Copper Minerals at Vesuvius Volcano (Southern Italy): A Mineralogical Review

This work is part of a project focused on the Somma–Vesuvius volcano and aimed at identifying Cu minerals related to mineralizing processes associated with magmatic activity in an active magmatic-hydrothermal system. A mineralogical survey was carried out on a set of samples represented by sublimates and fumarolic products from the collection of the Mineralogical Museum of the University of Naples Federico II (Italy). These samples are mainly related to most recent eruptive episodes of Vesuvius activity, from 1631 onward. Copper-bearing minerals were characterized, as well as associated minerals, by X-ray diffraction (XRD) scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). An investigation on the structural complexity of Cu-mineral assemblages with different temperature formations was also carried out using the TOPOS software package. The main copper phases are sulfates, followed by vanadates, hydroxyhalides, oxides, carbonates, silicates and finally, phosphates. New mineral occurrences for Vesuvius, both Cu-bearing and Cu-free, are described. Nevertheless, the fumarolic/alteration minerals at Vesuvius cannot be considered of economic relevance as a copper reservoir, this type of mineralizations are significant for copper crystal chemistry and for the knowledge of the mineralogical variants. The obtained datasets can be of interest for the knowledge of volcanic byproducts of copper ore deposits (i.e., porphyry copper systems) and of (base) metal segregation processes.

Fumaroles with salammoniac with ammonium sulfates and fluorides (the Deville's "water vapor and salammoniac-bearing fumaroles"), found almost exclusively on the lower part of lava flows, rarely close to the crater (T ≈ 300 • C). The most abundant mineral is salammoniac and in small amounts, mascagnite and chryptohalite also occur; 4.
Sulfur-bearing low T fumaroles (the Deville's "water vapor and and H 2 S + native S-bearing fumaroles"; [11] and references therein), with T rarely > 100 • C; these fumaroles follow the acid ones and are characterized by abundant H 2 O(gas) with variable quantity of H 2 S. Silicates of the host rocks are transformed to opal, and the most abundant minerals are native sulfur, gypsum, K-Al-Fe sulfates [alunogen, alum-(K), metavoltine and voltaite] and sassolite.
The post-1944 fumaroles were studied, in particular, by Parascandola [38], who reported maximum temperatures of about 800 • C down to 460 • C between 1948 and 1960 [14,[39][40][41]. The current temperatures of the fumaroles at the eastern rim of the crater are between 70 • C and 80 • C [41]. Chiodini et al. [14] reported a chronogram of the maximum temperatures measured in the crater area after the last eruption, which took place in 1944; this chronogram clearly shows a hot period from 1944 to 1960 when temperatures of 600 to 800 • C were recorded, followed by a cold period. Temperatures close to the boiling point of water at the crater level (95 • C) were attained during the 1990s. This temperature decline was accompanied by remarkable changes in the mineralogy of sublimates and alteration products deposited at the fumarolic vents [10,38]. Oxide of Cu (tenorite), as well as Na chloride (halite) and Pb (cotunnite) and were the most important minerals at temperatures of 500 to 800 • C, whereas the present mineral assemblage includes sulfur, gypsum, and sulfates, such as alunite, alum-(K), metavoltine, pickeringite, and halotrichite [14].
According to the literature [10,11], a number of Cu-bearing minerals, as malachite, azurite and cuprorivaite and also some chalcopyrite occurrences were detected in Somma ejecta or lavas as well. A list of the Cu-bearing minerals found at Somma-Vesuvius so far is given in Table 1. It is worth noting the occurrence of thirteen type locality (or co-type locality) copper species, ranging from sulfides to silicates.

Materials and Methods
In the present study, we selected 32 samples (Table 2), all belonging to the "Vesuvian Collection" of the Mineralogical Museum of the Naples University (Italy), where copper minerals typically occur as encrustations and/or tiny patinas, coatings and/or void filling associated with the historical activity of Vesuvius. Museum samples were selected on the basis of the catalog description and/or a visual examination, trying to select promising and representative lithotypes. Hence, for this study, we considered small subsamples of each Museum specimen in order to preserve their integrity as much as possible. The studied fractions were carefully separated from the host rocks, trying to select those richer in copper minerals, at least at a first macro-to-mesoscopic evaluation. A further selection of different mineral assemblages was also carried out by means of a binocular stereo microscope for the subsequent mineralogical analyses. Table 2. List of the investigated Somma-Vesuvius samples from the Mineralogical Museum of Naples University (Italy) with Cu-bearing minerals, faithfully reporting both the description and the locations (when present) reported in the original labels. Minerals in italics correspond to discredited species (see Table 3). Due to the high solubility in water of many minerals (i.e., soluble salts), likely forming the various associations, polished sections were prepared by using only oil-based suspensions and pastes and/or ethyl alcohol for sample preparation and final cleaning. Mineralogical analyses were carried out by X-ray powder diffraction (XRD) using a Seifert-GE ID3003 diffractometer, with CuKα radiation, Ni-filtered at 40 kV and 30 mA, in the 3-80 • 2θ range, a step scan 0.02 • , and time of 10 s/step at the Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse (DiSTAR) University of Naples Federico II (Italy), and the RayfleX (GE) software package. The identification of different phases in multimineralic samples by XRD was sometimes tricky due to reflections overlapping.

# Sample ID Museum Classification
Few selected samples were analyzed at the Institute of Crystallography of CNR (IC-CNR, Bari, Italy) and their X-ray powder diffraction patterns were collected at room temperature by using an automated Rigaku RINT2500 rotating anode laboratory diffractometer (50 kV, 200 mA) equipped with the silicon strip Rigaku D/teX Ultra detector. An asymmetric Johansson Ge (111) crystal was used to select the monochromatic Cu Kα1 radiation (λ = 1.54056 Å). Measurement was executed in the transmission mode by introducing the sample in a glass capillary (0.5 mm diameter) (put in rotation), which was mounted on the axis of the diffractometer. In order to reduce the effect of possible preferred orientation, the capillary was rotated during measurement to improve the randomization of the orientations of the individual crystallites. A qualitative phase analysis was carried out by the software QUALX2.0 [42], whereas structure solution was executed by the software EXPO [43].
Micromorphological and chemical analyses by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were also carried out using a JEOL JSM5310 electron microscope (located at DiSTAR) equipped with an Oxford energy dispersive spectrometry (EDS) INCA X-stream pulse processor and the 4.08 version Inca software. The operating conditions were an acceleration voltage of 15 kV, 50-100 µA filament current, variable spot size and a working distance of 20 mm; the reference standards used for quantitative microanalysis were anorthoclase, Si, Al and Na; diopside, Ca; microcline, K; rutile, Ti; fayalite, Fe; olivine, Mg; serandite, Mn; sphalerite, Zn; benitoite, Ba; celestite, Sr; fluorite, F; halite, Cl; pyrite, S; galena Pb; pure metal, Cu. Detection limits of the analyzed elements are below 0.1%.
The crystal structural representation was prepared via VESTA program [44]. Information-based complexity measures for crystal structures of minerals were calculated using TOPOS software [45] using the formula proposed in the [46]. The information about complexity Cu-minerals from Vesuvio volcano is given in Table S1. Table 3 lists the Cu-bearing minerals cited in the text, as well as the correspondence with the obsolete names for some species of minerals and their ideal formulas.  In Table 4, we report the assemblages of the Cu-bearing minerals and associated minerals which refer to subsamples selected from the investigated museum specimens (Table 2). Due to the complex nature (i.e., too tiny grain size of crystals or intimate intergrowths in the micro-aggregates) and the great variety of the Cu-bearing mineral associations, the mineralogical characterization was often tricky and hence, the identification of some phases was made by means of only a qualitative EDS analysis or a micro-morphological SEM evaluation, or, if possible XRD reflection(s), so that a more complete validation can be achieved.

Hydroxylhalides
The most abundant basic cupric chloride is atacamite (Tables 3 and 4). It was mainly found in green to blue-green-yellowish samples and rich in cupric chlorides at a visual examination, and occurs in three macroscopically different types ( Figure 1): (i) green-blue-yellow thin crusts coating the lava host (Figure 1a,e,f), (ii) green encrustations of finely crystalline material (Figure 1c  Paratacamite (corresponding to the discredited "atelite", Table 3) is very subordinate (Table 4). In sample 17029 E5568, this hydroxysalt forms tiny euhedral crystals (Figure 2e), containing 6.25 wt% ZnO (ca. 5 wt% Zn, Table 5). In sample 16256 E6787, paratacamite grows on tenorite and shows a lower content of Zn (2.54 wt% ZnO). In sample 11061 D1512, a (weak) XRD reflection at 5.47 Å was observed, and EDS spectra show the presence of Zn, which are consistent with paratacamite. Paratacamite (corresponding to the discredited "atelite", Table 3) is very subordinate (Table 4). In sample 17029 E5568, this hydroxysalt forms tiny euhedral crystals (Figure 2e), containing 6.25 wt% ZnO (ca. 5 wt% Zn, Table 5). In sample 16256 E6787, paratacamite grows on tenorite and shows a lower content of Zn (2.54 wt% ZnO). In sample 11061 D1512, a (weak) XRD reflection at 5.47 Å was observed, and EDS spectra show the presence of Zn, which are consistent with paratacamite.

Oxides
In the investigated samples, the copper oxides are represented by tenorite and cuprite (Table 3); tenorite was detected in eight samples, whereas cuprite in one specimen only (Table 4 and Figure 3). In the investigated samples, the copper oxides are represented by tenorite and cuprite (Table 3); tenorite was detected in eight samples, whereas cuprite in one specimen only (Table 4 and Figure 3).    The tenorite composition is presented in Table 6 and, as for cuprite, it is fairly stoichiometric, with minor amounts of Fe, Mg, Mn and Zn.  The tenorite composition is presented in Table 6 and, as for cuprite, it is fairly stoichiometric, with minor amounts of Fe, Mg, Mn and Zn.

Carbonates
Azurite was recognized in two samples, labeled as "Azurite in 1631 lavas" in the Museum catalog (Tables 2 and 3). It characteristically occurs as thin patinas on lava host with a deep blue color ( Figure 5) and always associated with atacamite (Table 4). Azurite was recognized in two samples, labeled as "Azurite in 1631 lavas" in the Museum catalog (Tables 2 and 3). It characteristically occurs as thin patinas on lava host with a deep blue color ( Figure 5) and always associated with atacamite (Table 4). Observed at SEM, azurite shows distinctly fibrous fine-grained forms (Figure 6a,b,d) or occurs as compact admixtures with atacamite ( Figure 6c). Table 7 presents compositional data of azurite, which commonly shows small Zn concentrations (0.18-0.82 wt% ZnO). Malachite was found only in one sample (Table 4) and restricted to a single area ( Figure 6d).  Observed at SEM, azurite shows distinctly fibrous fine-grained forms (Figure 6a,b,d) or occurs as compact admixtures with atacamite ( Figure 6c). Table 7 presents compositional data of azurite, which commonly shows small Zn concentrations (0.18-0.82 wt% ZnO). Malachite was found only in one sample (Table 4) and restricted to a single area ( Figure 6d). Azurite was recognized in two samples, labeled as "Azurite in 1631 lavas" in the Museum catalog (Tables 2 and 3). It characteristically occurs as thin patinas on lava host with a deep blue color ( Figure 5) and always associated with atacamite (Table 4). Observed at SEM, azurite shows distinctly fibrous fine-grained forms (Figure 6a,b,d) or occurs as compact admixtures with atacamite ( Figure 6c). Table 7 presents compositional data of azurite, which commonly shows small Zn concentrations (0.18-0.82 wt% ZnO). Malachite was found only in one sample (Table 4) and restricted to a single area ( Figure 6d).   Table 7 shows the malachite composition, characterized by a high Zn amount (ca. 7.4 wt% ZnO). Even though this copper carbonate was identified in a very limited area of one sample, the chemical compositions (see Table 7) coupled to (weak) XRD reflections at 3.87 and 2.70 Å are consistent with malachite.
Chalcanthite was found in three samples (Tables 3 and 4); SEM observations show subhedral crystals (Figure 8b), stoichiometric and with only traces of Fe, Mg, Mn and Ca (Table 8).
Chalcocyanite was observed in anhedral masses ( Figure 8c) in three samples (Tables 3 and 4); as for chalcanthite, this sulfate only shows trace amounts of Fe, Mg, Mn, Zn and Ca (Table 8). Cryptochalcite, occurring in three samples (Table 4), forms subhedral individuals and was found as filling of open spaces and covering external surfaces ( Figure 8d). In composition, it always shows significant Zn amounts, in the range 1.42-3.73 wt% ZnO (Table 8), as normally reported for this mineral in the literature [48,49].
Cyanochroite was found in sample 21911 only (Table 4), where forms tiny aggregates of euhedral crystals ( Figure 8e) with an almost stoichiometric composition (Table 8).
Euchlorine was identified in three samples, two of which labelled as euchlorine-rich in the Museum catalog (Tables 2 and 4). This mineral occurs in quite compact masses or in euhedral crystals ( Figure 8f).  Brochantite was detected only in sample 16256 E6768 (Table 4), where it occurs in small anhedral masses with linarite (Figures 8a and 9c). Small amounts of Zn (ca. 1 wt% ZnO) were found (Table 8).
Chalcanthite was found in three samples (Tables 3 and 4); SEM observations show subhedral crystals (Figure 8b), stoichiometric and with only traces of Fe, Mg, Mn and Ca (Table 8).
Chalcocyanite was observed in anhedral masses ( Figure 8c) in three samples (Tables 3 and 4); as for chalcanthite, this sulfate only shows trace amounts of Fe, Mg, Mn, Zn and Ca (Table 8). Cryptochalcite, occurring in three samples (Table 4), forms subhedral individuals and was found as filling of open spaces and covering external surfaces ( Figure 8d). In composition, it always shows significant Zn amounts, in the range 1.42-3.73 wt% ZnO (Table 8), as normally reported for this mineral in the literature [48,49].
Cyanochroite was found in sample 21911 only (Table 4), where forms tiny aggregates of euhedral crystals ( Figure 8e) with an almost stoichiometric composition (Table 8).  Euchlorine was identified in three samples, two of which labelled as euchlorine-rich in the Museum catalog (Tables 2 and 4). This mineral occurs in quite compact masses or in euhedral crystals ( Figure 8f). Kröhnkite was likely detected in sample 11269 D1720, where crystals with short prismatic habits are observed ( Figure 9a); small but distinct XRD reflections at ca. 6.34 and 3.29 Å, typical of this phase, are recognized, whereas the composition shows minor contents of Fe, Mg, Mn, Na and Ca (Table 9).     Kröhnkite was likely detected in sample 11269 D1720, where crystals with short prismatic habits are observed ( Figure 9a); small but distinct XRD reflections at ca. 6.34 and 3.29 Å, typical of this phase, are recognized, whereas the composition shows minor contents of Fe, Mg, Mn, Na and Ca (Table 9).
Kröhnkite can be possibly present in other four samples, for which only qualitative EDS spectra could be obtained, hence, its occurrence needs further validation (Table 4).
Leightonite, observed in three samples (Table 4), is represented by crystals with a platy habit ( Figure 9b) and a nearly stoichiometric composition (Table 9).
According to the historic Museum classification, two Cu-bearing sulfates, i.e., boothite (corresponding to the discredited "cupromagnesite") and chlorothionite, had to be present in samples 11004 D1455 and 17032 E5571 ("cupromagnesite") and 17029 E5568 and 17030 E5569 (chlorothionite), (Table 2). However, these sulfates were not surely identified (at least in the subsamples selected for this study), because in the XRD patterns, the diagnostic reflections were very weak (3.02 Å for boothite in sample 17032 E5571, and 3.02 Å for chlorothionite in samples 17029 E5568 and 17030 E5569), and at the same time, it was not possible to obtain good-quality quantitative chemical analyses, but only some EDS qualitative spectra (Table 4).
Finally, dravertite, a rare Cu-Mg bearing sulfate of recent discovery [50], could likely occur in sample 12756 E1295, even though only qualitative EDS spectra were obtained in a very limited area ( Figure 9f) and a weak reflection at 2.65 Å was found in the XRD pattern of a multimineralic sample, affected by many reflections overlappings.

Vanadates
The cupric vanadates are found in sample 18531 E7062 labelled as "cuprite" and in the samples with the discredited "vesbine", as expected [10,51], i.e., 17718 E6249, 17719 E6250 and 17722 E6253 (Tables 2 and 4, Figures 1e, 3c and 10). 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals boothite in sample 17032 E5571, and 3.02 Å for chlorothionite in samples 17029 E5568 and 17030 E5569), and at the same time, it was not possible to obtain good-quality quantitative chemical analyses, but only some EDS qualitative spectra (Table 4). Finally, dravertite, a rare Cu-Mg bearing sulfate of recent discovery [50], could likely occur in sample 12756 E1295, even though only qualitative EDS spectra were obtained in a very limited area ( Figure 9f) and a weak reflection at 2.65 Å was found in the XRD pattern of a multimineralic sample, affected by many reflections overlappings.

Vanadates
The cupric vanadates are found in sample 18531 E7062 labelled as "cuprite" and in the samples with the discredited "vesbine", as expected [10,51], i.e., 17718 E6249, 17719 E6250 and 17722 E6253 (Tables 2 and 4, Figures 1e, 3c and 10). Volborthite was only detected in sample 18531 E7062 (Figure 3c), where it forms a tiny light green-yellowish patina close to atacamite efflorescences; at SEM scale, it occurs in platy crystals of maximum ca. 10 μm in length (Figure 11a). The chemical composition of this mineral is given in Table  10 and shows trace amounts of Fe, Mg, Mn, Zn and P (respectively, up to 0.33, 0.23, 0.26, 0.09 and 0.11 wt% in oxides).
In the investigated samples 17718 E6249, 17719 E6250 and 17722 E6253, the so-called "vesbine"rich fraction is mainly formed by tiny concretions (Figure 11b-e) composed of mottramite, the Cu-Pb-bearing vanadate, and chrysocolla plus vanadinite (see the following section 4.2). These copper minerals form rounded to botryoidal-like masses (Figure 11b), that in polished sections correspond to a texture with a distinct colloform growth banding (Figure 11c-e). The mottramite composition Volborthite was only detected in sample 18531 E7062 (Figure 3c), where it forms a tiny light green-yellowish patina close to atacamite efflorescences; at SEM scale, it occurs in platy crystals of maximum ca. 10 µm in length (Figure 11a). The chemical composition of this mineral is given in Table 10 and shows trace amounts of Fe, Mg, Mn, Zn and P (respectively, up to 0.33, 0.23, 0.26, 0.09 and 0.11 wt% in oxides).
In the investigated samples 17718 E6249, 17719 E6250 and 17722 E6253, the so-called "vesbine"-rich fraction is mainly formed by tiny concretions (Figure 11b-e) composed of mottramite, the Cu-Pb-bearing vanadate, and chrysocolla plus vanadinite (see the following Section 4.2). These copper minerals form rounded to botryoidal-like masses (Figure 11b), that in polished sections correspond to a texture with a distinct colloform growth banding (Figure 11c-e). The mottramite composition (Table 10) shows significant amounts of As (1.55-2.74 wt% As 2 O 5 ), and variable contents of Fe, Mg, Mn, Zn, Al and Ca.
In sample 17722 E6253, together with the above-mentioned minerals, starovaite, a rare Cu-K-bearing vanadate of recent discovery [52], was also found (Tables 3 and 4). This mineral shows tiny crystals (maximum dimension ca. 4 µm in length) with an euhedral prismatic habit (Figure 11f,g); it occurs mainly in association with chrysocolla. Its chemical composition is very similar to the species found in the literature [52], with small contents of Mn and Zn (Table 10). However, the identification of starovaite was made by EDS only, as its characteristic reflections were not detected in the XRD patterns, due to the low amounts of this mineral in the sample.
Lastly, tsumebite (Tables 3 and 4) was detected in sample 16256 E6787, as cavity filling in elpasolite-rich matrix (Figure 11h). Its composition is given in Table 10. However, as for starovaite, tsumebite was characterized only by EDS analyses, but it was not possible to recognize its occurrence in the XRD spectrum, due to scarce amounts of this mineral.

Silicates
The Cu-bearing silicates found in the investigated samples are chrysocolla (Figures 1e and 10) and litidionite ( Figure 12).
Chrysocolla occurs mainly in efflorescences in association with the copper vanadates or atacamite, as observed in the previous sections, where it is indistinguishable to the naked eye (Figures 1e and  10); this silicate was clearly identified by SEM observations, as reported in Figures 2b, 11b-g and  13a,b, and forms few micrometer-sized zoned concretions. Table 11 shows the chemical composition of chrysocolla; it always contains some Ca amounts, in the range 1.0-1.8 wt% CaO, and variable contents of Fe, Mg, Mn, K and Zn, whereas trace amounts of Pb and V can be related to vanadate impurities. In sample 17722 E6253, together with the above-mentioned minerals, starovaite, a rare Cu-Kbearing vanadate of recent discovery [52], was also found (Tables 3 and 4). This mineral shows tiny crystals (maximum dimension ca. 4 μm in length) with an euhedral prismatic habit (Figure 11f,g); it occurs mainly in association with chrysocolla. Its chemical composition is very similar to the species found in the literature [52], with small contents of Mn and Zn (Table 10). However, the identification of starovaite was made by EDS only, as its characteristic reflections were not detected in the XRD patterns, due to the low amounts of this mineral in the sample.
Lastly, tsumebite (Tables 3 and 4) was detected in sample 16256 E6787, as cavity filling in elpasolite-rich matrix (Figure 11h). Its composition is given in Table 10. However, as for starovaite, tsumebite was characterized only by EDS analyses, but it was not possible to recognize its occurrence in the XRD spectrum, due to scarce amounts of this mineral.

Silicates
The Cu-bearing silicates found in the investigated samples are chrysocolla (Figures 1e and 10) and litidionite (Figure 12).  Table 11 shows the chemical composition of chrysocolla; it always contains some Ca amounts, in the range 1.0-1.8 wt% CaO, and variable contents of Fe, Mg, Mn, K and Zn, whereas trace amounts of Pb and V can be related to vanadate impurities.

Associated Minerals
As illustrated in Table 4, a variety of minerals can be associated with the Cu-bearing phases, among which selected Cu-free halides, sulfates, molibdates, phosphates, vanadates and silicates are reported in Figures 14 and 15 and in Table 12.

Associated Minerals
As illustrated in Table 4, a variety of minerals can be associated with the Cu-bearing phases, among which selected Cu-free halides, sulfates, molibdates, phosphates, vanadates and silicates are reported in Figures 14 and 15 and in Table 12.       Elpasolite, K 2 NaAlF 6 , is a cubic Na-K aluminofluoride found in five samples (Table 4) commonly as clusters of octahedra (maximum dimension ca. 15 µm), which crystallize on the surfaces or fill cavities of host rocks (Figure 14a,b). The chemical composition of elpasolite is reported in Table 12 and shows the following ranges: 11.07-11.98 wt% Al 2 O 3 , 9.28-11.02 wt% Na 2 O, 29.07-29.33 wt% K 2 O and 47.88-50.88 wt% F. This fluoride is quite abundant in two samples, i.e., 16256 E6787 and 17030 E5569), hence, it was possible to obtain the cell parameters on an elpasolite-rich fraction of the first specimen by powder XRD (see Methods); the obtained data from the crystal structural refinement are the isometric system, Fm3m space group a = 8.11289(7) Å, V = 533.98(6) Å 3 and Z = 4. These data are perfectly comparable with the literature data [53].
Sbacchiite, Ca 2 AlF 7 , is a Ca-aluminofluoride and was only found in sample 16256 E6787, in needle-like crystals of ca. 30 µm in length together with fluorite and anglesite (Figure 14c). Its composition (Table 12) is very similar to the species recently discovered in fossile fumarole of the AD 1944 eruption of Vesuvius [41].
Thermessaite, K 2 AlF 3 (SO 4 ), is a K-bearing aluminofluoride with SO 4 [54], detected in two samples (Table 4). This mineral occurs in prismatic and partially dissolved crystals (Figure 14e,f). The chemical composition is very similar to the type-locality species [55], with only trace amounts of Ca, Mg and Al (  [10], are some of the Cu-free sulfates randomly detected in the investigated samples (Table 4). Apthitalite and syngenite form euhedral prismatic crystals in sample 17030 E5569 and 17029 E5568, respectively (Figure 15a,b); palmierite is found in anhedral masses in three samples (Table 4 and Figure 15c). The chemical formulas of these sulfates are presented in Table 12. Moreover, thenardite (Na 2 SO 4 ) could be present in sample 21836 (Table 4), where EDS qualitative spectra and morphological observations seem to point to this type of sulfate.
Concerning the species belonging to phosphates, molibdates and vanadates, pyromorphite [Pb 5 (PO 4 ) 3 Cl] and wulfenite [PbMoO 4 ] occur only in two different samples (Table 4), respectively, in acicular and rounded individuals (Figure 15d,e) and both showing a nearly stoichiometric composition (Table 12). Vanadinite, Pb 5 (VO 4 ) 3 Cl, was already described in the mottramite-bearing samples (i.e., the "vesbine" samples; see Section 4.1.5), where zoned concretions are formed with mottramite and chrysocolla (Figure 11b-e). Compared to the ideal formula, the chemical composition of the Somma-Vesuvius species shows small contents of As, in the range 0.84-1. 36 [56,57], was identified in the litidionite-bearing samples 17923 E6454 and 17926 E6457 (Table 4 and Figure 12). At the SEM scale, it is very similar to its cupric counterpart (Figure 15f) ; a phase indicated as opal was also identified in the literature in these type of rocks ( [10,11] and references therein); however, further chemical, XRD and spectroscopic investigations are ongoing on the whole litidionite-related paragenesis.

General Mineralogical Remarks on Copper Minerals and Associated Phases
Various types of halides, both Cu-bearing and Cu-free, occur in the investigated samples (Table 4). Atacamite is the most common (hydroxyl)halide in museum specimens, which span from the AD 1631 lavas (Scala, Camaldoli, Uncino; Figure S1) to the products of the AD 1870-1872 and 1880-1906 periods (Tables 2 and 4). A rare occurrence of distinct and well-crystallized individuals of atacamite has been found in a lava fragment by Russo et al. [58] (Villa Inglese, Torre del Greco; Figure S1), who also reported, for the atacamite-bearing 1631 lavas of Camaldoli and Villa Inglese, a more likely medieval age (AD 938 and 1037). In the studied samples, this mineral is coeval with the Cu-vanadates and silicates, azurite and some oxides (hematite, cuprite). The origin of atacamite is commonly related to the supergene oxidation zone of Cu deposits (i.e., porphyry copper, [7]), especially under arid and saline conditions [59,60]. However, it is also related to fumarolic deposition and to weathering of sulfides in subsea black smoker deposits in deep seawater seafloor hydrothermal sites; in these areas, atacamite and paratacamite are the most stable copper salts at the pH and Eh of cold deep seawater, undersaturated in CaCO 3 , and appear to be the ultimate sink for the Cu leached by the hydrothermal systems from the oceanic crust [61]. At Somma-Vesuvius, this mineral has a fumarolic genesis, being deposited during the latest phases of the magma cooling mainly along the lava fractures, as described in the literature ( [35,58] and references therein). It was also observed [35,53] that atacamite is associated with azurite, "vesbine" and various oxides (hematite, hausmannite and rarely cuprite), as quite similarly observed in the present research.
Paratacamite, the trigonal polymorph of atacamite, was identified in very few occurrences and in the same atacamite-bearing samples, although in restricted parts. It occurs as tiny clusters of crystals or as alteration of tenorite in products of the 1872-1882 period of the historical activity. In agreement with Russo and Punzo [10], the transformation of tenorite to paratacamite is due to the reaction of fumarolic HCl with the copper oxide, i.e., 2CuO + HCl + H 2 O = Cu 2 (OH) 3 Cl. The compositional features of this basic cupric chloride show the presence of Zn in its crystal lattice; it was demonstrated by Braithwaite et al. [62] that paratacamite is defined as a mineral with the rhombohedral structure, and composition Cu 3 (Cu,M 2+ )(OH) 6 Cl 2 , in which Cu/M 2+ atomic is~11-7, corresponding to~1/3 to 1/2 the occupancy of the non-Jahn-Teller site by M 2+ , which is an essential stabilizing non-Jahn-Teller-distorting cation of suitable radius, such as Zn 2+ , Ni 2+ , Co 2+ , Fe 2+ , Cd 2+ and Mg 2+ .
Interestingly, together with the Cu-halides, several Cu-free varieties have been found in various mineral assemblages of the studied Somma-Vesuvius specimens, mainly belonging to chlorides and fluorides; halite (NaCl), sylvite (KCl) and fluorite (CaF 2 ) often occur together, and also in various associations with elpasolite, sbacchiite, challacolloite, thermessaite. In particular, elpasolite belongs to cubic perovskites [63], as the mineral parascadolaite discovered for the first time at Vesuvius [64]; other rare fluorides were also very recently found, such as the already cited sbacchite [41] and verneite [65]. These fluorides are formed by the reaction of HF with the surrounding rocks, indicating environments with T > 500 • C. Challacolloite was found in the 1906 fumaroles of HT [11,12,35], but no temperature data were recorded; this mineral can form at a temperature range of 360 • C (Vulcano island, Italy) to 550-390 • C (Satsuma-Iwojima volcano, Japan) [12]. Thermessaite is a rare mineral discovered at Vulcano island, southern Italy [55] in medium temperature fumarole (around 300 • C) in sub-mm colorless prismatic crystals associated with alunite, sassolite, anhydrite, and metavoltine; it was also identified at Vesuvius in fumaroles from recent activity by Campostrini et al. [54].
Among the cupric oxides, cuprite is restricted to one sample, labelled as "1631 lava. As many other oxidized copper minerals, cuprite is quite common in the weathered portions of many copper sulfide deposits, whereas is rare in volcanic environments. At Vesuvius, cuprite was detected in the lavas of Camaldoli and Scala [10] (Figure S1), in associations with atacamite. Tenorite is more widespread compared to cuprite, both in the investigated samples and in the occurrences from the literature ( [13] and references therein); in fact, in the studied samples, tenorite was detected in various rocks related to 1631 to 1906 eruptions, whereas according to the literature [10], this oxide was identified in scoriae of the 1760 eruption and in all the eruptions from 1825 onward, with particularly abundant and large crystals in the products of the 1944 eruptive episode. Tenorite is typical of the HT fumaroles (K-Na salts bearing, T > 400 • C) and originates by interaction of gaseous CuCl 2 and water vapor, according to the reaction CuCl 2 + H 2 O = CuO + 2HCl [10,15]. As stated before, tenorite can be subsequently altered by HCl brines to form paratacamite.
The Cu-bearing carbonates are represented by azurite and malachite, only found in two samples and mainly in association with atacamite. As also reported in the literature, these minerals are rare at Vesuvius and have been found in the so-called 1631 lavas (azurite with atacamite and "vesbine"), in an ejectum of Lagno Macedonia with magnetite and malachite, and in fumarolic products of the 1872 eruption together with connellite, or in various ejecta (malachite) [10]. The general scarcity of carbonates (Cu-bearing or not) in the fumarolic mineralizations is not surprising (notwithstanding the abundant CO 2 in volcanic gases), considering that the typical high acidity of the fumarolic environments makes most carbonates unstable [35]. While azurite composition in the studied samples is close to the ideal formula, malachite is distinctly Zn-bearing, with a partial substitution of Cu 2+ by Zn 2+ , giving the formula: (Cu 1−x M x ) 2 (OH) 2 CO 3 (M = Zn, Mg, Co, Ni, etc.) for members of the malachite-rosasite group [66,67].
Cu-bearing sulfates are represented by the largest variety of species in the investigated Somma-Vesuvius samples (followed by the whole of halides) and have been recognized in all the products of the recent activity. Even though these minerals belong to different systematic subgroups of sulfates, they can be gathered in main three types, i.e., (i) anhydrous sulfates (chalcocyanite, cryptochalcite, ±dravertite), (ii) anhydrous sulfates with hydroxyl or halogens (brochantite, euchlorine, linarite, ±chlorothionite), and (iii) hydrous sulfates (chalcanthite, cyanochroite, kröhnkite, leightonite, natrochalcite, ±boothite). These sulfates can be composed of Cu only, or of Cu-K, Cu-Na, Cu-K-Na, Cu-Pb and Cu-Mg (±anions, ±H 2 O). As also observed by Pekov et al. [6], the alkali cations have an important role in the chemical feature of the fumarolic minerals, just as the sulfates; indeed, according to these authors, 46 of 86 sulfates and other oxysalts plus chlorides contain K, Na and Cs, whereas only 16 of 174 copper minerals from the oxidation zones of sulfide ores show alkali elements (K and/or Na). Hence, the alkali cations represent a key factor in determining the unique crystal-chemical nature of fumarolic copper minerals. A remarkable feature was indicated by Balić-Žunić et al. [35] for the sulfates found in fumaroles, i.e., the characteristic existence of several hydrous forms associated with the anhydrous ones. These varieties have different stability fields, that depend on the humidity and temperature conditions and hence, can appear in various zones of the same fumarole. The anhydrous sulfates are generally unstable and readily hydrate under atmospheric conditions [35]. This is true also for the Cu-bearing sulfates, for which it is possible to observe the anhydrous phases and the hydrous counterparts; an example is chalcocyanite [(Cu(SO) 4 ], which rapidly changes to chalcanthite [CuSO 4 ·5(H 2 O)] [35]. In the studied specimens, these two sulfates were also found in the same sample (127656 E1295), probably in relation to an incomplete alteration (hydration) process of chalcocyanite. However, it is quite common to find complex and various mixtures of the above-mentioned two or three types of sulfates in the same sample, for instance (Table 4): It is interesting to note that SEM studies also support the observation that hydrous Cu sulfate generally appears to be as later phases compared to the other sulfates in the paragenetic sequence. Among the identified varieties, kröhnkite and leightonite were recently reported as new occurrences among Vesuvius fumarolic mineral by Campostrini et al. [54], whereas brochantite, cryptochalcite and natrochalcite detected in the present study are, to the authors' knowledge, the first recorded occurrences at Vesuvius. The same is valid for boothite and dravertite, if their presence was confirmed by our ongoing investigations.
Cu-bearing vanadates characterize few studied samples, commonly indicated as vesbine-bearing and restricted to the occurrences related to 1631 lavas. They are represented by OH-or H 2 O-bearing minerals, as mottramite and volborthite, and by the anhydrous phase, starovaite. Mottramite is the prevailing cupric vanadate, at least in the investigated samples, and can be found in mineral associations, i.e., together with vanadinite + chrysocolla + wulfenite, vanadinite + chrysocolla + tenorite, or chrysocolla + starovaite + atacamite. Volborthite occurs with atacamite just in one sample. Both starovaite (the second worldwide occurrence) and vanadinite are the first occurrences at Vesuvius, to the authors' knowledge. According to Russo and Punzo [10] and Russo et al. [58], it is more reliable the attribution of vesbine to volborthite, even though mottramite and vesigniéite have been also indicated as other possible mineral components; indeed, the present study pointed out that the so-called vesbine of Vesuvius is a complex mixture of various minerals, alternatively composed of Cu, Cu-Pb, Pb and Cu-K vanadates and of Cu-bearing silicates. According to Pekov et al. [6], Cu vanadates are important minerals in volcanic exhalations and their number is related to the fumarolic environment of two significant occurrences, i.e., Izalco (San Salvador) and Tolbachik (Kamchatka) volcanoes, is almost twice the number of supergene Cu vanadates. As for sulfates, the H-free vanadates can be considered more specific to the volcanic domain.
Tsumebite, a rare Cu-Pb phosphate, was detected in our study for the first time at Vesuvius in a complex and multimineralic assemblage; this mineral typically occurs as the secondary phase in the oxidized zone of As-bearing Cu-Pb deposits, with other secondary oxidized minerals. In fact, according to Pekov et al. [6], copper phosphates (as well as carbonates) are even unknown in volcanic exhalations, unlike in supergene formation, where they are numerous and widespread. Moreover, Balić-Žunić et al. [35] affirmed that phosphate group gives limited or null contribution to the mineralogy of the European fumarolic occurrences. In light of this, Vesuvius occurrence has an exceptional character.
The copper silicates identified in the studied samples are chrysocolla and litidionite, related to different type of rocks. Chrysocolla was only found in the thin encrustations on 1631 lava with atacamite or with the Cu-vanadates. Following the literature [10], at Vesuvius, this silicate was extremely rare, and observed as light green encrustations in a lava fragment of the 1872 eruption (Le Novelle quarry, Ercolano; Figure S1), with atacamite, apatite, hematite, magnetite and pyroxene, of presumably pneumatolytic origin. Chrysocolla is commonly found in weathered portions of many copper sulfide deposits, whereas it is very poorly represented, as Cu phosphates and carbonates, in volcanic exhalation products. Litidionite is even rarer and restricted to a unique occurrence, i.e., to thermally modified pyroclastic fragments by the fumarolic activity related to the 1872 eruption. The litidionite-bearing paragenesis is mainly composed of tridymite, calcinaksite (the second recorded worldwide occurrence), wollastonite and diopside and is typical of high-temperature alteration processes at the rock-fumaroles interface. Interestingly, the first discovery of this mineral was in a calcic xenolith hosted by an alkaline basalt of Bellerberg volcano (Eifel, Germany) as the product of contact metamorphism (metasomatism); the Bellerberg paragenesis consists of calcium silicates and CHS phases (wollastonite, gehlenite, browmillerite, tobermoreite, ettringite, etc.), which are also typical components of cement clinker and cement materials [56,57].
Finally, in the studied samples, we did not find Cu sulfides, such as chalcopyrite, covellite, chalcocite, or Cu sulfosalts; only sporadic grains of galena were observed. Base metal-bearing sulfides are quite rare in fumarolic products at Vesuvius [10,68], and can be present in products of different ages (see Table 2 for copper sulfides). Indeed, Balić-Žunić et al. [35] argued that metallic sulfides are confined to deeper parts of a volcanic system with its high-temperature hydrothermal conditions and rarely appear as sublimates, hence, they commonly occur in small amounts on the surface of the fumarole deposits.

Genetic Considerations
The Cu-bearing mineral assemblages observed in the investigated samples (or sub-samples) from Vesuvius are often very heterogeneous and can involve minerals that crystallized in different temperature conditions. In fact, Vesuvius can be distinguished by an oscillation in term of oxidizing vs. reducing conditions, mainly in the periods immediately following the eruptions [35], as well as of temperature values. However, following the literature [10,11,14,[35][36][37][38][39][40][41], most of the minerals found in the present research may be formed from high-to moderate-temperature fumaroles, at a temperature ranging from 300 • C to more than 650 • C. At higher temperatures, chlorides and fluorides (e.g., halite, sylvite, fluorite, elpasolite, sbacchiite) and alkali sulfates (e.g., thénardite) prevail. On the contrary, Cu-Cl oxyhalides and hydrohyhalides (as atacamite and paratacamite), Cu-bearing sulfates (e.g., chalcocyanite, euchlorine) and tenorite, together with a set of Pb-bearing minerals (e.g., cotunnite, pseudocotunnite, palmierite), hematite and sulfides probably formed at lower temperatures. This latter group can likely include other phases, even though with different precipitation temperatures, such as linarite, the Cu-bearing and Cu-free vanadates, chrysocolla, and tsumebite.
According to Pekov et al. [6], similar temperature ranges can be assumed for the origin of Cu minerals of the Tolbachik volcano (Kamchatka, Russia); these authors distinguished two main groups of minerals, formed either in the hot zones of fumarolic system, corresponding to a T > 200 • C, mainly in the range~400-700 • C, or in the moderately hot zones of fumarolic system, with a T < 200 • C and mainly in the interval of 70-150 • C. This study shows that anhydrous sulfates, as chalcocyanite, cryptchalcite, euchlorine and dravertite can be formed by hot fumarolic system. Instead, hydrous sulfates, i.e., cyanochroite, leightonite, natrochalcite and kröhnkite, can be related to lower temperature conditions and belong to the second group. In agreement with these observations, starovaite should belong to the high T group, being formed before chrysocolla, that likely had lower precipitation temperatures.
Litidionite and its peculiar paragenesis can be formed as a product of very localized high-temperature (exhalation-related) alteration processes of hosting silicates and with the introduction of other chemical components by fluids (e.g., Cu 2+ ), also in agreement with that observed for the Eifel calcinaksite-bearing rocks; these phenomena can occur likely at a temperature of > 600 • C.
Other minerals randomly detected in the investigated samples, such as gypsum, anhydrite, alum-K, alunite, calcite and opal, are related to the last phases of mineral deposition, characterized by lower temperatures of exhalation phenomena, with a temperature rarely higher than 100 • C (see Section 2).
In their thorough review, Pekov et al. [6], have carried out a comparison between the crystal-chemical characteristics of Cu minerals found in volcanic fumarole environment and those formed under supergene conditions (i.e., the oxidation zone of sulfides ores, at T-P surface condition and under the influence of aqueous solutions). Even though the bulk of oxygen-and halogen-bearing Cu minerals occur primarily in the oxidation zones of sulfide ores (supergene environment), these authors report that more than one hundred new minerals were recently discovered in volcanic exhalation environment, showing great diversity and originality of motifs formed by the Cu 2+ -centred coordination polyhedral for both the mineral sets. Our research confirms this statement, demonstrating that a large variety of the Cu minerals formed from fumarolic activity at Vesuvius. Pekov et al. [6] also showed that in particular, H-free copper sulfates and vanadates of fumarolic origin are numerous and structurally diverse, but are unknown in the supergene environment, which is instead characterized by Cu 2+ -based sulfates, vanadates and chlorides containing OH-bearing groups.

Crystal Structure and Complexity Considerations
A new approach for quantifying the structural complexity of a crystalline matter is based on the TOPOS software package [45]. The information-based complexity measures made possible to widely use the compelxity parameter as a tool for studying the evolution of mineral formation in various systems [46,[69][70][71]. Mineral structures can be classified into very simple (0-20 bits, i.e., binary digits), simple (20-100 bits), intermediate (100-500 bits), complex (500-1000 bits), and very complex (>1000 bits). According to the proposed quantitative approach, the crystal structure can be viewed as a reservoir of information encoded in its complexity [46]. Regarding this topic, we give a brief description of crystal structures of Cu-minerals from Vesuvius volcano and discuss their complexity in the context of different periods of volcanic activity.
Oxide minerals represent the simplest group from their structural complexity of Somma-Vesuvius Cu-minerals minerals. The crystal structure of tenorite Figure 16 exhibits a framework based on CuO 4 planar groups polymerized through shared vertexes. On the polyhedral approach, the crystal structure of curpite can be described as based on the O-centered tetrahedral framework, whereas each [OCu 4 ] tetrahedra is connected by shared vertexes. The dominant motif of the kröhnkite structure is the chains of [SO4] tetrahedra connected with single Cu octahedra, extending along the c axis; these chains are linked together by Na atoms coordinated by seven anions at distances between 2.39 and 2.57 Å [78]. The chalcanthite structure is based on infinite chains of distorted Cu-octahedra with an edge shared Mg-octahedra. These chains are held together by corner-shared sulfate tetrahedra and a hydrogen bonding system [79]. In the cyanochroite structure, each Cu atom is coordinated by six water molecules and connected via hydrogen bonds with [SO4] tetrahedra and K atoms [80]. In this work, boothite is the second complex mineral and its structure is similar to that of cyanochroite based on Cu-octahedra with six water molecules and hydrogen-bonded with [SO4] tetrahedra [81]. Generally, the leightonite structure has a framework character. To date, only a disordered model has been proposed [82]. In this model, Cu atoms have square-planar coordination, but octahedral if considering mixed K/O sites. All equatorial O atoms in the copper octahedra are connected with corner-shared sulfate groups. The Ca atoms are eight-coordinated, whereas six oxygens are shared for sulfate groups.
In the chlorothionite crystal structure, the Cu atoms are square-coordinated. In chlorothionite, Cu is coordinated by two Cl and two O atoms, whereas oxygen atoms are edge for adjacent [SO4] tetrahedra and chlorine atoms bonded with K [83]. There are two copper silicate minerals in the investigated samples, but the crystal structure of chrysocolla still unsolved. The crystal structure of litidionite consists of tubular [Si 8 O 20 ] chains with a hexagonal cross-section, which are interconnected by [CuO 4 ] planar groups [56]. The structural cavities are filled by the Na and K atoms.
Hydroxyhalides include atacamite and its polymorph paratacamite. Generally, both minerals have the same net-like motif formed chains of CuO 4 planar groups connected with perpendicular chains based by [CuO 5 Cl] distorted octahedra. The paratacamite structure is more ordered, one half distorted octahedra changed by [CuO 6 ] and the other half changed by CuO 4 planar groups, whereas Clanions filled voids [73,74].
There are three vanadates found at the Vesuvius volcano. In the crystal structure of mottramite distorted Cu-octahedra connected by shared edges forms infinite chains along the b axis. The V 5+ cations occupy slightly distorted tetrahedra connected by three oxygen vertexes with [CuO 6 ]-based chains and one oxygen involved in hydrogen bonding. The Pb 2+ cations are seven-coordinated with bonds in the range 2.45-2.80 Å [75]. There are five independent Cu sites in the crystal structure of starovaite, which has tetragonal pyramidal or trigonal bipyramidal coordination. They form complex ∞[Cu 5 O 13 ] sheets connected via [VO 4 ] tetrahedra in the framework. The cages in the framework are occupied by 10-coordinated K atoms [76]. Volborthite is a well known type of layered structure with octahedral layers based on [CuO 6 ] distorted octahedra with 1/3 empty sites. Such layers are connected with [V 2 O 5 ] nesovanadate groups that form channels populated by water molecules [77].
The dominant motif of the kröhnkite structure is the chains of [SO 4 ] tetrahedra connected with single Cu octahedra, extending along the c axis; these chains are linked together by Na atoms coordinated by seven anions at distances between 2.39 and 2.57 Å [78]. The chalcanthite structure is based on infinite chains of distorted Cu-octahedra with an edge shared Mg-octahedra. These chains are held together by corner-shared sulfate tetrahedra and a hydrogen bonding system [79]. In the cyanochroite structure, each Cu atom is coordinated by six water molecules and connected via hydrogen bonds with [SO 4 ] tetrahedra and K atoms [80]. In this work, boothite is the second complex mineral and its structure is similar to that of cyanochroite based on Cu-octahedra with six water molecules and hydrogen-bonded with [SO 4 ] tetrahedra [81]. Generally, the leightonite structure has a framework character. To date, only a disordered model has been proposed [82]. In this model, Cu atoms have square-planar coordination, but octahedral if considering mixed K/O sites. All equatorial O atoms in the copper octahedra are connected with corner-shared sulfate groups. The Ca atoms are eight-coordinated, whereas six oxygens are shared for sulfate groups.
In the chlorothionite crystal structure, the Cu atoms are square-coordinated. In chlorothionite, Cu is coordinated by two Cl and two O atoms, whereas oxygen atoms are edge for adjacent [SO 4 ] tetrahedra and chlorine atoms bonded with K [83].
The crystal structures of dravertite, linarite, chalcocyanite, natrochalcite, tsumebite consist of Cu or mixed Cu-Mg (dravertite) layers of edge-shared octahedral chains. In terms of structure, dravertite can be considered as a cation-ordered derivative of chalcocyanite, with alternating Cu-and Mg-centred octahedra in cationic chains [50]. In chalcocyanite, the structure of octahedral chains parallel to [010] is connected to a framework by [SO 4 ] tetrahedra [84]. In the linarite structure, octahedral Cu-chains are connected with vertex-shared [SO 4 ] tetrahedra and eight-coordinated Pb atoms [85]. The sulfate groups are connected through Pb atoms; thus, they form a double (Pb-S) layer between Cu-layers. Similarly to other members of the brackebuschite supergroup, the tsumebite crystal structure contains a cubic closest-packed array of O and Pb atoms with infinite chains of edge-sharing distorted Cu octahedra decorated by two unique [SO 4 ] and [PO 4 ] tetrahedra [86]. The Cu-S layers in the natrochalcite structure are close to those in chalcocyanite and are built from [Cu 2 OH(SO 4 ) 2 ·2H 2 O] sheets and are interconnected by Na + ions and hydrogen bonds [87]. Cu-polyhedra is indicated in blue, Mg-octahedra-orange, S-tetrahedra-yellow, P-tetrahedrapink. Oxygen atoms are shown as red spheres, chlorine-green, potassium-purple, led-gray, sodium-yellow. Hydrogen-bonding systems are shown as dashed lines.
The crystal structures of dravertite, linarite, chalcocyanite, natrochalcite, tsumebite consist of Cu or mixed Cu-Mg (dravertite) layers of edge-shared octahedral chains. In terms of structure, dravertite can be considered as a cation-ordered derivative of chalcocyanite, with alternating Cu-and Mgcentred octahedra in cationic chains [50]. In chalcocyanite, the structure of octahedral chains parallel to [010] is connected to a framework by [SO4] tetrahedra [84]. In the linarite structure, octahedral Cuchains are connected with vertex-shared [SO4] tetrahedra and eight-coordinated Pb atoms [85]. The sulfate groups are connected through Pb atoms; thus, they form a double (Pb-S) layer between Culayers. Similarly to other members of the brackebuschite supergroup, the tsumebite crystal structure contains a cubic closest-packed array of O and Pb atoms with infinite chains of edge-sharing distorted Cu octahedra decorated by two unique [SO4] and [PO4] tetrahedra [86]. The Cu-S layers in the natrochalcite structure are close to those in chalcocyanite and are built from [Cu2OH(SO4)2·2H2O] sheets and are interconnected by Na + ions and hydrogen bonds [87].
In the brochantite structure, two distorted Cu octahedra are interconnected through common edges to build infinite planar double chains running along the c axis. These chains are interconnected by sharing vertices to build 'zig-zag' chains running along c. Each [SO4] tetrahedra is connected to three adjacent double chains [88]. Cu-polyhedra is indicated in blue, Mg-octahedra-orange, S-tetrahedra-yellow, P-tetrahedra-pink. Oxygen atoms are shown as red spheres, chlorine-green, potassium-purple, led-gray, sodium-yellow. Hydrogen-bonding systems are shown as dashed lines.
In the brochantite structure, two distorted Cu octahedra are interconnected through common edges to build infinite planar double chains running along the c axis. These chains are interconnected by sharing vertices to build 'zig-zag' chains running along c. Each [SO 4 ] tetrahedra is connected to three adjacent double chains [88].
For the structures containing an "additional" oxygen atom, it is convenient to use an approach to understand the structure in terms of anion-centered coordination polyhedra [89]. In the euchlorine structure, "additional" non-sulfate oxygen atoms are considered to form two independent oxocentered [OCu 4 ] tetrahedra which share a common Cu1-Cu1 edge, thus forming an [O 2 Cu 6 ] dimer. Two S-centered sulfate tetrahedra are attached 'face-to-face' to the dimers. And one [SO 4 ] tetrahedra provides the linkage of these clusters in two dimensions to form [Cu 3 O(SO 4 ) 3 ] layers parallel to the bc plane. Potassium and sodium atoms are located in the interlayer [90].
The  2 coplanar to the ab plane and connected via SO 4 tetrahedra. The first layer consists of clusters formed by four edge-sharing octahedra with signifificant Janh-Teller distortion. The second layer is formed by two isolated Cu-centered tetragonal pyramids alternating with two Cu-centered trigonal bipyramids. Inside the layers, Cu-polyhedra are connected via corner-shared [SO 4 ] tetrahedra, interstices filled by K atoms [49].
The complexity totals for Cu-minerals are 1449, 2101 and 550 bits for temperature ranges of 50-200 • C, 200-400 • C and 400-700 • C, respectively (Figure 18a). At first sight, this is not in agreement with Goldsmith's simplexity principle [91]. The observed splash of complexity for middle temperature Cu-minerals is connected with the presence of "additional oxygen" in formula. The additional oxygen in the structure makes possible the organization of Cu-clusters, which can also be described in terms of oxy-centered crystallography [6]. If we consider the complexity of all minerals in assemblages including Cu-free minerals, the totals would be 1863, 2245 and 1126 bits (Figure 18b). The means of complexity for middle-temperature minerals is still higher than for low-temperature ones. This contradiction with the simplexity principle is probably due to a lack of information about Cu-free minerals, or due to destruction of some water-soluble minerals under atmosphere conditions. oxocentered [OCu4] tetrahedra which share a common Cu1-Cu1 edge, thus forming an [O2Cu6] dimer. Two S-centered sulfate tetrahedra are attached 'face-to-face' to the dimers. And one [SO4] tetrahedra provides the linkage of these clusters in two dimensions to form [Cu3O(SO4)3] layers parallel to the bc plane. Potassium and sodium atoms are located in the interlayer [90].
The crystal structure of cryptochalcite is based on the heteropolyhedral framework [Cu5O(SO4)5] composed of two types of alternating Cu-S-O polyhedral layers [Cu2(SO4)2] and [Cu3O(SO4)]2 coplanar to the ab plane and connected via SO4 tetrahedra. The first layer consists of clusters formed by four edge-sharing octahedra with signifificant Janh-Teller distortion. The second layer is formed by two isolated Cu-centered tetragonal pyramids alternating with two Cu-centered trigonal bipyramids. Inside the layers, Cu-polyhedra are connected via corner-shared [SO4] tetrahedra, interstices filled by K atoms [49].
The complexity totals for Cu-minerals are 1449, 2101 and 550 bits for temperature ranges of 50-200 °C, 200-400 °C and 400-700 °C, respectively (Figure 18a). At first sight, this is not in agreement with Goldsmith's simplexity principle [91]. The observed splash of complexity for middle temperature Cu-minerals is connected with the presence of "additional oxygen" in formula. The additional oxygen in the structure makes possible the organization of Cu-clusters, which can also be described in terms of oxy-centered crystallography [6]. If we consider the complexity of all minerals in assemblages including Cu-free minerals, the totals would be 1863, 2245 and 1126 bits (Figure 18b). The means of complexity for middle-temperature minerals is still higher than for low-temperature ones. This contradiction with the simplexity principle is probably due to a lack of information about Cu-free minerals, or due to destruction of some water-soluble minerals under atmosphere conditions. The maximal values of total complexity correspond to the samples attributed, at least according to the museum labels, to the 1868-1870, 1872-1875 and 1880-1885 eruptions. The main contribution here is given by cryptochalcite and boothite minerals. According to Ivanyuk et al. [92], a relationship between massif sizes and the number of minerals known in them can exist; similarly, the relationship between different means of total complexity for various periods of Somma-Vesuvius volcanic activity and the eruption power (size) is currently under investigation. At the surface conditions, ancient samples should contain a smaller number of water-soluble minerals than younger eruptions. This The maximal values of total complexity correspond to the samples attributed, at least according to the museum labels, to the 1868-1870, 1872-1875 and 1880-1885 eruptions. The main contribution here is given by cryptochalcite and boothite minerals. According to Ivanyuk et al. [92], a relationship between massif sizes and the number of minerals known in them can exist; similarly, the relationship between different means of total complexity for various periods of Somma-Vesuvius volcanic activity and the eruption power (size) is currently under investigation. At the surface conditions, ancient samples should contain a smaller number of water-soluble minerals than younger eruptions. This should lead to a decrease of the total number of minerals and consequently, to a decrease of total complexity during the geological time. On the other hand, some minerals could be a product of the alteration of initial chlorides at surface conditions. Such low-temperature (≤50 • C) minerals (i.e., boothite) contain a complex hydrogen bonding system, which leads to an increase of the total complexity for the low-T assemblages [90].

Concluding Remarks
Our investigation on Cu-bearing minerals from a collection of Somma-Vesuvius volcano minerals shows that the most widespread phases are the sulfates, followed by vanadates, hydroxyhalides, oxides, carbonates, silicates and finally, phosphates. This is not surprising in a fumarole environment, as also observed by Balić-Žunić et al. [35] for the whole of European volcanic occurrences, where the prevalence of sulfates as well as halides mineralizations over the other species is shown. Interestingly, as in the literature, in the investigated samples, the minerals formed by other base metal (Pb, Zn) are less widespread and represented by few mineral varieties compared to the Cu-rich minerals. This could be due to different factors, such as crystal-chemical reasons. Indeed, Pekov et al. [6] affirmed that minerals composed of Zn, Mg, Fe, Pb, etc., in the same fumaroles do not demonstrate the distinctive crystal-chemical features shown by Cu-bearing minerals, nor do they present such great diversity of both mineral species and structural aspects observed in the cupric phases. These authors stated that this difference is likely due to the influence of the Jahn-Teller effect on the crystal-chemical behavior of the Cu 2+ cation [93], especially when its coordination is formed only by O 2− and Cl − ligands with no OH − groups or H 2 O molecules. However, the predominance of Cu minerals over the Pb-Zn-bearing phases of the mineral assemblages of the Somma-Vesuvius volcano in a geochemical and volcanological perspective is currently under investigation.
From the structural complexity point of view, the most complex minerals are related to the sulfate mineralization (cryptochalcite and boothite). The diversity of Cu mineralization in the Vesuvius volcano (different structural types) is connected with the different combinations of four-, five-and sixfold Cu-centred polyhedra in both fumarolic and supergene minerals. The most complex structures crystallize at temperatures up to 400 • C and the main factor, which lead to increasing complexity, is the presence of "additional oxygen" in the mineral formula.
Another aspect of interest provided by this study is the recognition of new copper minerals in the Somma-Vesuvius parageneses (i.e, cryptochalcite, natrochalcite, starovaite, tsumebite), as well as of interesting Cu-bearing vs. Cu-free minerals and related assemblages (i.e., litidionite-calcinaksite association). It is worth noting that some of these minerals are common secondary phases in supergene environments connected to primary base metal-bearing sulfides. However, more accurate studies on the selected copper minerals and related assemblages are in progress in order to decipher their crystal chemistry in connection with the petrological/geochemical/volcanological evolution. Hence, the present research contributes to a further update of the complex and unique mineralogy of this volcano and especially concerning the sublimate/alteration minerals, formed in a particular environment of high temperature and atmospheric pressure. Even though Vesuvius is one of the most studied volcanoes in the world, and remarkable in respect to the fumarolic minerals as well, the data provided by this research, as well as by the recent mineralogical surveys (i.e., [41,58,64,65,68]), lead to the prediction that new mineral discoveries will be carried out in the Cu-bearing assemblages.
Lastly, despite the fumarolic/alteration, Cu-bearing minerals at Vesuvius cannot be considered of economic relevance as a copper reservoir, as this kind of mineralizations is significant for the crystal chemistry of this base metal and for the definition of its mineralogical variants. All these datasets can be of interest for the knowledge of alteration byproducts of copper ore deposits (i.e., porphyry copper systems) and of (base) metal segregation processes.