Maria Rosaria Ghiara

Geochemistry of thermal waters on the island of Ischia (Campania, Italy)

Abstract The stratigraphic and structural situation on the island of Ischia (southern Italy), the recent volcanic activity and the presence of hot springs and fumaroles, suggest the existence of a geothermal field. The chemical and isotopic compositions of the waters from several springs and wells were examined to obtain information on deep temperatures and to formulate a geothermal model of the island. δD values range from −33.60 to −12.50‰ and δ18O from −7.10 to −1.71‰ , relative to SMOW. These variations have mainly been attributed to the presence of seawater, as confirmed by the general shift to more positive values with the increase of Cl content. Water-rock reactions, evaporation and subsurface boiling also contribute to the δ18O−δD trend. The chemical analyses reveal the presence of alkaline sulphate chloride water (seawater), bicarbonate waters and waters interpreted as the result of mixing. The chemical and isotopic composition of the latter are dependent on water-rock interactions, water circulation rates and eventual evaporation and condensation phenomena. The silica geothermometer, which seems to be the most suitable for determining the deep temperatures of these waters, gave values of about 200°C, even for mixing models. Our data suggest the following geothermal model: the heat flow heats up a deep reservoir, causing steam to rise through faults and fractures and transfer heat to a shallower aquifer. The temperatures of 200°C obtained by the geothermometers are not the maximum reservoir temperatures, but are probably water-rock equilibrium temperatures for the shallower aquifers. The high boron contents and the isotopic data confirm the presence of steam in the system.

Multi-methodological investigation of kunzite, hiddenite, alexandrite, elbaite and topaz, based on laser-induced breakdown spectroscopy and conventional analytical techniques for supporting mineralogical characterization

Gem-quality alexandrite, hiddenite and kunzite, elbaite and topaz minerals were characterized through a multi-methodological investigation based on EMPA-WDS, LA-ICP-MS, and laser-induced breakdown spectroscopy (LIBS). With respect to the others, the latter technique enables a simultaneous multi-elemental composition without any sample preparation and the detection of light elements, such as Li, Be and B. The criteria for the choice of minerals were: (a) the presence of chromophore elements in minor contents and/or as traces; (b) the presence of light lithophile elements (Li, Be and B); (c) different crystal chemistry complexity. The results show that LIBS can be employed in mineralogical studies for the identification and characterization of minerals, and as a fast screening method to determine the chemical composition, including the chromophore and light lithophile elements.

Occurrence of clinoptilolite and mordenite in Tertiary calc-alkaline pyroclastites from Sardinia (Italy)

Clinoptilolite and mordenite occur as diagenetic products of medium-grained, moderately welded and poorly sorted pyroclastic flows belonging to the Tertiary calc-alkaline volcanism of Sardinia. Both clinoptilolite and mordenite occur within pyroclastic flows of the same stratigraphic unit. Mordenite frequently occurs in the late volcanic sequences from Anglona area (northern Sardinia). Textural features indicate that zeolites are products of glass alteration. Thin sections show either complete alteration of glassy shards by clinoptilolite and mordenite or unaltered shards with clinoptilolite or mordenite confined to the cineritic matrix. During the zeolitization process, interacting fluids were important in the mobilization and distribution of alkali elements. The compositional variations of clinoptilolite and mordenite within a single sample showed trends that suggest steps in a continuous process probably evolving towards equilibrium conditions.

Chemical alteration of volcanic glasses and related control by secondary minerals: Experimental studies

A detailed experimental mineralogical and geochemical study on hydrothermal alteration processes of volcanic glasses with a different chemical composition and leucites sampled in the Roman Comagmatic Region was carried out. 2g samples of different grain sizes and 50 ml of deionized water or seawater were sealed in bronze Teflon-lined autoclaves and placed in a rotating sample-holder at 200°C. The internal pressure was 16.2 bars. At arbitrary intervals, the pressure vessel was quenched to room temperature and both solids and solutions were separated by filtration. The solids were used to identify possible alteration products by means of X-ray powder diffraction, SEM and microprobe analyses. In all deionized water systems the contact solution reached pH basic values, but in the various systems the consumption of H+ ions occurred in different ways. This is probably linked to different concentrations of released cations. In seawater systems, however, pH values initially fell sharply but subsequently increased slightly. After 2 days, cation concentrations were clearly buffered by reaction products in all solutions. These were mainly zeolites and clay minerals. The following crystallization sequences in all glass/deionized water experiments were observed: the first reaction product was represented by phillipsite, followed by analcime and illite in the alkali-trachytic run; chabasite and analcime in the phonolitic-tephritic run; analcime and finally feldspar in shoshonitic and trachytic runs. Phillipsite and smectite crystallized together only in the basaltic run. In leucite/deionized water systems, however, the first reaction phase was illite, followed by analcime and then phillipsite. The reaction products in glass/seawater systems were smectite and anydrite. The chemical composition of synthetic zeolites was clearly controlled by the chemistry of initial glasses. These synthetic zeolites are like the natural ones in volcaniclastic products from central and southern Italy. This suggests that extensive zeolitization processes of these volcaniclastic rocks may occur through interaction of volcanic glasses with fluids at a very low salinity and a temperature close to 200°C.

Crystal-chemical investigation of Fe3(PO4)2· 8 H2O vivianite minerals

Abstract Crystal-chemistry of Fe3(PO4)2 · 8 H2O vivianite minerals was investigated by means of a multi-methodological approach based on electron microprobe analysis in wavelenght dispersive mode, single-crystal X-ray diffraction and infrared spectroscopy. To this aim, three specimens coming from Commentry (France) (1), Settefrati (Frosinone, Italy) (2) and Ibex Mine (Colorado, USA) (3), have been subject to EMPA analysis, indicating chemical composition (Fe2.94Mg0.06Mn0.01)Σ3.02(P1.99O8) · 8 H2O (1), (Fe2.96Mg0.01Mn0.04)Σ3.01((P1.96Si0.03Al0.02)Σ2.01O8) · 8 H2O (2) and (Fe2.65Zn0.27Mn0.06Na0.03Mg0.02)Σ3.03(P1.99O8) · 8 H2O (3); successively they have been subject to structural refinement by means of single-crystal X-ray diffraction within monoclinic space group C2/m, with the following unit cell constants: a = 10.100(3), b = 13.494(3), c = 4.709(3) Å; β = 104.24(4)° and V = 622.1(7) Å3 (1); a = 10.116(3); b = 13.473(3), c = 4.7064(7) Å; β = 104.33(4)° and V = 621.5(2) Å3 (2); a = 10.083(2), b = 13.468(2), c = 4.718(1) Å, β = 104.47(2)° and V = 620.3(2) Å3 (3). Iron cations occupy two special positions, making up single and dimerical octahedral groups, joined by (PO4) tetrahedra groups, in order to form infinite layers down b, making up, via consistent O—H … O hydrogen bonds, the three-dimensional framework. The present vivianite samples were subject to geochemical and crystal chemical comparisons, the latter also by means of bond valence analysis with literature available phases. Last, IR spectra were performed for the three phases, in order to provide information on bending and stretching modes of the minerals, as well as to investigate H bonds by means of Libowitzki formula application.

Crystal-chemical and structural characterization of fluorapatites in ejecta from Somma-Vesuvius volcanic complex

Abstract The mineralogy and crystal chemistry of apatites occurring in 14 ejecta of historical eruptions (1631 and 1872 A.D.) of the Somma-Vesuvius volcanic complex were investigated by a multi-methodological approach including polarized optical microscopy, scanning electron microscope, electron microprobe analysis in wavelength-dispersive mode, laser ablation inductively coupled plasma mass spectroscopy, and single-crystal X‑ray diffraction. Five different groups of apatite, with different mineralogical and crystal-chemical features, were identified. Apatite crystals occur with well-developed hexagonal prismatic habit and, more rarely, with skeletal acicular forms. The crystals are yellow (type 1), transparent and colorless (type 2 and type 3), green (type 4) and aquamarine colored (type 5), with different paragenesis: macro-crystalline aggregates of clinopyroxenes, phlogopite, and apatite (type 1; type 4); clusters of apatite with micro-crystalline clinopyroxene, minerals of the cancrinite group, feldspars, and opaque minerals (type 2); aggregates of apatite, sellaite, wagnerite, gypsum, and phlogopite (type 3); aggregates of davyne, nepheline, mica group minerals, and apatite (type 5). Chemical analyses of apatites show variable amounts of Na, REE, Mg, Sr, and Fe replacing Ca, not negligible amounts of Si and S substituting P, and a significant substitution of Cl and OH instead of F. Five crystals representative of each apatite-type were studied by single-crystal X‑ray diffraction. Their crystal structure was refined in the hexagonal P63/m space group. A significant variation of the unit-cell parameters with the composition was observed. A comparative crystal-chemical analysis between apatites from Somma-Vesuvius and those from other localities is carried out.

Crystal-chemical investigation of Al 2(PO 4)(OH) 3augelite from Rapid Creek, Yukon, Canada

Abstract The structural characterization of Al 2(PO 4)(OH) 3augelite, a natural orthophosphate coming from Rapid Creek, Yukon, Canada, was carried out by means of a multi-methodological approach based on WDS-EMPA chemical analysis, single crystal X-ray diffraction, and infrared spectroscopy. The structure was solved in the C2 /mmonoclinic space group, with the following unit cell constants: a= 13.0740(10), b= 7.9690(7), c= 5.0910(7) Å, β= 112.282(5)°, V= 490.81(9) Å 3. Phosphorus atoms display tetrahedral (PO 4) coordination, while aluminum cations display octahedral and unusual five-fold coordination. The framework of the mineral can be viewed as the repetition of cationic arrays made up by AlO 6/AlO 5polyhedra, bridged by PO 4groups, and further joined by O–H···O hydrogen bonds: such interactions were also correlated to IR spectroscopy analysis, by means of Libowitzky formula applied on OH stretching vibrations.

Crystal-chemical study of wavellite from Zbirov, Czech Republic

Abstract The crystal chemistry of wavellite from Zbirov (Czech Republic), ideally Al3(PO4)2(OH,F)3·5H2O, was addressed by means of a multi-methodological approach based on electron microprobe analysis (EMPA) using wave-dispersive spectroscopy, single-crystal X-ray diffraction, powder and singlecrystal infrared spectroscopy and Raman spectroscopy. The EMPA data showed the presence of significant F replacing OH in the sample studied. The structure was solved in the Pcmn orthorhombic space group, with the following unit-cell constants: a = 9.6422(7), b = 17.4146(15), c = 7.0094(2) Å, V = 1176.98(10) Å3. Phosphorus atoms display tetrahedral (PO4) coordination, while Al cations display octahedral coordination. The mineral framework can be viewed as the repetition of cationic arrays made up of AlO6 polyhedra, bridged by PO4 groups and further joined by O-H…O hydrogen bonds. The single-crystal unpolarized Fourier transform infrared (FTIR) spectrum shows combination bands indicating the presence of both OH and H2O in the structure. Both FTIR and Raman spectra show a broad absorption extending from 3600 to 2800 cm-1 resulting from the overlapping of several components due to the water molecules and the OH group. The frequencies observed are comparable to those expected on the basis of the Libowitzky relationship for the range of D-H …A bond systems in the structure.

Fibrous minerals from Somma-Vesuvius volcanic complex

A survey on fibrous minerals coming from the densely populated area of Campania around the Somma-Vesuvius volcanic complex (Italy) was performed by means of a multi-methodological approach, based on morphological analyses, EMPA/WDS and SEM/EDS applications, and unit-cell determination through X-ray diffraction data. Such mineralogical investigation aims to provide suitable tools to the identification of fibrous natural phases, to improve the knowledge of both geochemical, petrogenetic and regional mineralogy of Somma-Vesuvius area, and to emphasize the presence of minerals with fibrous habit in all volcanic environments. The survey also fits well in the calls of health and environment of Horizon 2020 program of the European Commission (Climate Action, Environment, Resource Efficiency and Raw Materials).