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The present work is part of a systematic mineralogical and petrographic characterization of mica-bearing xenoliths from Somma-Vesuvius volcano (Roman Magmatic Province, southern Italy). Skarns, composite skarns-marbles and cumulates from Pompeii Plinian eruption (AD 79), and skarns and syenite from Avellino eruption (3945 ± 10 cal yr BP) were investigated to define the crystal chemistry of the Somma-Vesuvius trioctahedral micas and to draw inferences on petrogenetic processes to which they were subjected. Xenoliths were characterized by means of polarized optical microscopy, scanning electron microscopy (SEM-EDS), X-ray powder diffraction (XRPD), and bulk-rock geochemical analyses. Mica crystals were studied using electron microprobe analysis (EMPA) and single-crystal X-ray diffraction (SCXRD). Micas from skarns are variably associated with Mg ± Ca silicates (clinopyroxene, vesuvianite, humite, clinohumite, chondrodite, forsterite, and garnet), other sporadic silicates (anorthite, sodalite, titanite, and britholite), apatite, calcite, various types of oxides, as well as rare sulfides and halides. In composite skarn-marble rocks, the mineral assemblages show some differences compared to skarns, as a lack of clinopyroxene and the presence of dolomite. Cumulate samples consist of mica and clinopyroxene, whereas syenite is mainly composed of mica, K-feldspar, feldspathoids, and clinopyroxene. Together with mica, apatite occurs in all the lithotypes. Trace element arrays are scattered for skarn and composite skarn-marble samples. The REE patterns have a general enrichment in light (La, Ce, Pr, Nd) and medium (Sm, Eu, Gd, Tb, Dy) rare earth elements, in some cases with slight positive Gd anomaly. Cumulate samples generally have low amounts of Ba, Sr, Zr, and Th, while syenite exhibits low concentrations of trace elements, except for Rb, Cs, and Tl. Mica crystals occurring in the studied xenoliths are phlogopite with different Al and Mg contents at the octahedral site, a negligible tetraferriphlogopite component and variable dehydrogenation degree. All samples belong to the 1M polytype (C2/m and C2 space group) and have a wide range of unit-cell parameters, especially of the c axis [5.3055(1) ≤ a ≤ 5.3218(1) Å, 9.1893(1) ≤ b ≤ 9.2188(4) Å, 10.1803(2) ≤ c ≤ 10.2951(2) Å]. The shortest c-cell parameter pertains to de-hydrogenated phlogopite from Avellino skarn, whereas OH-rich phlogopite from Pompeii composite skarns-marbles has a c-cell parameter that approximates that of the end-member phlogopite. Overall, it is observed that the crystal chemistry of the micas studied here extends the known range of the other Vesuvian micas from the literature. The Ti-depletion and the wide degree of dehydrogenation of phlogopites from skarns and composite skarns-marbles suggest that the studied samples originated under variable pressure conditions. In addition, the presence of humite in the mineral assemblage seems to indicate the occurrence of devolatilization reactions. The scarce mica occurrence in cumulate and mainly in syenite, instead, may depend on pressure conditions in the magma storage system exceeding the mica stability. [ABSTRACT FROM AUTHOR]

The evaluation of caprock integrity and reservoir efficiency is critical for safe CO2 geological storage management. It is therefore important to investigate geochemical reactions between CO2-rich fluids and host rocks and their contribution in retaining CO2 at depth. This study deals with diffusive reaction experiments on shales and carbonate samples cored from an offshore structure in the Malaysian basin, a potential target for CO2-enhanced gas recovery. The aim is to evaluate the CO2 reaction front velocity in a typical shaly caprock and the mineral response of the reservoir. Rock samples were characterized in terms of texture, chemistry, and mineralogy by X-ray diffraction, electron microscopy (SEM), microanalysis (EDS), infrared spectroscopy (FT-IR), rock geochemistry (XRF), and mercury injection capillary permeability (MICP). Performed analyses show mineralogical alteration induced by CO2 as it penetrated into the samples. Carbonate dissolution and weathering of pyrite to form secondary carbonates belonging to siderite-ankerite series were observed along two reaction fronts. Estimated diffusion coefficients of CO2 are two orders of magnitude lower than CO2(aq) molecular diffusion in pure water and from half to an order of magnitude lower than diffusivity computed on unaltered sample, highlighting the important effect of gas–water–rock reactions on the CO2(aq) diffusivities in shales and carbonates. Results obtained in this study provide an insight regarding the effect of geochemical reactions on CO2 transport and represent a further discussion point on the diffusion coefficients. [ABSTRACT FROM AUTHOR]

The Tolfa Volcanic District (Latium, Central Italy) shows variable and locally very intense hydrothermal alteration with sulfide mineralizations and alum and kaoline deposits intensely explored in the post-medieval period. The District is part of a volcanic complex of Pliocene-Pleistocene age that emplaced on a basement made of limestone, flysh and marine-to-continental sediments. The hydrothermal alteration products has received attention in the oldest literature (Field and Lombardi, 1972; Lombardi and Sheppard, 1977; Cortecci et al., 1982), providing some petrological and isotopic studies of sulfates and clays, with scattered and incomplete rock geochemistry composition as well. However, most recent studies (e.g., Cinti et al., 2011) essentially focused on the geochemistry of thermal springs, aquifers and pools.Here, the Tolfa hydrothermal alteration area has a renewed attention. We have conducted a field campaign in the area between Tolfa, Allumiere and Santa Severa. The field allows detecting various alunite, kaoline and mineralized limestone quarries, and one boiling pool as well. We also observed various degree of hydrothermal alteration and the argillified zones. The sedimentary basement with limestones, sediments and schist-like deposits described in the literature in the Fiorotta area. A total of 21 samples have been collected based on field observations and analyzed for mineralogical and petrological purposes. We use Optical and Electron Microscopy, X-Ray Dffraction, Infrared Spectroscopy, and Rock Geochemistry to characterize the alteration materials. The collected data are useful (i) to understand the quality and importance of the raw materials largely used in the past economy, and (ii) to compare hydrothermal alteration from extinct and quiescent volcanic districts, such as Campi Flegrei and Ischia. In such perspective, the collected samples are object of on-going study finalized to implement the data set. Therefore, the samples will be used in larger and long-term researches that we are developing. Cited literatureCalderoni G, Ferrini V, Masi U (1985). Distribution and significance of Pb and Tl in the sulfides and host rocks from the hydrothermal mineralization of the Tolfa Mountains (Latium, Central Italy). Chemical Geology, 51, 29-39Calderoni G, Masi U, Ferrini V (1986). Abundance and significance of Eu, Mn and Zn in the sulfides and host rocks from the hydrothermal mineralization of the Tolfa Mountains. Rend. Soc. Ital. Min. Petrol. Vol. 4l (2), 359-368.Cortecci G, Lombardi G, Reyes E, Turi B (1981). A sulfur isotopic study of alunites from Latium and Tuscany, Central Italy. Mineralium Deposita, 16(1), 147-156.Field C, Lombardi G (1972). Sulfur isotopic evidence for the supergene origin of alunite deposits, Tolfa district, Italy. Mineralium Deposita 7.2, 113-125.Locardi E, Mittempergher M (1971). Exhalative supergenic uranium, thorium and marcasite occurrences in Quaternary volcanites of Central Italy. Bulletin of Volcanology 35(1), 173-184.Lombardi G, Sheppard SMF (1977). Petrographic and isotopic studies of the altered acid volcanics of the Tolfa—Cerite area, Italy: the genesis of the clays. Clay Mineral, 12, 147-161. [ABSTRACT FROM AUTHOR]