Pellino, Annamaria (2024) Crystal chemistry and genesis of copper minerals of Somma-Vesuvius. [Tesi di dottorato]

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Collection description

The subject of this PhD thesis is the study of the Cu-bearing minerals and their associated phases in samples mainly related to exhalative processes of the recent activity of Somma-Vesuvius volcano. The studied samples belong to the Vesuvian Collection of the Royal Mineralogical Museum of Naples University Federico II, Italy (RMMN), which preserves the most important collection of Somma-Vesuvius samples. The study aim was twofold: (1) to carry out a detailed crystal-chemical investigation on the complex paragenesis of Cu-bearing minerals and their associated phases, considering samples of the Vesuvian Collection of the RMMN, in order to shed light on Cu (and other metals) enrichment and minerogenesis in hydrothermal systems of volcanic active areas; (2) to increase the knowledge of the Vesuvius systematics by investigating rare mineral associations. The 42 samples selected from the RMMN collection have been sorted in three main types of mineralogical associations: (i) Cu-bearing silicates and related minerals (i.e. litidionite-bearing samples), (ii) Cu-bearing vanadates and related minerals (i.e. vesbine-bearing samples), and (iii) Cu-bearing sulfates samples and related minerals. The studied samples can be related to products of medieval eruptions to 1891 deposits. The Cu-bearing silicate assemblages considered in this thesis consist of the typical blue encrustations characterized by the rare mineral copper silicate litidionite, CuKNaSi4O10. These assemblages are originated by thermally modified pyroclastic fragments related to the 1872 eruption, as a result of high-temperature alteration processes at the rock-fumaroles interface. The litidionite-bearing assemblage has been characterized in detail by SEM-EDS, WDS, SC-XRD, XRPD, FTIR and Raman spectroscopy. Litidionite mainly occurs together with kamenevite, REE-bearing perovskite, diopside, rutile, wollastonite, trydimite, calcinaksite, spinel, magnetite, ilmenite, apatite, and silica glass. Minor amounts of atacamite and halite also occur. Together with litidionite, a new Ti-bearing litidionite variety have been revealed by the present study; it appears on the latest stages of partial crystallization of Ti-bearing silica glass, also associated to a Pb-enriched litidionite. The incorporation of Ti into the litidionite structure is accompanied by the complex heteropolyhedral substitution according to the scheme: VTi4+ + VII–VIII□ + IVAl3+ ↔ VCu2+ + VII-VIII(Na,K)+ + IVSi4+. Ti-bearing litidionites have significant variability in their chemical composition, with TiO2 content reaching up to 12.06 wt.% (0.56 apfu). Two configurations are possible for this maximal TiO2 content: CuTiK□Na2Si7AlO20 (Z = 1) or CuTiK2Na□Si7AlO20 (Z = 1). The refinement of the site-occupation factors confirmed the presence of Ti at a five-coordinated M site. The crystal structure of Ti-litidionite has been refined in the P1 space group, a = 6.9699(7), b = 7.9953(10), c = 9.8227(10) Å, α = 105.186(9), β = 99.458(8) and γ = 114.489(10) to R1 = 0.064 for 1726 unique observed reflections. Incorporation of Ti into litidionite with creation of vacancies at the K site makes it possible to consider this phase as a prospective material for the selective removal of radionuclides from waste aqueous solutions. According to our recent studies, calcination of the ion-exchanged forms of titanosilicates up to 1000°C results in the formation of Synroc-type titanate ceramics. In the litidionite assemblage, further investigations particularly focused on the silicate minerals led to the discovery of the new mineral enricofrancoite, ideally KNaCaSi4O10, approved by the Commission on New Minerals and Mineral Names (CNMMN). The mineral is named in honor of Enrico Franco, a distinguished mineralogist known for his contributions to the mineralogy in the Somma-Vesuvius area. The new mineral was characterized by optical and physical methods, as well as by SEM-EDS, WDS, SC-XRD, XRPD, FTIR and Raman spectroscopy. Enricofrancoite occurs as euhedral and platy crystals which are transparent colorless or light blue with a vitreous lustre. Mohs hardness is 5.5. Dmeas is 2.63(3) g/cm3 and Dcalc is 2.63 g/cm3. The mineral is optically biaxial (−), α = 1.542(5), β = 1.567(5),γ = 1.575(5); 2V(meas) = 60(2)° and 2Vcalc = 58°. The mean chemical composition (wt.%, electron-microprobe data) is: SiO2 64.81, Al2O3 0.03, TiO2 0.08, FeO 0.07, MgO 1.71, CaO 10.64, CuO 2.22, Na2O 8.56, K2O 11.41. The absence of water-related bands in the IR spectrum supports its characterization as an anhydrous phase. The empirical formula based on 10 O apfu is: K0.90Na1.03(Ca0.71Mg0.16Cu0.10)Σ=0.97Si4.02O10. Enricofrancoite is triclinic, space group P-1, unit-cell parameters refined from the single-crystal data are a = 7.0155(4) Å, b = 8.0721(4) Å, c = 10.0275(4) Å, α = 104.420(4)°, β = 99.764(4)°, γ = 115.126(5)°, V = 472.74(5) Å3. The crystal structure of enricofrancoite closely resembles that of litidionite, featuring a complex heteropolyhedral layered framework. In litidionite-related structures, a notable characteristic is the presence of one-dimensional infinite anionic tubes [Si8O20]8−∞ running along the [100] direction. Enricofrancoite can be categorized as a loop-branched dreier double chain {lB, 21∞} [3Si8O20] according to the Liebau classification or a 3T8 silicate tube-containing mineral as per the structure hierarchy for silicate minerals. Enricofrancoite is an H2O-free analogue of calcinaksite with 5-coordinated Ca2+ at the M site. The Cu-bearing vanadates of this study are represented by so-called vesbine samples. Vesbine refers to a discredited mineral believed to contain vesbium (a new element similar to vanadium) by the famous Neapolitan mineralogist Arcangelo Scacchi (director of the RMMN from 1844 to 1891). The vesbine samples were reported by A. Scacchi in relation to the 1631 lavas, but according to literature these lavas are more likely related to the medieval activity. These samples consist of fumarole-related yellow-green patinas coating some historical lavas. Subsequent studies showed that vesbine corresponded to a mixture of copper vanadates and halides. To identify in detail these V-rich assemblages, a selection of the RMMN samples have been analyzed by combined optical microscopy, SEM-EDS, XRPD, FTIR and TEM-HRTEM-EDS. Results reveal complex mineral associations, including vanadates, halides, carbonates, oxides, silicates, tungstates/molybdates and sulfates (Cu-bearing and Cu-free). The samples, mainly yellow-greenish patinas on lavas, exhibit varying morphologies from thin films to irregular encrustations. Two main groups, Group I (yellow patinas) and Group II (yellow-green-blue patinas), have been identified based on macroscopic observations and mineralogical investigations. Group I is characterized by dominant volborthite, associated with minor mottramite, vanadinite, atacamite, and stolzite-wulfenite while group II displays atacamite-vanadinite-mottramite associations, often with volborthite, azurite, and malachite in distinct colored bands. XRPD analyses have permitted to clarify ambiguous mineral identifications such as the different polymorphs of Cu2(OH)3Cl and confirm the presence of the rare botallackite, which is the first recorded occurrence at Vesuvius. Several different additional non-essential elements have been detected in the vanadates, including Mn, Zn and As. The occurrence of wulfenite- and stolzite-rich phases indicates the presence of Mo and W, along with Pb, in the mineralizing fluids. Mn-rich phases, commonly in mixtures with silicates and vanadates, were also observed. These minerals are formed by a combination of different processes, including rock-fluid interactions, gas-water interactions, and alteration/oxidation of primary fumarolic minerals. Temperatures for the depositions of the vanadates-bearing assemblages are interpreted to be in the range of 100 to 400 °C. The RMMN sulfates-bearing samples studied in this dissertation are represented by products dated 1813, 1848, 1870, 1871, 1886, and 1891, as inferred from the Museum catalog. These samples, characterized by combined SEM-EDS, WDS, XRPD, LA-ICP-MS, and FTIR (as well as by means of very preliminary CHNS elemental analyses), are composed of complex mixtures of prevailing sulfates variously associated with many other phases. Sulfates exhibit the highest diversity, with 16 Cu-bearing species observed. The remarkable variety of sulfates in the investigated Vesuvius samples can be attributed to the existence of numerous hydrous forms in relation with anhydrous high-T phases. The stability fields of these phases are significantly influenced by factors such as temperature and humidity, making them highly unstable under standard atmospheric conditions. Indeed, the hydration process of Cu-sulfates is evidenced by the finding of high-T phases and their respective hydrated form in the samples, indicating an incomplete alteration process probably caused by a change of the humidity condition. OH-H2O-bearing Cu-sulfates are abundant, with kaliochalcite being the most widespread, followed by kröhnkite, cyanochroite, leightonite, and antlerite. Chalcanthite is also common but with an H2O content around 12 wt.%, lower than the stoichiometric composition. These H-bearing sulfates are secondary minerals formed through the hydration of high-temperature sublimates or from mixed systems due to the interaction of primary minerals, volcanic gases, and atmospheric water vapors. Anhydrous copper-bearing sulfates are relatively less abundant and are consistently found alongside OH-H2O-bearing varieties. Of these, eleomelanite and fedotovite identified in this study represent the first recorded occurrence for Vesuvius. Pb- and Ba-bearing sulfates, anglesite and baryte, are present in varying amounts across nearly all samples. Anglesite is the main Cu-free sulfate; two samples, featuring high concentration of Ba, reveal a significant baryte solid solution. Other alkali sulfates, both OH/H2O-bearing and H-free, are detected in variable amounts together with the main components. As regards As-bearing phases, although As is found as a trace element in several fumarolic minerals of Vesuvius, in this study still poorly identified (possibly new?) Cu- and Al-arsenates are observed. Additionally, the finding of a copper phosphate, potentially corresponding to antipovite, would be the second Cu-phosphate occurrence at Vesuvius. Considering the whole mineral assemblages, Cu-hydrochlorides occur in all the fumarolic assemblages. The litidionite-bearing associations indicate a formation temperature exceeding 600 °C. The presence of aphthitalite and tenorite in the 1848 sample indicates a formation temperature around 650 °C. Pb-phases and atacamite are formed in the range of 300-400 °C to 650 °C. With decreasing fumarole temperature, minerals like leightonite (formed at <200 °C), kaliochalcite (formed at T < 150 °C), and cyanochroite (as an alteration mineral formed by interaction with water) are deposited. The RMMN fumarolic samples of the museum whose labels show the dates of 1880 and 1881exhibit similar thermal behaviors, with the formation of a high-temperature assemblage in the lower part of the range of 300-400 to 650 °C, followed by the gradual formation of lower-temperature minerals. The fumarolic patinas and encrustations from Vesuvius reveal diverse associations of sulfates, chlorides, hydroxychlorides, oxides, silicates, vanadates, phosphates, arsenates, tungstates, and molybdates. Interestingly, phases containing one or more base metals as specie-defining cations are the most widespread, with Cu-phases being the most abundant ones. Elements such as Zn, Pb, Mn, Ti, Ba, As and P are very common as impurities in different Cu-bearing phases, even in significant amounts, as demonstrated by the characterization of Ti- and Pb-rich litidionites and Mn-rich atacamite and volborthite, in this last cases possibly related to the presence of unresolved Mn-oxides. Cu-minerals botallackite, dioptase, eleomelanite, fedotovite and of Cu-free stolzite and steklite are reported for the first time at Vesuvius, with eleomelanite representing the second world occurrence after the type locality. Tl-, Cd-, As- and N-bearing phases are still under investigations, representing possible new occurrence for Vesuvius. Finally, this study confirms that the knowledge of mineralogical associations of the RMMN samples of Vesuvius is far from being complete, and ongoing studies will contribute to increasingly define the systematics of this volcano.

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