G. Padalino

The bentonites in pelotherapy : thermal properties of clay pastes from Sardinia (Italy)

Abstract On the basis of a previous detailed characterization of the Sardinian bentonites for pelotherapy applications [Cara, S., Carcangiu, G., Padalino, G., Palomba, M., Tamanini, M., 2000. The bentonites in pelotherapy: chemical, mineralogical and technological properties of materials from Sardinia deposits (Italy). Appl. Clay Sci. 16, 117–124 (this issue)], three commercial bentonites have been selected for detailed studies. The thermal properties of the pastes, prepared with these bentonites at different moisture, have been compared with the properties of a peloid, commonly used in the spa of Benetutti (northern Sardinia). The cooling kinetics of the bentonites along with the thermal mud have been studied using an experimental apparatus that reproduces the conditions during the application of a cataplasm. A mathematical model for the cooling kinetics has been obtained by means of a theoretical cooling equation, with instrumental constants derived from a reference paste (TiO 2 at 50% moisture). The heat capacity of the pastes introduced into the model was calculated from chemical and mineralogical data. The methodology developed can be used in other laboratories of the thermal centres to determine the quality and ability of the bentonitic materials for pelotherapy.

The bentonites in pelotherapy: chemical, mineralogical and technological properties of materials from Sardinia deposits (Italy)

Abstract Bentonite muds are suited for use in pelotherapy due to their typical properties of high swelling (>25 ml/2 g), specific heat (Cp) and ease of handling, generally obtained at a material viscosity of 400 cps. This paper reports chemical, mineralogical and technological properties of Sardinia bentonites, to identify the most suitable materials for pelotherapy pastes. Their characteristics have been compared to the peloid properties commonly used in the spa of Benetutti (Northern Sardinia). On the basis of the mineralogical, chemical and technological characterization of the samples, besides their proximity to the most important regional spas and/or thermal sources, Costa Paradiso, Pedroseddu, Laconi, Busachi, Siurgus-Donigala and Giba bentonites are the most suitable Sardinia materials for pelotherapy. The results of this study also show that a complete characterization of clay materials is necessary in pelotherapy paste preparation. The utilization and/or addition of bentonites, largely available in Sardinia at relatively low cost, can improve the thermal mud quality for curative purposes.

Mineralogical and Stable Isotope Studies of Kaolin Deposits: Shallow Epithermal Systems of Western Sardinia, Italy

Large kaolin deposits hosted by Miocene silicic pyroclastic rocks in northwestern Sardinia represent hydrothermal alteration formed within 200 m of the Miocene paleosurface. Boiling hydrothermal fluids ascended steeply dipping faults that are enveloped by altered rock. The broadly stratiform kaolin deposits constitute advanced argillic alteration that was produced in a steam-heated zone near the paleosurface overlying the deeper hydrothermal systems. The deeper zones represent two distinct types of epithermal systems: weakly acidic (inferred low-sulfidation) systems at Tresnuraghes and acidic (high-sulfidation) systems at Romana. Tresnuraghes is characterized at depth by chalcedony ± quartz ± barite veins within a 50-m-wide zone of K-feldspar-quartz-illite alteration and overlying local occurrences of chalcedony sinter, which define the paleosurface. Kaolin deposits near the paleosurface are characterized by zonation outward and downward from an inner shallow zone of kaolinite 1T-opal ± dickite ± alunite (<20- μ m-diam grains) to an outer deeper kaolinite 1M-montmorillonite-cristobalite. This zonation indicates formation by descending acidic fluids. The system evolved from ascending weakly acidic or neutral fluids that boiled to produce H2S-rich vapor, which condensed and oxidized within the near-surface vadose zone to form steam-heated acid-sulfate waters and kaolin alteration. At Romana, veins at depth contain chalcedony or quartz and minor pyrite and are enclosed in up to 20-m-wide zones of kaolinite 1T-quartz alteration. Near hydrothermal vents along the paleosurface, chalcedonic silica is enclosed within a zone of kaolinite 1T-alunite (<50- μ m-diam grains)-quartz-opal± dickite ± cristobalite. Kaolin quarries near the paleosurface display outward and downward zoning to kaolinite 1T-opal ± cristobalite and then to montmorillonite-kaolinite 1T ± opal, consistent with formation by descending low pH fluid. The siliceous and advanced argillic alteration along steep conduits formed from acidic ascending magmatic-hydrothermal fluids, whereas the near-surface kaolin formed from steam-heated meteoric waters. Alteration mineral assemblages and stable isotope data provide evidence of the temperature and source of hydrothermal fluids. Barite from Tresnuraghes (average δ 18O = 17.1‰, δ 34S = 18.8‰), one alunite sample from Romana ( δ 18O = 12.0‰, δ D = –3‰, δ 34S = 16.7‰), and quartz from both localities ( δ 18O = 15.9–22.0‰) formed in hydrothermal feeders. Source fluids were likely mixtures of meteoric water and minor magmatic fluid, similar to other epithermal systems. Kaolinite-dickite minerals from the kaolin deposits ( δ 18O = 16.6–21.4‰, δ D = –43 to –53‰) formed from steam-heated meteoric water having δ D = – 20 per mil, consistent with the presence of anomalous Hg and fine-grained Na- and Fe-poor alunite. The laterally extensive kaolin deposits in Sardinia, and possibly similar deposits elsewhere in the world, appear to represent the uppermost parts of large hydrothermal systems that may be prospects for gold at depth.

Preliminary geochemical exploration in semiarid climate: The case of a porphyry-type occurrence in Sardinia (Italy)

Abstract In the southwestern part of Sardinia some subvolcanic andesitic bodies occur, related to rift structures. Weak alteration phenomena and the presence of pyrite disseminations led the authors to investigate the area through petrographical studies and geochemical prospecting of rocks and soils. The geochemical survey showed that, given the local morphological and climatic conditions, rock and soil sampling is well correlated; furthermore, correlations relating Cu with Ba and Sr proved effective in defining the anomalous areas. The main anomaly area found by these studies could represent a porphyry copper body.