 nm line of a solid-state laser, attenuated to 25% (i.e. 6.25 mW) in order to avoid any potential sample damage. 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 (3) with variable counting times, ranging from 30 to 60 s. Back-scattered radiation was analysed with a 1200 gr/mm grating monochromator. Raman spectra were processed using Fityk  (Wojdyr, 2010), subtracting the background and fitting the spectra to theoretical peak shapes using Voigt functions. Experimental precision can be estimated at ± 2 cm -1 .

X-ray crystallography
Single-crystal X-ray diffraction data were collected at the Dipartimento di Scienze della Terra, Università di Pisa, Italy, using a Bruker Apex II diffractometer operating at 50 kV and 30 mA and equipped with an air-cooled Photon II CCD detector. Graphite-monochromatised MoKα radiation was used. The detector-to-crystal working distance was set to 50 mm. Intensity data were integrated and corrected for Lorentz, polarisation, background effects and absorption using the APEX3 software package (Bruker AXS Inc., 2016). Crystal structure refinement was performed using Shelxl-2018 (Sheldrick, 2015) starting from the atomic coordinates given by Kharisun et al. (1997) for kintoreite. Neutral scattering curves were taken from the International Tables for Crystallography (Wilson, 1992). The following scattering curves were used: Pb at the Pb site, Fe at the Fe site, P at the P site, and O at the O(1)-O(3) sites. After several cycles of isotropic refinement, the R 1 factor converged to 0.0674, thus confirming the correctness of the structural model. Taking into account chemical data, the site occupancy at the Fe site was refined using the scattering curves of Fe vs. Al, improving the quality of refinement to R 1 = 0.0447. Owing to the similar scattering factors for P (Z = 15) and S (Z = 16), only the scattering curve of the former was used. Introducing the anisotropic displacement parameters for all atoms decreased the R 1 to 0.0376. However, Pb was negatively defined and consequently the final model was refined using anisotropic displacement parameters for all atoms but Pb. A maximum residual, interpreted as an H atom bonded to O(3), was located in the difference-Fourier map and a restraint on the O(3)-H bond distance was added. The refinement converged to R 1 = 0.0415 for 226 unique reflections with F o > 4σ (F o ) and 31 least-square parameters. Details of data collection and structure refinement are in Table 2, whereas atomic coordinates and displacement parameters are shown in Table 3. Bond distances and bond-valence sums, calculated according to the bond parameters of Gagné and Hawthorne (2015), are given in Tables 4 and 5, respectively. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Chemical data
The back-scattered electron image of the grain studied revealed a compositional inhomogeneity (Fig. 1b), with dark and bright domains, related to different Fe/Al atomic ratios. Electron microprobe data were recalculated on the basis of 14 O atoms per formula unit (apfu), calculating the amount of H 2 O in order to constrain the sum of (P + S) to 2 apfu. To the best of our knowledge, the sample from the Tripi mine has the highest S content among kintoreite occurrences reported in literature. Kharisun et al. (1997) described a kintoreite with a significant As and S substitution for P, i.e. 0.40 As apfu and 0.30 S apfu. On the contrary, several findings from Slovakia and the Czech Republic have negligible S contents (Pauliš et al., 2012;Števko et al., 2016;Vrtiška et al., 2019Vrtiška et al., , 2021. The substitution of Fe 3+ by Al 3+ in the octahedral site agrees with the wide substitution observed between kintoreite and plumbogummite, ideally PbAl 3 (PO 4 )(PO 3 OH)(OH) 6 , by other authors (e.g. Vrtiška et al., 2016).

Micro-Raman spectroscopy
The Raman spectral features of kintoreite from the Tripi mine are shown in Fig. 2. Between 100 and 1200 cm -1 (Fig. 2a), two regions can be distinguished. The first one, between 900 and 1200 cm -1 , is characterised by stretching modes of P-O bonds and, probably, of S-O bonds. In the second region, between 100 and 650 cm   bands agree with the general features described by Frost et al. (2006). The interpretation of the weak band at 820 cm -1 , attributed to the occurrence of (AsO 4 ) groups by Frost et al. (2006), is, on the contrary, uncertain; indeed, As was found to be below the detection limit in the sample studied from the Tripi mine. Moreover, Plášil et al. (2014) interpreted some bands observed at 722, 812 and 859 cm -1 in the Raman spectrum of hydroniumjarosite as due to δ(OH) modes, in keeping with the chemistry of the kintoreite studied.
In the region between 1500 and 1700 cm -1 , a weak band at 1603 cm -1 was observed (Fig. 2b); Frost et al. (2006) reported a Raman band at 1595.6 cm -1 , attributed to the occurrence of H 2 O groups. Indeed, this could be interpreted as due to the O-H-O bending of H 2 O groups. The Raman spectrum of the studied material in the region between 2800 and 3400 cm -1 is different from that reported by Frost et al. (2006). In fact they observed a broad band related to the convolution of three components at 2967.8, 3224.6, and 3391.1 cm -1 . On the contrary, in kintoreite from the Tripi mine (Fig. 2c). four weak bands were observed, three of them forming a relatively broad band between 2800 and 3000 cm -1 , and the fourth one occurring as a relatively sharp band at 3060 cm -1 .

Cation sites
Lead is hosted in a distorted icosahedron and is displaced from the origin, being distributed in a ring-like structure in the (0001) plane at the 18h position. In hinsdalite, PbAl 3 (PO 4 )(SO 4 )(OH) 6 , and plumbogummite, Pb occurs at the 18f position (Kolitsch et al., 1999), as in the refinement of kintoreite reported by Grey et al. (2009). On the contrary, Kharisun et al. (1997) proposed a model where Pb is located on a 36i position. Moreover, this element is located at the origin in corkite, PbFe 3 (PO 4 )(SO 4 )(OH) 6 , and osarizawaite, Pb(Al 2 Cu 2+ )(SO 4 ) 2 (OH) 6 , but with anomalously large displacement parameters Tadini, 1980, 1987). The distorted twelve-fold coordination shown by Pb in kintoreite from the Tripi mine is characterised by six short bonds (Pb-ϕ distance shorter than 2.80 Å) and six longer ones. This distortion is probably due to the activity of the 6s 2 lone-pair of Pb 2+ . The average <Pb-ϕ> distance is 2.833 Å, to be compared with 2.842 Å and 2.846 Å for kintoreite studied by Kharisun et al. (1997) and Grey et al. (2009), respectively. These values can be compared with those observed in other Pb-bearing alunitesupergroup minerals, e.g. 2.79 Å in hinsdalite and plumbogummite (Kolitsch et al., 1999), 2.823 Å in corkite (Giuseppetti and Tadini, 1987) and 2.858 Å in beudantite (Szymański, 1988). In these phases, the <Pb-O(2)> distances are longer than the <Pb-O(3)>, e.g. 2.87 vs. 2.80 Å in beudantite (Szymański, 1988), 2.92 vs. 2.73 Å in corkite (Giuseppetti and Tadini, 1987) and 2.82 vs. 2.78 Å in gallobeudantite (Jambor et al., 1996). In kintoreite from the Tripi mine, these values are 2.90 and 2.77 Å, respectively, to be compared with those given by Kharisun et al. (1997) and Grey et al. (2009)  Iron, replaced partially by Al and a minor vacancy, is the main G constituent and is hosted in a quite regular octahedron, formed by four equatorial O(3) sites and two apical O(2) sites. Average <G-ϕ> distance is 1.972 Å, shorter than those reported by Kharisun et al. (1997) and Grey et al. (2009), i.e. 2.011 Å in both structural models, owing to the extensive Al-to-Fe 3+ replacement. Using the ionic radii proposed by Shannon (1976), the observed average bond distance would correspond to the Fe site occupancy (Fe 0.70 Al 0.30 ). This would correspond to the site population (Fe 2.10 Al 0.90 ), to be compared with (Fe 1.89 Al 0.97 □ 0.14 ) and (Fe 2.21 Al 0.53 □ 0.26 ) for the dark and bright domains, respectively. The discrepancy between electron microprobe data and calculated site population is probably due to the unaccounted occurrence of a vacancy. Refined site scattering at the Fe site is 63.2 electrons per formula unit (epfu), to be compared with 61.8 epfu calculated for the dark domain and 64.4 epfu for the brighter ones. The observed value is intermediate between these two values. The bond-valence sum at the Fe site was calculated on the basis of the site occupancy (Fe 0.68 Al 0.32 ) refined from single-crystal X-ray diffraction data and it corresponds to 2.90 vu.
Phosphorus and minor S are tetrahedrally coordinated, with an average <T-ϕ> distance of 1.526 Å. This distance is shorter than that reported by Kharisun et al. (1997), i.e. 1.563 Å, related to the partial substitution of P 5+ by the larger As 5+ cation. In 'pure' kintoreite studied by Grey et al. (2009), the corresponding distance is 1.537 Å, in agreement with the grand <P-ϕ> distance given by Huminicki and Hawthorne (2002). In the sample from the Tripi mine, P is replaced partially by S. On the basis of the observed average distance, using the ideal distances reported by Huminicki and Hawthorne (2002) and Hawthorne et al. (2000) for <P-ϕ> and <S-ϕ>, respectively, the site occupancy (P 0.83 S 0.17 ) for the P site can be proposed. In comparison those observed in the dark and bright domains are (P 0.785 S 0.215 ) and (P 0.755 S 0.245 ), respectively. The bond-valence sum, calculated assuming the site occupancy (P 0.78 S 0.22 ), is 5.15 vu, in accord with the partial replacement of P 5+ by S 6+ . In kintoreite, P and S are disordered, as observed in other mixed (P,S) phases belonging to the alunite supergroup, e.g. woodhouseite, CaAl 3 (PO 4 )(SO 4 )(OH) 6 (Kato, 1977), svanbergite, SrAl 3 (PO 4 )(SO 4 )(OH) 6 (Kato and Miura, 1977) and hinsdalite (Kolitsch et al., 1999). The ordering of P and S in corkite reported by Giuseppetti and Tadini (1987) is considered questionable (e.g. Bayliss et al., 2010).

Anion sites and H bonds in kintoreite
Three anion sites occur in the crystal structure of kintoreite, i.e. O(1), O(2) and O(3).
O(1) is bonded to (P,S) only, with a relatively short distance of 1.510 Å. Its bond-valence sum indicates a significant underbonding, 1.34 vu. O(2) is corner-shared between one (P,S)-centred tetrahedron and an (Fe,Al)-centred octahedron. Moreover, it is Using the relations proposed by Ferraris and Ivaldi (1988), a bond strength of 0.18 vu can be calculated for this H bond. In this way, the bond-valence sum at O(3), that acts as a donor, ranges between 0.92 and 1.08 vu, whereas O(1), acceptor of three H bonds, increases its bond-valence sum up to 1.88 vu. However, as stressed by Cooper and Hawthorne (2012) (2) angle, ≈121°, is very small, close to the lower limit of observed O-H⋅⋅⋅O bonds and indicative of a very weak bond (Brown, 1976). These values can be compared with those given by Cooper and Hawthorne (2012) Fig. 4.
Micro-Raman spectroscopy reveals the occurrence of possible H-O-H bending modes in kintoreite, thus supporting the presence of H 2 O groups. Although H 2 O groups may be the result of the protonation of (OH)groups, as suggested by Szymański (1985), a more reliable hypothesis is that the occurrence of H 2 O is related to the presence of octahedral vacancies. Their occurrence has been suggested by previous investigators (e.g. Jambor and Dutrizac, 1983;Szymański, 1985) and they seem to be coupled with the protonation of equatorial H 2 O groups of octahedral sites, as described by Nielsen et al. (2007).
The sample from the Tripi mine has an exceptionally small unit-cell volume, i.e. 759.8(3) Å 3 , with a = 7.2191(10) and c = 16.834(3) Å. Assuming the unit-cell parameters of the specimen studied by Grey et al. (2009) as a reference for ideal kintoreite, the specimen from the Tripi mine has Δa = -1.1%, Δc = -0.1% and ΔV = -2.2%. As discussed by other authors (e.g. Stoffregen et al., 2000), the a unit-cell parameter is affected mainly by the Fe 3+ -Al substitution, whereas variations of the unit-cell parameter c are largely related to substitution at the twelve-fold coordinated site. Consequently, the contraction of the Tripi mine sample is probably due to the high Al content of the material studied, supporting the existence of a substitutional series with the Al-isotype plumbogummite.

Conclusions
Kintoreite from the Tripi mine represents the first wellcharacterised Italian occurrence of this uncommon member of the alunite supergroup. Using a multi-analytical approach (electron microprobe analysis, single-crystal X-ray diffraction, and micro-Raman spectroscopy), the crystal chemical features of this mineral species were determined, detailing the H-bond system and suggesting the occurrence of H 2 O groups, probably due to the presence of an octahedral vacancy. In this way, the plasticity of the alunite structure to accommodate several substitutions is further confirmed. This is particularly important as kintoreite is able to host PTE such as Pb. Indeed, in the Alì area, high Pb content of geogenic origin in soils has been reported by Cosenza et al. (2015).
Notwithstanding the current lack of mineralogical studies aiming at constraining the mineralogy of secondary assemblages from the Peloritani Mountains, it is very likely that alunite-supergroup minerals may play a central role in determining the fate of PTE in this former mining district. The finding and characterisation of kintoreite could represent a first step in assessing the environmental role of these phases in this sector of northeastern Sicily.