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

Abstract

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.

1. Introduction

Copper is a base metal and the 26th most abundant element in the Earth’s crust, with concentrations ranging from 26 to 28 ppm in the continental crust [1] and less than 44 ppm in the lower oceanic crust [2]. As a transition metal, Cu is moderately siderophile and strongly chalcophile, with approximately two thirds of terrestrial Cu thought to be stored in the core [3]. In the silicate Earth, it behaves as a chalcophile element and usually forms stable sulfides, among which chalcopyrite (CuFeS₂) is the most common Cu-bearing mineral; copper is also a common constituent, together with silver, in natural sulfosalts [4]. However, in nature, copper occurs in a great variety of minerals; according to the mindat.org database [5], it is essentially contained in 562 valid mineral species, in decreasing order of abundance, hosted in sulfides–sulfosalts, phosphates–arseniates–vanadates, sulfates, oxides, halides, silicates, carbonates, borates and native elements. According to Pekov and co-workers, at least 690 mineral species can be described, of which Cu prevails over the other constituents in one independent position in the crystal structure [6].

Copper is an economically important base metal. It forms large and rich ore deposits with exceptional mineral diversity; major ore sulfide minerals include, in addition to the aforementioned chalcopyrite CuFeS₂ (35% Cu), bornite Cu₅FeS₄ (63% Cu), covellite CuS (66% Cu) and chalcocite Cu₂S (80% Cu). About three-quarters of the world’s copper is provided by porphyry copper systems [7], mainly centered on calc-alkaline porphyry magmatic systems in subduction zone settings ([8,9] and references therein). Lesser but significant contributions come from massive sulfide, skarn, and other types of deposits. The top Cu producers are Chile, USA, Indonesia and Peru [7,9].

Somma–Vesuvius is an active Quaternary volcano which belong to the alkaline potassic-ultrapotassic Roman Magmatic Province of southern Italy. Here, copper is associated with historically active fumaroles and occurs in a variety of minerals, primarily sulfates and halides, but also in silicates, carbonates, sulfides, oxides and vanadates and commonly as encrustations and patinas on lavas [10,11,12,13,14,15]. The majority of Cu-bearing minerals are sulfates and halides, followed by silicates, carbonates, sulfides, oxides and vanadates, which commonly occur as encrustations and patinas on lavas. The present work represents the starting point of a large project focused on the Somma–Vesuvius volcano and aimed at identifying Cu minerals related to mineralization processes associated with magmatic activity in an active magmatic-hydrothermal system (porphyry-like?) and augmenting the understanding of geodiversity within this unusual paragenetic environment. This first step deals with a mineralogical survey of a set of samples represented by sublimation and fumarolic products belonging to the vast sample 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, and were catalogued among the specimens containing one or more Cu-bearing phases. Together with the copper-bearing minerals, a characterization of the associated minerals has also been carried out in order to give a more complete overview of the whole assemblages. Similar crystal-chemical investigations are ongoing on Somma–Vesuvius Cu-bearing samples/assemblages of different types, not only fumaroles/encrustations/sublimates-related. Hence, the final aim will be to discuss the nature of Cu-bearing minerals and associations in the context of the related mineralizing events.

2. Geological and Volcanological Setting of Somma–Vesuvius and Mineralogical Background

Somma–Vesuvius (Figure S1) is a moderate size stratovolcano consisting of an older edifice dissected by a summit caldera, i.e., Mt Somma, and a recent cone, i.e., Vesuvius that grew within the caldera. It is located in the Campanian plain, bordering the bay of Naples. The complexity of this area and its evolution are connected to the tectonic evolution of the southern Apennine chain of the Italian peninsula, related to the opening of the Tyrrhenian basin during Plio-Pleistocene ([16] and references therein). The formation of the Campanian plain was accompanied by the uplift of the central part of the southern Apennines, in a regional stress regime generating NW–SE and NE–SW-trending faults and establishing the conditions for magma formation and rising to the surface. With the exception of the oldest volcanic rocks with calc-alkaline composition, erupted magmas younger than 400 ka are alkaline; at Vesuvius, the composition of volcanic products is mainly potassic to high potassic. Mt Somma consists of a pile of thin lava flows interbedded with spatter and cinder deposits post-dating the 39 ka old Campanian ignimbrite of Campi Flegrei [17,18]. The caldera is a nested poly-phased structure which is formed by several collapses related to the main explosive eruptions [19]. About 22 ka ago, the mainly effusive activity of Mt Somma abruptly changed into highly explosive activity, with the occurrence of high-magnitude plinian eruptions, i.e., Pomici di Base, Greenish pumice, Mercato (8.9 ka) and Avellino (3.9 ka) [20,21,22,23,24,25,26,27,28,29]. Activity became more frequent after the Avellino pumice, with at least eight explosive eruptions and some lava flows which preceded the 79 AD Pompeii pumice eruption [23].

The Vesuvius edifice began to form after 79 AD, growing discontinuously during periods of open-conduit activity. High intensity, explosive eruptions sporadically occurred, the two largest being the subplinian events of 472 AD (Pollena eruption) and 1631. The most recent period (1631–1944) was characterized by alternating strombolian activity and violent eruptions; a detailed study on the eruptions ages of the recent activity (mainly post-1631) is given, among others, by Arrighi et al. ([30] and references therein). Summit or lateral lava effusions and semi-persistent mild explosive activity with small lava fountains occurred, together with gases and vapor emission from the crater, interrupted by pauses lasting from months to a maximum of seven years. In the recent period, the eruption of 1906 is the largest explosive eruption of the twentieth century. At the end of the eruption, the top of the cone was truncated, with the formation of a vast crater ([11] and references therein). The Vesuvius area was characterized by quiescent periods alternated by prolonged volcanic activity, effusions of lava and low energy explosions after this explosive event. The last eruption of Vesuvius occurred in March 1944. The 1944 eruption marked the end of an open-conduit phase and the beginning of a quiescence period with obstructed conduit. Presently, Mt. Vesuvius is in a quiescent period and affected by relatively low level volcanic-hydrothermal activity in the inner slopes and the bottom of the crater. According to Caliro et al. [13] the activity is mainly characterized by (i) widespread fumarolic emissions, accompanied by diffuse soil CO₂ degassing in the crater area, and (ii) CO₂-rich groundwaters along the southern flank of Vesuvius and in the adjacent plain and seismic activity with epicentres clustered inside the crater. Fumarolic fluids discharging by the crater rim fumaroles are of relatively low temperatures (<75 °C) and are mainly composed of atmospheric components. Fumaroles from the crater bottom have H₂O and CO₂ as major components, followed by H₂, H₂S, N₂, CH₄, CO, and He (in order of decreasing content), and undetectable SO₂, HCl, and HF and a discharge temperature of about 95 °C [13]. The chemical composition of the fumaroles of the crater bottom relate to the hydrothermal type, with the typical presence of H₂S [14]. According to Chiodini et al. [14], CH₄ and NH₃ contents suggest that the origin of these fluids is related to a high-temperature hydrothermal system located underneath the Vesuvius crater area. These authors argued that NaCl brines reside in the high-temperature reservoir and have an influence on the chemical composition of the gases discharged by the fumaroles of the crater bottom. Fumarolic H₂O is either meteoric water, enriched in ¹⁸O through high-temperature water–rock oxygen isotope exchange, or a mixture of meteoric and arc-type magmatic water. Fumarolic CO₂ is mainly generated by decarbonation reactions of marine carbonates, but the addition of small amounts of magmatic CO₂ is also possible. Regarding the magma composition, the Somma–Vesuvius products display, with very few exceptions, a potassic to ultrapotassic composition, exhibiting a wide variability from nearly silica-saturated to silica-undersaturated. Joron et al. [31] and Ayuso et al. [32] distinguished three different groups of rocks: (i) slightly silica-undersaturated, older than the Mercato eruption (35 to 18 ka); (ii) mildly silica-undersaturated, from the Mercato to the Pompeii (79 AD) eruptions (10 to 2 ka); (iii) highly silica-undersaturated, younger than Pompeii eruption. Variations in the content of some major and trace elements are consistent with this division (e.g., [32]), although the general incompatible trace element patterns remain similar among rocks of these three periods of activity ([16] and references therein).

As already mentioned, the vast majority of the Cu-bearing minerals at Vesuvius are found in alteration products, encrustations and sublimate formations deposited at the fumarolic vents, mainly related to most recent eruptive episodes of Vesuvius activity, from 1631 onward [10,11,12,33,34]. An overview of the fumarolic minerals in the main European active volcanic provinces (Italy, Iceland, Greece), including Somma–Vesuvius, was carried out by Balić-Žunić et al. [35]. Remarkably, Pekov et al. [6] have recently carried out a very important crystal-chemical survey on the copper minerals from volcanic exhalations occurring in active areas; this work mainly deals with the copper mineralization in the Tolbachik volcano (Kamchatka, Russia), the “word-record-holder” of the fumarolic copper minerals, with 90 different minerals found there, of which Tolbachik is the type locality.

A first detailed study of fumarolic mineral assemblages vs. temperature (T) conditions at Somma–Vesuviusis due to Lacroix [36], who divided fumarole activities into four typologies [10,11,12]:

• High T fumaroles, with Na and K-bearing salts, with temperature > 650 °C, i.e., the so-called Deville’s “dry fumaroles” [37]. The most important minerals are chlorides and alkali sulfates (halite, sylvite, thenardite, aphthitalite, etc). At a lower temperature, the mineral variety increases and Cu–Cl oxyhalides and hydroxyhalides prevail (eriochalcite, paratacamite, melanothallite), minerals formed by sulfuric acid (chalcocyanite, dolerophanite, euchlorine, chlorothionite and secondary chalcanthite), tenorite [as a result of H₂O(gas) and CuCl₂(gas) interaction], Pb-bearing minerals, like cotunnite and rarely, pseudocotunnite and palmierite; in the 1906 eruption, abundant galena also formed, together with pyrite, chalcopyrite, pyrrhotite, magnetite and hematite. The lowest temperature recorded for this kind of fumaroles is between 300 and 400 °C;
• Acid fumaroles (the Deville’s “acid fumaroles”), which follow the previous ones after magma cooling. Chloride minerals are present, as molisite, chloraluminite, erythrosiderite, kremersite, along with realgar and native sulfur. Their temperature is between 100 and 300 °C;
• 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;
• Sulfur-bearing low T fumaroles (the Deville’s “water vapor and and H₂S + native S-bearing fumaroles”; [11] and references therein), with T rarely > 100°C; these fumaroles follow the acid ones and are characterized by abundant H₂O(gas) with variable quantity of H₂S. 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. Summary of the Cu-bearing minerals of Somma–Vesuvius from the literature [5,10,34].

Systematics	Name	Formula	Location 1		Occurrence 2
sulfides	chalcopyrite	CuFeS2	Mt. Somma		leuco-tephritic ejecta, fumaroles, scoriae cavities of 1906 eruption
	covellite	CuS	Atrio del Cavallo	TL	1823 and 1906 fumaroles
halides	ammineite	CuCl2·2NH3	Vesuvius		post-1944 fumaroles
	atacamite	Cu2(OH)3Cl	Vesuvius		green patinas in 1631 lavas, 1872 lavas, 1906 lavas
	cumengeite	Pb21Cu20Cl42(OH)40·6H2O	Vesuvius		LT fumaroles
	eriochalcite	CuCl2·2H2O	Vesuvius	TL	1869 HT fumaroles
	hydromelanothallite	Cu2(OH)2Cl2·H2O	Vesuvius
	melanothallite	Cu2OCl2	Vesuvius	TL	1869 HT fumaroles
	mitscherlichite	K2CuCl4·2H2O	Vesuvius	TL	fumaroles, Vesuvius crater
	paratacamite	(Cu,Zn)2(OH)3Cl	Vesuvius		1944 fumaroles, alteration of piypite
	tondiite	Cu3Mg(OH)6Cl2	Vesuvius	co-TL	1906 lava
oxides	cuprite	Cu2O	Vesuvius		encrustations with tenorite (lavas of Camaldoli)
	tenorite	CuO	Vesuvius	TL	1760 scoriae, HT fumaroles of 1825, 1826, 1855, 1872, 1906, 1929, 1944
carbonates	azurite	Cu3(CO3)2(OH)2	Vesuvius		1631 lavas, 1872 lavas
	malachite	Cu2(CO3)(OH)2	Mt. Somma		clinopyroxene-rich ejectum, Pollena
sulfates	antlerite	Cu3SO4(OH)4	Vesuvius		alteration of dolerophanite by H2SO4 in fumaroles
	brochantite	Cu4(SO4)(OH)6	Vesuvius		post-1944 fumaroles
	chalcanthite	CuSO2·5H2O	Vesuvius		1855, 1872, 1881, 1895, 1906, 1944 fumaroles
	chalcocyanite	CuSO4	Vesuvius	TL	1868 HT fumaroles
	chlorothionite	K2CuSO4Cl2	Vesuvius	TL	encrustations in 1872 lavas
	connellite	Cu19(SO4)(OH)32Cl·3H2O	Vesuvius		1872 fumaroles
	cyanochroite	K2Cu(SO4)·6H2O	Vesuvius	TL	1855, 1872, 1906 fumaroles
	dolerophanite	Cu2OSO4	Vesuvius	TL	1868 fumaroles
	euchlorine	KNaCu3O(SO4)3	Vesuvius	TL	1868, 1892, 1893 fumaroles
	linarite	CuPbSO4(OH)2	Vesuvius		1881–1882 scoriae
	piypite	K4Cu4O2(SO4)4(Na,Cu)Cl	Vesuvius		1869 fumaroles
vanadates	mottramite	PbCu(VO4)(OH)	Vesuvius
	volborthite	Cu3V2O7(OH)2·2(H2O)	Vesuvius		1631 and 1868 lavas
silicates	chrysocolla	(Cu2−xAlx)H2−xSi2O5(OH)4·nH2O	Vesuvius		1872 lava
	cuprorivaite	CaCuSi4O10	Vesuvius	TL	Vesuvian lavas (unknown locality)
	litidionite	KNaCuSi4O10	Vesuvius	TL	1873 lapilli (Vesuvius crater)

¹ TL = type locality or co-type locality. ² The ages of the occurrences refer to the cited literature.

. It is worth noting the occurrence of thirteen type locality (or co-type locality) copper species, ranging from sulfides to silicates.

3. Materials and Methods

In the present study, we selected 32 samples (

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).

#	Sample ID	Museum Classification	Location *
1	A578	Cu chloride	Vesuvius
2	5357 B2192	chalcanthite	Vesuvius
3	10741 D1192	chalcanthite	May 1855
4	10817 D1268	azurite	Vesuvius, 1631 lava (Scala)
5	10820 D127	azurite	Vesuvius, 1631 lava (Scala)
6	10907 D1358	euchlorine with metamorphized hydrocyanite	crater, February 1870
7	11004 D1455	conglomerate with cupromagnesite	ejecta, April 1872
8	11061 D1512	malachite, pseudocotunnite	Vesuvius crater, July 1875
9	11269 D1720	tenorite	crater, July 1881
10	11271 D1722	atelite	Vesuvius crater, June 1880
11	12020 E559	tenorite	Vesuvius crater, 1885
12	12105 E644	Cu-bearing cotunnite	Vesuvius, March 1870, November 1868 lava, “Fosso della Vetrana”
13	12756 E1295	metamorphized hydrocyanite in chalcanithe	Vesuvius crater, 1870
14	12912 E1457	euchlorine	Vesuvius, 1870
15	16256 E6787	linarite	Vesuvius crater, 1882
16	17004 E5543	metamorphized tenorite (atelite)	Vesuvius crater, December 1872
17	17029 E5568	chlorothionite	Vesuvius crater, January 1873
18	17030 E5569	chlorothionite	Vesuvius crater, January 1873
19	17032 E5571	cupromagnesite	Vesuvius, April 1872 ejecta
20	17556 E6095	tenorite	Vesuvius, 1872 lavas
21	17602 E6141	Cu-bearing anglesite	Vesuvius, September 1877, fumarole of 1872 lava
22	17718 E6249	vesbine	Vesuvius, 1631 lava
23	17719 E6250	vesbine	Vesuvius, 1631 lava (Camaldoli)
24	17722 E6253	vesbine with atacamite	Vesuvius, 1631 lava (Uncino)
25	17724 E6255	atacamite	Vesuvius, 1631 lava (Camaldoli)
26	17729 E6260	greenish atacamite	Vesuvius, 1631 lava (Camaldoli)
27	17923 E6454	neocyanite	Vesuvius crater, October 1880
28	17926 E6457	litidionite	Vesuvius crater, June 1873
29	18531 E7062	cuprite	Vesuvius, 1631 lava
30	21836	tenorite	Vesuvius, 1906 eruption
31	21846	anglesite	Vesuvius, 1906 eruption
32	21911	metamorphosed hydrocyanite	Vesuvius, eruptive vent at the cone base, August 1891

* The timing indicated here refers to both the eruptions age (i.e., 1631 lava) and to the entry dates in the Museum collection (i.e., January 1873).

), 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.

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.

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.

4. Results

4.1. Characterization of the Cu-Bearing Minerals

Table 3. Chemical formula of the main Cu-bearing minerals found and/or cited in this study together with the obsolete names.

Mineral	Discredited Name	Formula *
atacamite		Cu2Cl(OH)3
azurite		Cu3(CO3)2(OH)2
boothite	cupromagnesite	CuSO4·7(H2O)
brochantite		Cu4(SO4)(OH)6
chalcanthite		CuSO4·5(H2O)
chalcocyanite	hydrocyanite	Cu(SO4)
chlorothionite		K2Cu(SO4)Cl2
chrysocolla		(Cu2-xAlx)H2-xSi2O5(OH)4·nH2O
cryptochalcite		K2Cu5O(SO4)5
cuprite		Cu2O
cyanochroite		K2Cu(SO4)2·6H2O
dravertite		CuMg(SO4)2
euchlorine		KNaCu3O(SO4)3
kröhnkite		Na2Cu(SO4)2·2H2O
leightonite		K2Ca2Cu(SO4)4·2H2O
linarite		PbCu(SO4)(OH)2
litidionite	neocyanite	KNaCuSi4O10
malachite		Cu2(CO3)(OH)2
mottramite	vesbine	PbCu(VO4)(OH)
natrochalcite		NaCu2(SO4)2(OH)·H2O
paratacamite	atelite	Cu3+3(Cu,Zn)(OH)6Cl2
starovaite		KCu5O(VO4)3
tenorite		CuO
tsumebite		Pb2Cu(PO4)(SO4)(OH)
volborthite	vesbine	Cu3V2O7(OH)2·2H2O

* IMA database.

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. Cu-bearing mineral assemblages of the studied samples, mainly occurring as incrustation or patinas, carried out by combined SEM-EDS and XRD analyses.

#	Sample ID	Main Mineral Assemblage *
1	A578	anglesite, atacamite, euchlorine
2	5357 B2192	chalcanthite, natrochalcite, cryptochalcite, anglesite, halite, leightonite
3	10741 D1192	chalcanthite, leghtonite, kröhnkite
4	10817 D1268	azurite, atacamite, malachite
5	10820 D1271	azurite, atacamite, chrysocolla, gypsum, calcite
6	10907 D1358	chalcocyanite, atacamite, alunite, opal, anglesite, galena, euchlorine
7	11004 D1455	atacamite, leightonite, sylvite, halite, gypsum
8	11061 D1512	atacamite, challacolloite, elpasolite, sylvite, fluorite, anglesite, palmierite, gypsum, galena, paratacamite
9	11269 D1720	tenorite, anglesite, cryptochalcite, elpasolite, palmierite, gypsum, anhydrite, thermessaite, kröhnkite
10	11271 D1722	atacamite, halite, gypsum, fluorite
11	12020 E559	tenorite, anglesite, hematite, elpasolite, fluorite, kröhnkite
12	12105 E644	cotunnite, anglesite
13	12756 E1295	chalcanthite, chalcocyanite, cryptochalcite, anglesite, gypsum, halite, kröhnkite, dravertite
14	12912 E1457	euchlorine, atacamite, anglesite, cryptochalcite, palmierite, barite, natrochalcite
15	16256 E6787	linarite, elpasolite, fluorite, anglesite, tenorite, paratacamite, pyromorphite, tsumebite, brochantite, sbacchiite, hematite, galena
16	17004 E5543	atacamite, pseudocotunnite, tenorite
17	17029 E5568	atacamite, anglesite, sylvite, halite, syngenite, paratacamite, challacolloite, chlorothionite
18	17030 E5569	atacamite, elpasolite, sylvite, halite, fluorite, aphthitalite, chlorothionite
19	17032 E5571	atacamite, sylvite, halite, challacolloite, boothite
20	17556 E6095	tenorite, atacamite, sylvite, halite, fluorite
21	17602 E6141	atacamite, anglesite, galena
22	17718 E6249	mottramite, chrysocolla, vanadinite, wulfenite
23	17719 E6250	tenorite, mottramite, chrysocolla, vanadinite, hematite
24	17722 E6253	mottramite, atacamite, chrysocolla, starovaite, anglesite, gypsum
25	17724 E6255	atacamite
26	17729 E6260	atacamite
27	17923 E6454	litidionite, calcinaksite, tridymite, wollastonite, diopside
28	17926 E6457	litidionite, calcinaksite, tridymite, wollastonite, atacamite, sylvite, glassy matrix
29	18531 E7062	cuprite, atacamite, fluorite, volborthite
30	21836	tenorite, anglesite, fluorite, halite, syngenite, linarite, thénardite, galena
31	21846	anglesite, tenorite, atacamite, gypsum, challacolloite
32	21911	chalcocyanite, cyanochroite, leightonite, gypsum, cotunnite, anglesite, thermessaite, alum-K, kröhnkite

* Minerals in Italics need to be validated.

, 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.

4.1.1. Hydroxylhalides

The most abundant basic cupric chloride is atacamite (Table 3 and Table 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,d), (iii) small rounded microcrystalline masses, green in color, spread out on the external surface of the host rock (Figure 1b). Atacamite rarely occurs as well crystallized clusters (Figure 2a), whereas it commonly forms compact masses covering the external surfaces of lavas (ca. 100 μm in thickness) or concretions in admixtures with chrysocolla (Figure 2b–d). The chemical composition of atacamite (

Table 5. Representative results of SEM-EDS analyses of atacamite and paratacamite from Somma–Vesuvius.

Sample ID	10820 1271			11061 D17722			17724 E6255			17729 E6260			16256 E6787	17729 E6260
wt%	atacamite												paratacamite
CuO	72.04	72.47	74.63	71.58	72.00	73.55	72.23	74.32	73.69	75.23	74.50	75.22	69.05	68.07
SiO2	-	-	0.11	-	-	-	-	-	-	0.12	0.25	-	-	-
Al2O3	-	0.12	0.22	-	-	-	-	-	-	-	-	-	-	-
CaO	-	-	0.13	-	-	0.30	-	0.68	-	-	-	-	-	-
FeO	1.05	-	-	-	0.45	0.45	-	-	-	0.90	0.33	-	0.21	0.23
MgO	-	-	-	-	-	0.11	0.78	-	-	-	-	-	-	0.12
MnO	-	1.26	-	-	-	0.26	1.48	1.92	-	0.13	-	-	-	0.09
ZnO	1.27	1.50	1.29	0.90	-	0.67	-	0.33	1.35	-	0.23	0.44	2.54	6.25
Cl	17.01	17.11	17.34	16.60	17.05	16.56	17.36	17.34	17.13	17.00	16.87	16.64	16.00	16.34
Total	91.37	92.46	93.72	89.08	89.50	91.90	91.86	94.60	92.17	93.38	92.18	92.30	87.80	91.10
–O=Cl2	3.84	3.87	3.92	3.75	3.85	3.74	3.92	3.92	3.87	3.84	3.81	3.76	3.62	3.69
	87.52	88.59	89.80	85.33	85.65	88.16	87.93	90.68	88.30	89.54	88.37	88.54	84.18	87.41

- below detection limit.

) shows small amounts of Fe, Mg, Mn, Ca and Si, and more frequently, Zn contents (0.2–1.5 wt% ZnO).

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.

4.1.2. 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).

Cuprite forms euhedral octahedrons (maximum dimension ca. 0.1 mm) covering the lava surface of sample 18531 E7062 (Figure 4a,b); its composition is quite stoichiometric, with only trace amounts of Fe, Mg and Mn (

Table 6. Representative results of SEM-EDS analyses of cuprite and tenorite from Somma–Vesuvius.

Sample ID	18531 E7062				16256 E6787
wt%	cuprite				tenorite
CuO	99.06	99.12	98.98	99.16	98.12	98.78	98.47	99.18
FeO	0.13	0.34	0.26	0.11	-	0.23	0.55	-
MgO	-	0.11		0.07	-	-	-	0.34
MnO	-	0.10	0.15	0.06	-	-	0.44	-
ZnO	-	-	-	-	0.34	-	-	-
SiO2	-	-	-	-	-	0.26	-	-
Total	99.19	99.67	99.39	99.40	98.46	99.27	99.46	99.52

- below detection limit.

). At SEM scale, tenorite generally occurs in platy to scaly individuals (Figure 4c,d); it is found as vug/surface fillings and also together with paratacamite (Figure 4e,f).

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.

4.1.3. Carbonates

Azurite was recognized in two samples, labeled as “Azurite in 1631 lavas” in the Museum catalog (Table 2 and Table 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. Representative chemical compositions of azurite and malachite from Somma–Vesuvius.

Sample ID	10817 D1268			10820 D1271			10817 D1268
wt%	azurite						Malachite *
CuO	68.59	68.43	68.13	66.15	66.77	68.99	73.46
FeO	-	-	0.07	-	-	0.32	0.23
MgO	-	-	0.07	-	-	-	0.24
MnO	-	-	-	0.13	-	-	-
ZnO	0.49	0.36	0.18	-	0.29	0.82	7.37
CaO	-	-	-	0.42	-	-	-
Total	69.08	68.80	68.45	66.28	67.06	70.13	81.30

- below detection limit. * Average of three point-analyses.

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.

4.1.4. Sulfates

Cu-bearing sulfates are the most abundant species identified in the investigated samples. These minerals generally occur in complex associations in extremely heterogeneous samples. However, at least ten different cupric sulfates are recognized (Table 3 and Table 4), namely brochantite, chalcanthite, chalcocyanite (“hydrocyanite”), cryptochalcite, cyanochroite, euchlorine, kröhnkite, leightonite, linarite and natrochalcite. Some representative rock lithotypes with Cu-bearing sulfates are shown in Figure 7.

Brochantite was detected only in sample 16256 E6768 (Table 4), where it occurs in small anhedral masses with linarite (Figure 8a and Figure 9c). Small amounts of Zn (ca. 1 wt% ZnO) were found (

Table 8. Representative chemical composition of Cu-bearing sulfates from Somma–Vesuvius: brochantite, chalcanthite, chalcocyanite, cryptochalcite, cyanochroite and euchlorine.

Sample ID	11269 D1720		5357 B2192		21911		16256 E6787				5357 B2192
wt%	kröhnkite		leightonite				linarite				natrochalcite
CuO	19.93	18.52	12.32	11.68	11.68	12.03	21.19	22.03	20.98	20.03	42.62	43.37
FeO	0.45		0.04	0.36	0.98	1.40	0.55	-	-	-	0.08	0.09
MgO	0.23	0.11	0.07	0.11	0.38	0.44	-	-	-	-	0.77	0.13
MnO	0.08		-	0.09	-	-	-	-	-	-	-	0.07
ZnO			-	-	-	-	-	0.37	-	0.68	-	0.11
CaO	0.05	0.12	18.13	17.64	16.18	17.99	-	-	-	-	-	-
Na2O	19.07	18.28	0.55	0.47	0.75	0.43	-	-	-	-	6.79	6.68
K2O	0.22		14.03	13.31	12.39	13.86	-	-	-	-	-	0.12
PbO			-	0.09	-	-	0.11	-	-	0.09	-	-
As2O5			-	-	-	-	-	-	-	-	-	-
P2O5			-	-	-	-	55.43	53.51	53.33	57.41	-	-
SO3	46.99	47.34	48.94	49.04	49.71	49.63	19.63	18.41	18.65	17.62	43.48	43.44
Total	87.02	84.37	94.084	92.79	92.07	95.78	96.91	94.32	92.95	95.83	93.74	94.01

- below detection limit.

).

Chalcanthite was found in three samples (Table 3 and Table 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 (Table 3 and Table 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 (Table 2 and Table 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. Representative chemical composition of Cu-bearing sulfates from Somma–Vesuvius: kröhnkite, leightonite, linarite and natrochalcite.

Sample ID	16256 E6787		10741 D1192		10907 D1358		5357 B2192		11296 D1720		12756 E1295		21911		12912 E1457
oxide wt%	brochantite		chalcanthite		chalcocyanite		cryptochalcite						cyanochroite		euchlorine
CuO	72.13	71.04	32.08	31.44	48.88	47.93	42.38	43.82	42.51	42.51	46.09	46.37	20.22	19.97	42.86	41.09
FeO	-	0.55	-	0.15	0.24	-	0.27	0.27	-	0.24	-	-	-	0.67	0.78	0.22
MgO	-	-	0.13	0.06	0.65	0.06	-	0.09	0.96	-	-	-	-	0.56	-	0.21
MnO	0.33	-	-	0.07	0.11	-	0.12	-	-	-	-	-	0.23	-	-	-
ZnO	1.08	0.97	-	-	0.09	0.09	1.55	1.42	1.83	1.71	3.74	3.15	-	-	0.56	-
Na2O	-	-	-	-	-	-	-	-	-	-	-	-	-	-	6.00	5.72
K2O	-	-	-	-	-	-	9.63	9.63	11.24	9.64	9.93	8.44	22.34	21.78	6.99	7.99
CaO	-	-	0.50	-	-	0.22	0.02	0.01	-	-	-	-	-	0.05	-	0.99
SO3	18.02	17.68	33.17	34.09	49.06	50.05	44.83	44.82	42.90	45.48	40.55	42.58	35.87	35.33	41.90	42.67
Total	91.56	90.24	65.88	65.81	99.03	98.35	98.81	100.06	99.44	99.58	100.31	100.54	78.66	78.36	99.09	98.89

- below detection limit.

).

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).

Linarite and natrochalcite are both recognized in two samples (Table 4); linarite forms acicular individuals or microcrystalline encrustations (Figure 9b,c) and is chemically nearly pure (Table 9). Natrochalcite was observed in sharp bipyramidal crystal (Figure 9e); as for linarite, its composition is almost stoichiometric (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.

4.1.5. 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 (Table 2 and Table 4, Figure 1e, Figure 3c and Figure 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. Chemical compositions of the selected Cu-bearing vanadates and phosphates from Somma–Vesuvius.

Sample ID	18531 E7062		17718 E6249				17719 E6250				17722 E6253					16256 E6787
oxide wt%	volborthite		mottramite								starovaite					tsumebite
CuO	51.24	50.97	18.03	19.86	20.81	18.89	24.00	21.50	21.75	20.87	49.70	49.46	49.88	50.89	49.88	12.09	13.93
FeO	0.25	0.33	1.39	2.97	1.09	2.03	-	-	-	-	-	-	-	-	-	-	0.14
MgO	-	0.23	-	-	0.76	-	-	-	-	-	-	-	-	-	-	0.12	-
MnO	0.12	0.26	-	1.55	-	1.67	-	-	-	-	1.26	1.00	-	-	2.73	-	-
ZnO	-	0.09	0.78	1.10	3.19	-	-	-	-	-	0.14	-	0.56	0.44	-	0.47	0.32
Al2O3	-	-	0.81	1.53	-	1.87	-	-	1.01	0.94	-	-	-	-	-	-	-
K2O	-	-	-	-	-	-	-	-	-	-	7.12	7.24	7.43	8.11	7.82	-	-
CaO	-	-	1.62	2.22	0.89	-	1.17	-	-	-	-	-	-	-	-	-	0.07
PbO	-	-	51.40	49.15	53.97	51.37	49.93	49.87	48.93	50.66	-	-	-	-	-	62.41	63.07
V2O5	39.06	38.68	19.32	17.23	16.09	18.12	20.63	22.54	20.99	20.51	38.71	39.67	39.62	39.45	38.56	-	-
As2O5	-	-	2.74	1.89	2.56	2.01	2.43	2.08	2.17	1.55	-	-	-	-	-	2.30	1.85
P2O5	-	0.11	-	-	-	-	-	-	-	-	-	-	-	-	-	10.76	8.54
SO3	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	11.00	10.46
Cl	-	-	-	-	-	-	-	-	-	-	0.51	0.63	0.69	1.05	0.57	-	-
–Cl2=O											0.11	0.14	0.16	0.24	0.13
Total	90.67	90.67	96.09	97.50	99.36	95.96	98.16	95.98	94.84	94.52	97.55	98.14	98.34	100.18	99.69	99.15	98.38

- below detection limit.

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% As₂O₅), 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 (Table 3 and Table 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 (Table 3 and Table 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.

4.1.6. Silicates

The Cu-bearing silicates found in the investigated samples are chrysocolla (Figure 1e and Figure 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 (Figure 1e and Figure 10); this silicate was clearly identified by SEM observations, as reported in Figure 2b, Figure 11b–g and Figure 13a,b, and forms few micrometer-sized zoned concretions.

Table 11. Chemical compositions of selected Cu-bearing silicates from Somma–Vesuvius. For litidionite, the analyses labelled as ‘core’ and ‘rim’ refer to the crystal in Figure 13d (average of three point-analyses).

Sample ID	17719 E6250				17722 E6253				19723 E6454				19726 E6457
wt%	chrysocolla								litidionite
									core	rim
SiO2	35.23	37.72	35.07	34.05	39.22	36.00	38.96	36.93	59.35	59.81	61.68	62.07	59.81	60.13	60.51	58.99
Al2O3	5.38	5.21	6.32	6.22	4.58	6.41	5.99	4.86	0.28	-	-	-	-	-	0.05	-
CuO	42.24	45.91	46.33	42.51	42.36	40.91	41.96	43.82	21.02	19.28	17.92	17.32	19.28	20.02	18.15	19.66
FeO	2.11	-	-	1.00	0.77	-	-	-	-	-	-	-	-	-	-	-
MgO	0.78	0.35	-	0.54	-	-	-	-	0.27	0.67	-	-	0.63	0.35	-	-
MnO	1.60	-	-	1.19	-	-	-	-	-	-	-	-	-	-	-	-
ZnO	-	0.66	-	0.22	0.87	-	-	-	-	-	-	-	-	-	-	-
Na2O	-	-	-	-		-	-	-	8.42	8.30	7.93	8.02	8.30	8.16	7.93	7.54
K2O	-	-	-	-	0.47	1.25	-	-	10.41	10.35	12.05	11.90	10.35	10.83	11.28	12.12
CaO	1.77	1.00	-	1.62	1.34	1.41	1.05	1.54	0.84	0.99	0.92	1.01	0.99	1.03	1.11	0.79
PbO	0.97	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
V2O5	0.77	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
Total	90.85	90.85	87.72	87.35	89.61	85.98	87.96	87.15	100.58	99.40	100.50	100.33	99.36	100.51	99.03	99.11

- below detection limit.

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.

Litidionite (neocyanite) characteristically occurs in the deep blue glassy crusts on lava fragments or small lapilli (Figure 12), deeply affected by the fumarolic activity (Table 2) [33]. In the analyzed samples (17923 E6454 and 17926 E6457), litidionite occurs with tridymite, calcinaksite, wollastonite and locally minor amorphous silica-rich phase (see Section 4.2), diopside, atacamite and halite. Observed at SEM scale, its crystal habit is fibrous, as in sample 17923 E6465 (Figure 13c), or can form euhedral and platy crystals, as in sample 17923 E6465 (Figure 13d). Table 11 shows the chemical composition of litidionite; some crystals of sample 17923 E6465 show a zonation, with higher Cu contents decreasing toward the rim (see Table 11); the compositional ranges are 58.99–62.07 wt% SiO₂, 17.32–21.02 wt% CuO, 7.54–8.42 wt% Na₂O, 10.35–12.12 wt% K₂O and 0.79–1.11 wt% CaO, with traces of Mg and Al.

4.2. 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 Figure 14 and Figure 15 and in

Table 12. Representative chemical compositions of selected miscellaneous minerals associated with Cu-bearing minerals from Somma–Vesuvius.

Sample ID	17030 E5569		16256 E6787		16256 E6787		11061 D1512			16256 E6787		17030 E5569		11269 D1720		16256 E6787
element wt%	elpasolite				sbacchiite		challacolloite		wt%	thermessaite		aphthitalite		palmierite		syngenite
Al	11.13	11.07	11.98	11.71	11.88	11.54	-	-	Al2O3	21.09	20.98	-	-	-	-	-	-
Na	9.31	9.28	11.02	10.98	-	-	-	-	Na2O	0.22	0.13	9.82	9.65	0.77	0.54	-	0.15
K	29.29	29.33	29.07	29.21	0.44	-	8.27	8.71	K2O	36.14	36.47	41.55	40.05	16.92	16.35	26.97	27.78
Ca	-	-	-	-	32.08	34.07	-	-	CaO	0.34	0.26	0.24	0.45	1.04	0.82	18.05	17.12
Pb	-	-	-	-	-	-	61.74	61.65	MgO	0.26	-	0.16	0.17	-	0.22	0.23	0.47
As	-	-	-	-	-	-	0.09	0.17	PbO	-	-	-	-	45.56	46.44	-	-
Cl	-	-	-	-	-	-	29.38	29.43	As2O5	-	-	-	-	0.55	0.67	-	-
F	50.72	50.88	47.98	48.67	55.65	54.12	-	-	P2O5	-	-	-	-	-	-	-	-
Total	100.45	100.56	100.10	100.57	100.05	99.73	99.48	99.96	SO3	28.84	27.65	48.14	48.09	35.43	35.09	47.23	48.18
									F	22.71	23.08	-	-	-	-	-	-
									Total	109.60	108.60	99.91	98.41	100.3	100.1	92.48	93.70
									–O=F2	9.54	9.69
										100.06	98.88
Sample ID	17719 E6250						17718 E6249			17923 E6454				17926 E6457
wt%	pyromorphite		vanadinite				wulfenite			calcinaksite
SiO2	-	-	-	-	-	-	-	-	-	63.13	63.68	61.92	61.44	62.70	62.14	63.28
Al2O3	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
Na2O	-	-	-	-	-	-	-	-	-	6.61	6.65	7.46	7.07	5.66	5.87	8.12
K2O	-	-	-	-	-	-	-	-	-	12.24	10.72	12.38	13.05	10.65	11.57	12.57
CaO	0.34	0.12	-	-	-	-	-	-	-	14.02	15.20	12.83	12.05	12.43	10.87	9.69
FeO	-	-	-	-	-	-	-	-	-	0.05	1.73	-	-	-	-	-
MgO	-	-	-	-	-	-	-	-	-	-	-	-	-	1.35	1.37	-
MnO	-	-	1.41	0.97	-	0.55	-	-	-	-	-	-	-	-	-	-
PbO	81.19	82.02	79.83	78.23	80.87	80.04	57.71	56.62	56.38	-	-	-	-	-	-	-
CuO	-	-	1.12	0.66	-	-	-	-	-	-	-	2.89	3.61	3.34	4.22	4.64
MoO3	-	-	-	-	-	-	27.87	28.94	28.13	-	-	-	-	-	-	-
V2O5	-	-	14.05	15.43	14.99	15.06	-	-	-	-	-	-	-	-	-	-
As2O5	0.52	0.44	0.84	1.23	1.36	0.99	-	-	-	-	-	-	-	-	-	-
P2O5	16.11	15.21	-	-	-	-	-	-	-	-	-	-	-	-	-	-
WO3	-	-	-	-	-		14.94	14.68	15.00	-	-	-	-	-	-	-
Cl	2.68	2.75	3.73	3.44	3.69	3.58	-	-	-	-	-	-	-	-	-	-
Total	100.50	100.54	100.98	99.96	100.91	99.67	100.5	100.20	99.51	96.05	97.98	97.48	97.22	96.13	96.05	98.29
–O=Cl2	0.60	0.62	0.84	0.77	0.83	0.80
	99.90	99.92	100.14	99.19	100.08	98.87

- below detection limit.

.

Elpasolite, K₂NaAlF₆, 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% Al₂O₃, 9.28–11.02 wt% Na₂O, 29.07–29.33 wt% K₂O 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) ų and Z = 4. These data are perfectly comparable with the literature data [53].

Sbacchiite, Ca₂AlF₇, 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].

Challacolloite, a K–Pb chloride with ideal formula KPb₂Cl₅ [11,35], was observed in four samples (Table 4), where tiny euhedral crystals are formed (Figure 14d) with a nearly stoichiometric composition (Table 12).

Thermessaite, K₂AlF₃(SO₄), is a K-bearing aluminofluoride with SO₄ [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 (Table 12).

Aphthitalite [K₃Na(SO₄)₂], palmierite [K₂Pb(SO₄)₂], and syngenite [K₂Ca(SO₄)₂·H₂O] [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 (Na₂SO₄) 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 [Pb₅(PO₄)₃Cl] and wulfenite [PbMoO₄] 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, Pb₅(VO₄)₃Cl, 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 wt% As₂O₅.

Calcinaksite, [KNaCa(Si₄O₁₀)·H₂O], the hydrous and Ca-dominant member of the litidionite group [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), with subhedral prismatic crystals. Its composition yields 61.44–63.68 wt% SiO₂, 5.66–8.12 wt% Na₂O, 10.72–13.05 wt% K₂O, 9.69–15.20 CaO as compositional ranges; significant contents of Cu, i.e., 2.89–4.64 wt% CuO, were detected in some areas of calcinaksite crystals and are inversely correlated to the Ca amounts. Together with prevailing tridymite, a silica-rich glassy phase (Figure 13d) shows a composition corresponding to SiO₂ 66.12, Al₂O₃ 1.41, TiO₂ 1.16, FeO 0.10, MgO 0.37, Na₂O 1.52, K₂O 6.47, CaO 0.91, CuO 3.33, Cl 0.78 (wt%); 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.

5. Discussion

5.1. 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 (Table 2 and Table 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 CaCO₃, 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 + H₂O = Cu₂(OH)₃Cl. 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 Cu₃(Cu,M²⁺)(OH)₆Cl₂, in which Cu/M²⁺_atomic is ~11-7, corresponding to ~1/3 to 1/2 the occupancy of the non-Jahn-Teller site by M²⁺, which is an essential stabilizing non-Jahn-Teller-distorting cation of suitable radius, such as Zn²⁺, Ni²⁺, Co²⁺, Fe²⁺, Cd²⁺ and Mg²⁺.

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 (CaF₂) 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 CuCl₂ and water vapor, according to the reaction CuCl₂ + H₂O = 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 CO₂ 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 Cu²⁺ by Zn²⁺, giving the formula: (Cu_1−xM_x)₂(OH)₂CO₃ (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, ±H₂O). 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 [CuSO₄·5(H₂O)] [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):

-

chalcocyanite–euchlorine (10907 D1358)

-

cryptochalcite–euchlorine–natrochalcite (12912 E1457).

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 H₂O-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.

5.2. 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., Cu²⁺), 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 Cu²⁺-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 Cu²⁺-based sulfates, vanadates and chlorides containing OH-bearing groups.

5.3. 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 CuO₄ 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 [OCu₄] tetrahedra is connected by shared vertexes.

Vesuvius carbonates are represented by azurite and malachite. The structure of azurite based upon Cu₂O₈-chains connected to the complex layers via CuO₄ planar groups copper layers with strong magnetic properties [72] linked by carbonate groups. In the structure of malachite, there are two independent copper sites in square-planar and octahedral coordination. The edge-shared octahedral columns connected with adjacent [CuO₄] groups form a zig-zag like pattern combined with each other by CO₃-groups. The importance of hydrogen bonding in malachite and azurite structures should be noted.

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 [Si₈O₂₀] chains with a hexagonal cross-section, which are interconnected by [CuO₄] 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 CuO₄ planar groups connected with perpendicular chains based by [CuO₅Cl] distorted octahedra. The paratacamite structure is more ordered, one half distorted octahedra changed by [CuO₆] and the other half changed by CuO₄ planar groups, whereas Cl^- anions 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 V⁵⁺ cations occupy slightly distorted tetrahedra connected by three oxygen vertexes with [CuO₆]-based chains and one oxygen involved in hydrogen bonding. The Pb²⁺ 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 ∞[Cu₅O₁₃] sheets connected via [VO₄] 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 [CuO₆] distorted octahedra with 1/3 empty sites. Such layers are connected with [V₂O₅] nesovanadate groups that form channels populated by water molecules [77].

The most complex Vesuvius Cu-minerals are related to the sulfates, which are represented by group of 14 minerals (Figure 17). Structurally, they can be divided into five subgroups based on the arrangement of Cu atoms: single Cu-octahedra (kröhnkite, chalcanthite, cyanochroite, boothite, leightonite); single Cu-centered squares (chlorothionite); octahedral chains of edge-shared Cu-octahedra (dravertite, linarite, chalcocyanite, natrochalcite, tsumebite); complex chains (brochantite); complex clusters (euchlorine, cryptochalcite).

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].

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 [SO₄] tetrahedra [84]. In the linarite structure, octahedral Cu-chains 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 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 [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].

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 [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 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].

6. 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 Cu²⁺ cation [93], especially when its coordination is formed only by O²⁻ and Cl⁻ ligands with no OH⁻ groups or H₂O 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.