Chromium solubility in anhydrous Phase B     

Luca Bindi  Ekaterina A. Sirotkina  Andrey V. Bobrov  Fabrizio Nestola  Tetsuo Irifune 《Physics and Chemistry of Minerals》2016,43(2):103-110

The crystal structure and chemical composition of a crystal of (Mg_14?x Cr _x )(Si_5?x Cr _x )O₂₄ (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr₂O₄–Mg₂SiO₄ at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å³ and Z = 2. The structure was refined to R ₁ = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe²⁺-bearing Anh-B. Taking into account that Cr³⁺ is not expected to be Jahn–Teller active, it appears that both the Cr³⁺–for–Mg and Cr³⁺–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg_13.66(8)Si_4.70(6)Cr_0.62(4)O₂₄ stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.  相似文献   

Photoluminescence properties of S<Subscript>2</Subscript> molecule trapped in Melanophlogite     

Fabrizio Messina  Michela Todaro  Gianpiero Buscarino  Lavinia Vaccaro  Marco Cannas  Franco M. Gelardi 《Physics and Chemistry of Minerals》2016,43(3):171-179

We studied the photoluminescence properties of a sample of SiO₂-clathrate Melanophlogite, a crystalline microporous material which is found in nature as a rare mineral. Upon β irradiation, the material displays an intense light emission under near-UV illumination. We studied in detail this optical activity by steady-state and time-resolved photoluminescence measurements as a function of temperature. The spectroscopic properties we find can be ascribed to a population of quasi-free molecules trapped within each of the two different types of cage available in the structure of this clathrate, although the spectroscopic properties of the guest molecules are affected by their interactions with the host matrix. Based on the available data, we attribute the observed photoluminescence to trapped S₂ molecules, emitting from their excited ³Σ _u ^? or ³Π _u electronic states, depending on the cage they are trapped in and on temperature. Our results have an impact on the fundamental understanding of host–guest interactions characteristic of microporous systems such as clathrates. Indeed, the data highlight that even a relatively weak coupling between quasi-free S₂ molecules and the two types of cages provided by the Melanophlogite host has a surprisingly complex influence on the optical properties of the guest.  相似文献   

Very hot,very shallow hydrothermal dolomitization: An example from the Maritime Alps (north‐west Italy–south‐east France)          下载免费PDF全文

Luca Barale  Carlo Bertok  Namam Salih Talabani  Anna d'Atri  Luca Martire  Fabrizio Piana  Alain Préat 《Sedimentology》2016,63(7):2037-2065

In the Maritime Alps (north‐west Italy – south‐east France), the Middle Triassic–lowermost Cretaceous platform carbonates of the Provençal Domain locally show an intense dolomitization. Dolomitized bodies, irregularly shaped and variable in size from some metres to hundreds of metres, are associated with tabular bodies of dolomite‐cemented breccias, cutting the bedding at a high angle, and networks of dolomite veins. Field and petrographic observations indicate that dolomitization was a polyphase process, in which episodes of hydrofracturing and host‐rock dissolution, related to episodic expulsion of overpressured fluids through faults and fracture systems, were associated with phases of host‐rock dolomitization and void cementation. Fluid inclusion analysis indicates that dolomitizing fluids were relatively hot (170 to 260°C). The case study represents an outstanding example of a fossil hydrothermal system, which significantly contributes to the knowledge of such dolomitization systems in continental margin settings. The unusually favourable stratigraphic framework allows precise constraint of the timing of dolomitization (earliest Cretaceous) and, consequently, direct evaluation of the burial setting of dolomitization which, for the upper part of the dolomitized succession, was very shallow or even close to the surface. The described large‐scale hydrothermal system was probably related to deep‐rooted faults, and provides indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin.  相似文献   

Diel microbial variations at a coastal Northern Adriatic station affected by Po River outflows     

Mauro Celussi  Alessandro Paoli  Fabrizio Bernardi Aubry  Mauro Bastianini  Paola Del Negro 《Estuarine, Coastal and Shelf Science》2008

Diel sampling was performed during an early spring survey in the Northern Adriatic Sea at a coastal station off the Po River delta. Samples were taken every 6 h at spring tide maxima and minima in the sub-superficial layer, at the maximum fluorescence depth (∼3 m). Variations in microbial community structure and its processes were assessed by considering heterotrophic bacteria, picocyanobacteria, viruses, exoenzymatic activities, microphytoplankton, nanoplankton and bacterial/cyanobacterial Denaturing Gradient Gel Electrophoresis (DGGE) profiles. A considerable diatom bloom, mostly supported by Skeletonema marinoi was detected. All microbial parameters except viruses, showed a sinusoidal trend with a 12 h period; only picocyanobacteria expressed relative maxima during high tide, showing a phase in opposition to the other parameters. No substantial changes in DGGE band patterns were detected. Even though the results showed bacterial activities to be influenced by the phytoplankton bloom, all microbial parameters' diel trends (except viruses) preferentially followed the tidal fluctuation rather than the light:dark cycle.  相似文献   

Upper Pleistocene tectonics in western Sardinia (Italy): Insights from the Sinis peninsula structural high     

Fabrizio Cocco  Stefano Andreucci  Daniele Sechi  Giulia Cossu  Antonio Funedda 《地学学报》2019,31(5):485-493

The Corsica‐Sardinia block is a lithospheric fragment whose recent role in the geodynamics of the central‐Western Mediterranean basin is still enigmatic. The most recent regional structure in Sardinia is the Plio‐Pleistocene Campidano Basin, which is considered in a ‘post‐rift’ stage since the Middle Pleistocene. New structural and stratigraphic geological surveys along with luminescence ages provide evidence to support an ongoing tectonic activity since the Marine Isotopic Stage 7 (MIS7; ca. 220 ka) on the Sinis peninsula, the structural high that bounds the north‐western side of the Campidano Basin. In particular, this paper reveals for the first time the presence of N–S striking normal faults system offsetting late Pleistocene aeolianites (130 ± 12 ka; 82 ± 9 ka).  相似文献   

Experimental models of extension of continental lithosphere weakened by percolation of asthenospheric melts     

Giacomo Corti  Marco Bonini  Fabrizio Innocenti  Piero Manetti  Giovanni B. Piccardo  Giorgio Ranalli 《Journal of Geodynamics》2007,43(4-5):465-483

Analogue models are used to investigate extension of a continental lithosphere weakened by asthenospheric melts percolating through the upper mantle, a process that has been hypothesised to control the opening of the Ligurian Tethys. Models were performed in a centrifuge apparatus and reproduced, by using materials such as sand and viscous mixtures, extension of 60-km thick, three-layer continental lithosphere floating above the asthenosphere. The percolated lithospheric mantle was assumed to be characterised by a rheological behaviour similar to that of the asthenosphere. Two sets of experiments investigated the influence on deformation of (1) the thickness of the percolated mantle and the associated strength contrast between the normal and weakened lithosphere, and (2) the lateral width of the weakened zone. Model results suggest that mantle percolation by asthenospheric melts is able to promote strong localised thinning of the continental lithosphere, provided that a significant thickness of the lithospheric mantle is weakened by migrating melts within a narrow region. Strain localisation is maximised for percolation of the whole lithospheric mantle and strong strength contrast between the normal and weakened lithosphere. Under these conditions, the thickness of the lithosphere may be reduced to less than 12 km in 3 Ma of extension. Conversely, localised thinning is strongly reduced if the thickness of the percolated zone is ≤1/3 of the thickness of the whole lithospheric mantle and/or the lithosphere is weakened over wide regions. Overall, model results support the working hypothesis that mantle percolation by asthenospheric melts is a controlling factor in the transition from distributed continental deformation to localised oceanic spreading.  相似文献   

Late Pleistocene to Holocene record of changing uplift rates in southern Calabria and northeastern Sicily (southern Italy, Central Mediterranean Sea)   总被引：3，自引：1，他引：2  

Fabrizio Antonioli  Luigi Ferranti  Kurt Lambeck  Steve Kershaw  Vladimiro Verrubbi  Giuseppe Dai Pra 《Tectonophysics》2006,422(1-4):23-40

A combination of published and new radiometric dates on uplifted Holocene fossil beaches from northeastern Sicily and southern Calabria (southern Italy) is compared with the altitude of the inner margin of the Last Interglacial (LIg) (Late Pleistocene, 124 ka) and older marine terraces in order to gain a regional-scale outline of uplift rates and their temporal changes in a region which is one of the fastest uplifting sectors of the Central Mediterranean Sea. Late Holocene radiocarbon dates from Ioppolo (southern Calabria) and Ganzirri (northeast Sicily), two newly discovered sites are here presented for the first time. The Holocene uplift rates are highest at St. Alessio and Taormina in eastern Sicily (2.4 mm/y) and at Scilla in southwestern Calabria (2.1 mm/y), two sites located across the Messina Straits and which separate the island of Sicily from mainland Italy. Uplift rates decrease towards the south and north from this centre of uplift. Late Holocene uplift rates show an apparent increase of between 64 and 124% when compared with the longer-term uplift rates calculated from the LIg highstand terraces. Furthermore, we discovered that the locations of fastest Late Pleistocene and Late Holocene uplift rates spatially coincide. To what extent the Holocene increase in uplift rates results from incomplete elastic strain release along the major extensional faults which frame the seismotectonic of the area, or indicate a true change in regional tectonic processes, is not resolved. Nonetheless, the heterogeneity of uplift, with a well-defined centre that crosses the Messina Straits, and its persistence at different time-scales indicates a tight connection between wider regional processes and fault-related displacement in controlling crustal instability in this area.  相似文献   

(Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxenes at high pressure: influence of cation substitution on elastic behavior and phase transition     

Angela Ullrich  Ronald Miletich  Tonci Balic-Zunic  Lars Olsen  Fabrizio Nestola  Manfred Wildner  Haruo Ohashi 《Physics and Chemistry of Minerals》2010,37(1):25-43

A compressional study of (Na,Ca)(Ti³⁺,Mg)Si₂O₆-clinopyroxenes was carried out at high pressures between 10⁻⁴ and 10.2 GPa using in situ single-crystal X-ray diffraction, Raman spectroscopy and optical absorption spectroscopy. Compressional discontinuities accompanied by structural changes, in particular, the appearance of two distinct Ti³⁺–Ti³⁺ distances within the octahedral chains at 4.37 GPa, provide evidence for the occurrence of a phase transition in NaTi³⁺Si₂O₆. Equation-of-state parameters are K ₀ = 115.9(7) GPa with K′ = −0.9(3) and K ₀ = 102.7(8) GPa with K′ = 4.08(5) for the low- and high-pressure range, respectively. The transition involves a C2/c–P [`1] \overline{1} symmetry change, which can be confirmed by the occurrence of new modes in Raman spectra. Since no significant discontinuity in the evolution of the unit-cell volume with pressure has been observed, the transition appears to be second-order in character. The influence of the coupled substitution Na⁺Ti³⁺↔Ca²⁺Mg²⁺ on the static compression behavior and the structural stability has been investigated using a sample of the intermediate composition (Na_0.54Ca_0.46)(Mg_0.46Ti_0.54)Si₂O₆. No evidence for a deviation from continuous compression behavior has been found, neither in lattice parameter nor in structural data and the fit of a third-order Birch–Murnaghan equation-of-state to the pressure–volume data yields a bulk modulus of K ₀ = 109.1(5) GPa and K′ = 5.02(13). Raman and polarized absorption spectra have been compared to NaTiSi₂O₆ and reveal major similarities. The main driving force for the phase transition in NaTi³⁺Si₂O₆ is the localization of the Ti³⁺ d-electron and the accompanying distortion, which is suppressed in the (Na,Ca)(Ti³⁺,Mg)Si₂O₆-clinopyroxene.  相似文献   

Influence of surface/groundwater interaction on pollution by pesticides in farmlands of the Fucino Plain,central Italy     

Eva Pacioni  Marco Petitta  Ezio Burri  Maria Fanelli  Piero Mazzeo  Fabrizio Ruggieri 《Environmental Earth Sciences》2010,61(1):201-216

This paper analyses flow and transport of pesticides from the unsaturated zone to groundwater so as to predict concentration of those contaminants in the Fucino Plain’s groundwater, by site investigations and numerical simulations. Pesticides were detected in surface water (peaks of 13 μg L⁻¹) and groundwater (peaks of 0.37 μg L⁻¹). Modelling tools made it possible to identify that pattern of precipitation, organic matter content, and root thickness are the key factors involved in vertical seepage of pesticides. Numerical simulations indicated that a significant fraction of contaminants is leached from the most surficial soil layers through runoff, while only a secondary fraction is mobilised towards groundwater. Likelihood of contaminating deep groundwater is fairly low, whereas surface waters show higher susceptibility. Results of the proposed conceptual hydrogeological model show that pesticides are more likely to be entrained by mixing of stream water with shallow groundwater in periods of high water exploitation from shallow wells.  相似文献   

Determination of the Physical Parameters of Binary Systems: A Statistical Approach     

Fabrizio Barone  Santi Mancuso  Leopoldo Milano  Guido Russo 《Astrophysics and Space Science》1998,260(4):469-491

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