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551.
Two parameters GO2– and HO2– are defined as the differences between respectively the Gibbs free energies and the enthalpies of formation of an oxide and its corresponding aqueous cation. The Gibbs free energies and enthalpies of formation of phosphates from their consituent oxides are shown to be linear functions of respectively GO2– and HO2– of their constituent cations. 相似文献
552.
A parameter GO2– is defined as the difference between the Gibbs free energy of formation from the elements of a given oxide and its corresponding aqueous cation. The Gibbs free energies of formation of compounds such as sulfates, nitrates or carbonates, from their constituent oxides, are shown to be linear functions of GO2– values of their constituent cations. 相似文献
553.
According to Koopmans' theorem, only the electrons associated with molecular levels of negative energy are stable. Many ions which cannot exist in the isolated state because certain occupied levels are positive, become stable in crystals under the stabilization effect arising from the crystal field. As examples, we have studied CO 3 2? in calcite, NO 3 ? in NaNO3, and several natural limpurities: O 3 ? in fluorite, [(OH)4]4?, [(OH)3F]4? and [(OH)2F2]4? in zircon and thorite, by means of the molecular self-consistent field, the crystal field being simulated by point charges. As expected, all the energies corresponding to the occupied levels are negative within the crystal field, contrary to what occurs in the isolated state. Informations concerning the structure and the size of the critical germ are obtained for CaCO3 and NaNO3. 相似文献
554.
Surficial sediments from 76 lakes from two western Quebec regions(Abitibi and Haute Mauricie) were sampled to identify the relationships betweendiatoms and environmental variables. Because the two regions containedradically different diatom communities, we then investigated which factors maybe responsible for the large community discrepancies in the two nearbygeographical areas. Standard lake chemistry variables showed little differencesbetween the regions, although epilimnetic light regimes were slightly lower inAbitibi. Nevertheless, lakes of the two regions with similar light regimes andchemistry still showed a clear separation in their diatoms, implying that otherimportant factors are influencing assemblages. We found that the calculatedconcentration of CO2 in the open water can explain some of thediscrepancy in diatom assemblages. A pCCA constrained to the concentration ofCO2 with alkalinity and pH as covariables explained 12.5% of speciesvariance and was significant. Given the lack of a relationship between DOC andCO2, and because the lakes are heavily supersaturated withCO2 in the calibration set, lake-to-lake variations inCO2 concentrations are likely due to groundwater inputs; thepossibility that this environmental variable may be influencing diatomcommunities might allow, in some cases, the reconstruction of historicalchanges in groundwater inputs to lakes. Finally, new calibration models werebuilt in Quebec by using weighted averaging partial least square(WA-PLS) techniques in order to infer pH, CO2, TP, TN, and, DOC fromdiatom assemblages preserved in the surface sediments. 相似文献
555.
Philippe Léone Charlotte Doussier-Brochard Gilles André Yves Moëlo 《Physics and Chemistry of Minerals》2008,35(4):201-206
Mn2+Sb2S4, a monoclinic dimorph of clerite, and benavidesite (Mn2+Pb4Sb6S14) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented
and compared with those of iron derivatives, berthierite (Fe2+Sb2S4) and jamesonite (Fe2+Pb4Sb6S14). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb2S4 shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature.
In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb2S4 interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]:
jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams
and persist down to 1.4 K. 相似文献
556.
557.
Boumédiène Derras Pierre Yves Bard Fabrice Cotton 《Bulletin of Earthquake Engineering》2014,12(1):495-516
We have used the Artificial Neural Network method (ANN) for the derivation of physically sound, easy-to-handle, predictive ground-motion models from a subset of the Reference database for Seismic ground-motion prediction in Europe (RESORCE). Only shallow earthquakes (depth smaller than 25 km) and recordings corresponding to stations with measured $V_{s30}$ properties have been selected. Five input parameters were selected: the moment magnitude $M_{W}$ , the Joyner–Boore distance $R_{JB}$ , the focal mechanism, the hypocentral depth, and the site proxy $V_{S30}$ . A feed-forward ANN type is used, with one 5-neuron hidden layer, and an output layer grouping all the considered ground motion parameters, i.e., peak ground acceleration (PGA), peak ground velocity (PGV) and 5 %-damped pseudo-spectral acceleration (PSA) at 62 periods from 0.01 to 4 s. A procedure similar to the random-effects approach was developed to provide between and within event standard deviations. The total standard deviation ( $\sigma $ ) varies between 0.298 and 0.378 (log $_{10}$ unit) depending on the period, with between-event and within-event variabilities in the range 0.149–0.190 and 0.258–0.327, respectively. Those values prove comparable to those of conventional GMPEs. Despite the absence of any a priori assumption on the functional dependence, our results exhibit a number of physically sound features: magnitude scaling of the distance dependency, near-fault saturation distance increasing with magnitude, amplification on soft soils and even indications for nonlinear effects in softer soils. 相似文献
558.
Many broadcast-spawning benthic invertebrates are subject to sperm limitation yet achieve high population densities, as for example dreissenid mussels (Dreissena polymorpha and Dreisssena bugensis) that were introduced into the Laurentian Great Lakes. The question remains whether biological or ecological/physical mechanisms reduce sperm limitation. Gamete dilution/longevity experiments were undertaken to determine whether dreissenid mussels are subject to sperm limitation, and computational fluid dynamic modeling was used to determine the potential influence of bottom roughness on sperm dilution in nature. Results indicated that dreissenid mussels may be sperm limited, but the extent to which sperm dilution affects them is lower than what was reported for other broadcast spawning invertebrates. Importantly, model mussel clusters influenced external fertilization by retaining sperm in downstream eddies but allowing downstream transport from one cluster to another. This, in addition to high sperm potency at low sperm concentrations, may help to explain the success of dreissenid mussels as invasive species. 相似文献
559.
Decadal evolution of a degassing magma reservoir unravelled from fire fountains produced at Etna volcano (Italy) between 1989 and 2001 总被引:1,自引:1,他引:0
Between 1989 and 2001, five eruptions at Etna displayed a regular alternation between repose periods and episodes rich in
gas, termed quasi-fire fountains and consisting of a series of Strombolian explosions sometimes leading to a fire fountain.
This behaviour results from the coalescence of a foam layer trapped at the top of the reservoir which was periodically rebuilt
prior to each episode (Vergniolle and Jaupart, J Geophys Res 95:2793–2809, 1990). Visual observations of fire fountains are combined with the foam dynamics to estimate the five degassing parameters characteristic
of the degassing reservoir, i.e. the number of bubbles, gas volume fraction, bubble diameter, reservoir thickness and reservoir
volume. The study of decadal cycles of eruptive patterns (Allard et al., Earth Sci Rev 78:85–114, 2006) suggests that the first eruption with fire fountains occurred in 1995 while the last one happened in 2001. The number of
bubbles and the gas volume fraction increase smoothly from the beginning of the cycle (1995) to its end (2001). The increasing
number of bubbles per cubic metre, from 0.61–20×105 to 0.1–3.4×109, results from cooling of the magma within the reservoir. The simultaneously decreasing bubble diameter, from 0.67–0.43 to
0.30–0.19 mm, is related to the decreasing amount of dissolved volatiles. Meanwhile, the thickness and the volume of the degassing
reservoir diminish, from values typical of the magma reservoir to values characteristic of a very thin bubbly layer, marking
the quasi-exhaustion of volatiles. The magma reservoir has a slender vertical shape, with a maximum thickness of 3,300–8,200 m
and a radius of 240 m (Vergniolle 2008), making its detection from seismic studies difficult. Its volume, at most 0.58–1.4 km3, is in agreement with geochemical studies (0.5 km3) (Le Cloarec and Pennisi, J Volcanol Geotherm Res 108:141–155, 2001). The time evolution of both the total gas volume expelled per eruption, and the inter-eruptive gas flux results from the
competition between the increasing number of bubbles and the decreasing bubble diameter. The smooth temporal evolution of
the five degassing parameters also points towards bubbles being produced by a self-induced mechanism within the magma reservoir
rather than by a magmatic reinjection prior to each eruption. The decadal cycles are therefore initiated by a magmatic reinjection,
in agreement with a typical return time of 14–80 years (Albarède 1993). Hence, the 1995 eruption results from a fresh magma being newly emplaced while the magma from the following eruptions is
progressively depleted in volatiles species until reaching a state of quasi-exhaustion in 2001. A magmatic reinjection of
0.13–0.6 km3 every few decades is sufficient to explain the expelled gas volume, including SO2. A scenario is also proposed for the alternation between gas-rich summit eruptions and gas-poor flank eruptions which are
observed during decadal cycles. The scenario proposed for Etna could also be at work at Piton de la Fournaise and Erta ’Ale
volcanoes. 相似文献
560.
Jessica Saïag Pierre‐Yves Collin Jean‐Pierre Sizun Frdric Herbst Ophlie Faÿ‐Gomord Carmela Chateau Smith Bruno Caline ric Lasseur 《Sedimentology》2019,66(7):2976-3007
Microtexture describes the type of particles and their arrangement in matrix samples at scanning electron microscopy scale. Although a microtexture classification exists for micritic limestone, it cannot be directly applied to chalk. This study therefore proposes a classification of chalk microtextures and discusses the origin of microtexture variability. Chalk was sampled at thirteen spatio‐temporal locations along the coastline of northern France (Cenomanian–Santonian). Four criteria are defined to describe, characterize and determine chalk matrix microtexture: (i) mineralogical content; (ii) biogenic fraction; (iii) micritic fraction; and (iv) cement fraction. From these criteria, two major groups are defined: Pure Chalk Microtexture Group, with seven classes, and Impure Chalk Microtexture Group, divided into two subgroups: Argillaceous Microtexture with four classes and Siliceous Microtexture with two classes. Microtexture variability is related both to initial sedimentation and to diagenesis. Sedimentological conditions (for example, climate and distance from shore) affect chalk composition (carbonate content and type of insoluble particles), thus influencing microtexture. Changes in Pure Chalk Microtexture are the result of increasing diagenetic intensity. This classification can also be used to characterize the microtexture of subsurface chalk reservoirs. Reservoir quality depends on the petrophysical and mechanical properties of reservoir rocks, which can be better understood by exploring their sedimentary and diagenetic history, revealed by the study of chalk microtexture variability. 相似文献