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91.
Matteo Ardit Michele Dondi Marco Merlini Giuseppe Cruciani 《Physics and Chemistry of Minerals》2012,39(3):199-211
The structural variations along the solid solution Sr2−x
Ba
x
MgSi2O7 (0 ≤ x ≤ 2), combined to the high-pressure characterization of the two end-members, have been studied. A topological change from
the tetragonal (melilite-type) to the monoclinic (melilite-related) structure along the join Sr2MgSi2O7 (e.g., P[`4]21 m P\bar{4}2_{1} m )–Ba2MgSi2O7 (e.g., C2/c) occurs with a Ba content higher than 1.6 apfu. Favored in the crystallization from a melt, the tetragonal form has a tetrahedral
sheet topology exclusively based on five-membered rings, which provide a regular “4 up + 4 down” ligand arrangement. In contrast,
the melilite-related structure, favored by solid-state reaction synthesis, is made by alternating six- and four-membered tetrahedral
rings, which give an asymmetric arrangement of alternated “5 up + 3 down” and “3 up + 5 down” ligands around Sr or Ba. This
latter configuration is characterized by an additional degree of freedom with Ba polyhedra hosted in the interlayer with a
more irregular and compact coordination and longer Ba–O bond distances. Further insights into the relationships between the
two melilite typologies were achieved by investigating the in situ high-pressure behavior of these systems. The synchrotron
high-pressure experiments allowed to calculate the elastic moduli for the Sr melilite-type end-member and for the Ba monoclinic
polymorph (Sr2MgSi2O7: K
T0 = 107, K
a=b
= 121, and K
c
= 84 GPa; m-Ba2MgSi2O7: K
T0 = 85, K
a
= 96, K
b
= 72, and K
c
= 117 GPa) and compare them with those reported in the literature for ?kermanite (Ca2MgSi2O7). The results show that, although the volume of Ba polyhedron in tetragonal polymorphs is larger than in the monoclinic forms,
the interlayer compressibility is significantly lower in the former structures due to the occurrence of very short Ba–O distances.
This unfavored Ba environment also makes tetragonal Ba2MgSi2O7 a metastable phase at room conditions, possibly favored by high pressure. However, no phase transition occurs from monoclinic
to tetragonal form due to kinetic hindrance in reconstructing the sheet topology. 相似文献
92.
The evaluation of the feasibility of ex situ carbonation in landfills utilizing raw natural substances (namely serpentinites as Mg-source and the CO2-rich fraction of biogas as C-source) was tested through a laboratory procedure comprising three steps. The first step is the acid attack of a serpentinite at 70 °C, by means of HCl 2 M, to get MgCl2-rich solutions. Attacks of different durations were performed to evaluate the time needed. The second step is the neutralization of the MgCl2-rich solution by addition of concentrated ammonia. The third (carbonation) step is mixing of the neutralized MgCl2-rich solution with a solution of ammonium carbonate. This was produced in a landfill by absorption of CO2 contained in biogas in a solution of ammonia. The neutralization of acid MgCl2-rich solutions caused the precipitation of ferrihydrite with secondary ammonium carnallite and salammoniac, whereas abundant precipitation of Amorphous Hydrated Impure Magnesium Carbonate (AHIMC), sometimes with minor nesquehonite, occurred in the third step. This solid carbonate acts as a stable CO2 sink up to 380 °C. The geochemical behavior of some minor elements was also investigated during the experimental processes revealing that Al, Cr and Ni were removed during neutralization (second step), in contrast to Ca which remained in the circumneutral MgCl2-rich solution and entered into the structure of AHIMC. During the carbonation step, precipitation of artinite, hydromagnesite, lansfordite, magnesite and nesquehonite was thermodynamically impossible as the aqueous phase was undersaturated with respect to these solid phases upon separation of AHIMC. 相似文献
93.
Giorgio Perrotta M. Stipa D. Silvi S. Coltellacci G. Curti G. Colonna T. Formica V. Casali T. Fossati F. Di Matteo M. Zelli M. Rinaldi L. Ansalone A. Di Salvo 《Experimental Astronomy》2011,32(1):63-82
The Mission MAGIA (Missione Altimetrica Geofisica GeochImica lunAre) was proposed in the framework of the ??Bando per Piccole Missioni?? of ASI (Italian Space Agency) in 2007. The mission was selected for a phase A study by ASI on February 7th 2008. The tight budget allocation, combined with quite ambitious scientific objectives, set challenging requirements for the satellite design. The paper gives a fast overview of the payloads complement and of the mission-constrained design drivers, including cost minimization, risk reduction, and AIT flexibility. The spacecraft architecture is then outlined, along with an overview of the key subsystems and trade-offs. Some details are given of a Moon gravitometric experiment based on a mother?Cdaughter satellite configuration with the daughter being a subsatellite released from the MAGIA satellite and intended to circle the Moon at a very low altitude. Budgets are appended at the end of the paper showing the key study results. 相似文献
94.
95.
Matteo Lelli Sergio Grassi Michele Amadori Fabrizio Franceschini 《Environmental Earth Sciences》2014,71(9):3907-3919
A detailed hydrogeochemical study of groundwater in the Cecina coastal plain (Livorno province, Italy) and its inner sectors was undertaken in 2008, as chemical analyses carried out on groundwater since 2006 have revealed Cr(VI) concentrations of up to 49 μg/L (well above the permissible limit of 5 μg/L). Ophiolite outcrops are present throughout the study area, and their fragments likely represent a significant portion of the existing multilayered aquifer skeleton. Waters delivered by the serpentinite outcrops have a typically Mg–HCO3 composition, whereas those of the coastal plain are prevailingly of the Ca/Mg–HCO3 type with significant Mg contents. Significant NO3 contamination characterises the studied coastal plain, and an interesting negative correlation exists between Cr(VI) and both NO3 and SO4 deriving from the widespread use of (NH4)2SO4 as a farm fertilizer. Chromium speciation calculations carried out using the EQ3NR code reveal that the prevailing Cr(VI) species in solution is CrO4 2?; however, CaCrO4° and MgCrO4° neutral complexes represent significant percentages (up to 42 %). These findings suggest that the mobility and consequently the bioavailability of Cr(VI) can be significantly enhanced by these neutral complexes, which are not considered to be affected by adsorption/desorption processes. The Cr(VI) source, investigated by means of the Mg/SiO2 molar ratio, seems to be represented mainly by Mg-bearing minerals of the chlorite group. Petrographic observations confirm the occurrence of this mineral group. The interaction between rainwater and the local serpentinite rock was simulated at different $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ conditions by reaction path modelling using the EQ3/6 software package. $P_{{{\text{O}}_{ 2} }}$ was varied in accordance with the assumption that redox conditions are determined in part by NO3. Results are in good agreement with experimental data on spring waters and subordinately with data on some coastal plain groundwater, which plot in a rather wide $P_{{{\text{CO}}_{ 2} }}$ and $P_{{{\text{O}}_{ 2} }}$ field. Although the dissolved Cr content is mostly of natural origin, fertilization may affect its fate. 相似文献
96.
XMM-Newton is a major X-ray observatory of the European Space Agency (ESA). Its observing time is open to astronomers from the whole scientific community on a peer reviewed competitive basis. The Science Operations Centre, located at ESA’s premises in Villafranca del Castillo, Spain, is responsible for the instrument operations, as well as for all the tasks related to facilitating the scientific exploitation of the data which the mission has been producing since its launch in December 1999. Among them, one may list:
Everyone can be an XMM-Newton observer. So far, astronomers from 36 countries submitted observing programs. Public data can be accessed by every scientist in the world through the XMM-Newton Science Archive (XSA).Despite all these efforts, one can’t help noticing an asymmetric level of scientific exploitation in the realm of X-ray astronomy between developing and developed countries. The latter have traditionally enjoyed the comparative advantage of deeper know-how, deriving from direct experience in hardware and mission development. The XMM-Newton Science Operations Centre’s efforts act to alleviate this situation through, for example, increasing the usage of the web for data and information dissemination, as well as by supporting actively such initiatives as the COSPAR Capacity-Building Workshops, specifically designed to create long-lasting bridges between researchers in developing and developed countries. 相似文献
• | distribution of scientific data in different formats, from raw telemetry, up to processed and calibrated high-level science products, such as images, spectra, source lists, etc; |
• | development and distribution of dedicated science analysis software, as well as of continuously updated instrument calibration; |
• | regular organisation of training workshops (free of cost), for potential users of XMM-Newton data, where the procedures and techniques to successfully reduce and analyze XMM-Newton data are introduced; |
• | access to the data through state-of-the-art, in-house-developed archival facilities, either through the Internet or via CD-ROM; |
• | continuously updated documentation on all aspects of spacecraft and instrument operations, data reduction and analysis; |
• | maintenance of a comprehensive set of project web pages; |
• | a competent and responsive HelpDesk, providing dedicated support to individual XMM-Newton users. |
97.
98.
Matteo Guainazzi 《Monthly notices of the Royal Astronomical Society》2002,329(1):L13-L17
In this paper, a BeppoSAX observation of the bright Seyfert 2 galaxy NGC 6300 is presented. The rapidly variable emission from the active nucleus is seen through a Compton-thin absorber. A Compton-reflection component with an unusually high reflection fraction , and the comparison with a reflection-dominated spectrum measured by RXTE two and half years earlier suggest that NGC 6300 belongs to the class of 'transient' active galactic nucleus, undergoing long and repeated periods of low activity. The spectral transition provides support to the idea that Compton-thick and Compton-thin X-ray absorbers in Seyfert 2 galaxies are decoupled, the former being most likely associated with the 'torus', whereas the latter is probably located at much larger distances. 相似文献
99.
Elisa Cenci Matteo PizzolonNicole Chimento Carlotta Mazzoldi 《Estuarine, Coastal and Shelf Science》2011
Deployed for diverse purposes, artificial substrata can, theoretically, act as fish attractors or as fish producers. We tested these hypotheses by comparing fish assemblages on two artificial hard substrata, the Ca’ Roman and Sottomarina jetties, before (1994 and 2003) and after (2005 and 2006) the deployment of a new breakwater in the area. Our results support the fish production hypothesis. A remarkable increase in richness and abundance of sedentary species was observed on the extant substrata with the presence of the new barrier. More mobile species showed less definite trends. The net result has been a total increase in abundance and richness in the area. While fish assemblage showed important temporal variation in composition before deployment (Ca’ Roman, comparison between 1994 and 2003), clear trends of increasing abundance were concurrent with the presence of the new barrier. This increase in abundance, particularly of sedentary species, may be attributed to enhanced connectivity and/or an improvement in larval retention due to the changes in water circulation connected with the new breakwater, or to an increase in juveniles and/or adults promoted by changes in environmental conditions. Indeed, changes in benthic cover were recorded, with an increase of uncovered rock and the appearance of previously unrecorded algae after breakwater deployment. 相似文献
100.
James S. Bolton Martin G. Haehnelt Matteo Viel Robert F. Carswell 《Monthly notices of the Royal Astronomical Society》2006,366(4):1378-1390
The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H i and He ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H i and He ii ionization rates. We further show that the large spatial variations and the anticorrelation with H i absorber density observed in the ratio of the measured He ii to H i column densities can be explained in a model where the H i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He ii ionization rate. A significant contribution to UV emission at the He ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He ii by QSOs. 相似文献