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1.
In a metamorphosed gabbro from Hokkaido, Japan, augite containing exsolved orthohypersthene and minor pigeonite has been altered to a variety of biopyriboles. High resolution transmission electron microscopy of slightly altered augite shows only narrow (010) lamellae of clinoamphibole which always contain even numbers of double chains. In more highly altered regions, all three pyroxenes are changed to double-, triple- and more highly polymerized multiple-chain biopyriboles, with chlorite d
001 14.5 Å) found only in orthopyroxene. Several (010) lamellae containing only one double chain have been observed, and their textural relationship to the surrounding single-chain host may explain how the rotated domains of biopyribole initially attain their orientation in the host pyroxene.A structural model is proposed for the polymerization of single chains in the tetrahedral layers of pyroxene which involves small movements of oxygens and tetrahedral atoms (Si, Al) with a minimum of bond breaking and re-forming, concommitant with hydroxylization of certain oxygens and the diffusion of Mg, Fe and/or Ca along the rift in the octahedral layer. 相似文献
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Multiple linear regression analysis has been applied to the geometric and chemical variables in sodic plagioclases in order to determine their relative effects on individual T-O bond lengths in the Al1+xSi3?xO8 tetrahedral framework. Using data from crystal structure analyses of low and high albite, An16 and An28, and assuming that low albite is completely ordered, 1 $$\begin{gathered} {\text{T}} - {\text{O = 1}}{\text{.568}} + {\text{[(0}}{\text{.122) x (Al content of the T site)]}} \hfill \\ {\text{ }} - {\text{[(0}}{\text{.037) x (}}\Delta {\text{{\rm A}l}}_{{\text{br}}} )] + [0.063){\text{ x }}(\Sigma {\text{[}}q{\text{/(Na,Ca}} - {\text{O)}}^{\text{2}} ])] \hfill \\ {\text{ }} + {\text{[(0}}{\text{.029) x (}} - {\text{1/cosT}} - {\text{O}} - {\text{T)]}} \hfill \\ \end{gathered}$$ where the Al content of a particular tetrahedral (T) site can be estimated from empirically-derived determinative curves, where Δ Albr is a linkage factor to account for the Al content of adjacent tetrahedral sites, where the formal charge on the (Na1?xCax) atom is q=1+x, and where T-O-T is the inter-tetrahedral angle involving the T-O bond. For sodic plagioclases it is essential to know only the anorthite content and the 2Θ131-2Θ1¯31 spacing (CuK α radiation) in order to determine the independent variables in this equation and thus to evaluate the individual T-O distances. The 64 individual T-O distances predicted for the four sodic plagioclases by this equation agree well with the observed T-O bond lengths (σ=0.004 Å; r=0.994), and the method has been used by way of example to rationalize the T-O bond lengths in analcime (cf. Ferraris, Jones and Yerkess, 1972). 相似文献
3.
Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average
depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx.
2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the
bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time,
using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed
with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m),
the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements,
but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu)
in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent
mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows
that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation
and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts
at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay
has increased, and the inverse circulation has also strengthened. 相似文献
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5.
ABSTRACTDroughts can have serious negative impacts on the water quality needed for irrigated agriculture. The Metropolitan region of Chile is a relevant producer of high-value crops and is prone to droughts. Standardized Drought Indices were used to characterize meteorological and hydrological droughts for the period from 1985 to 2015. To understand the relationship between droughts and water quality, we evaluated the correlations between daily discharge and surface water quality observations. The threshold level method was used to compare physicochemical parameters during hydrological drought periods with the Chilean water quality thresholds for agricultural uses. A significant (p < 0.05) negative relationship between discharge and electrical conductivity and major ions was found in most of the basin. Hydrological stations located in irrigation districts exceeded the official thresholds for these parameters during hydrological drought periods seriously threatening irrigated agriculture of the region. 相似文献
6.
Energy gaps and electrical conductivities in the ferrous silicates, Fe2SiO4 and FeSiO3, depend primarily on Fe-O bonding and may be studied by ultraviolet and soft X-ray spectroscopy. We have measured FeLII–III X-ray band spectra under conditions of “minimal” (I4, at 4.0 keV) and “high” (I10, at 10.0 keV) self absorption to determine 3d orbital energy levels, to delineate d states in the valence band, and to construct band gap models. Absorption spectra, I4/I10, were computed to determine vacant orbital levels in the gap. A difference function (I4–I10) has been proposed to identify X-radiation at photon energies above the measured LIII absorption edge, including high-energy, double-vacancy satellites and radiative transitions involving the anti-parallel (spin-down) d 6 electron in the ground state. The proposed band gap model for Fe2SiO4 is consistent with that of Nitsan and Shankland (1976), including an intrinsic transition of 6.5 eV and an energy gap of 7.8 eV. The 3d orbital energy level electronic structures are in general agreement with levels computed by Tossell et al. (1974) for [FeO6]10? in FeO using an SCF Xα cluster MO method. A high-energy, double-vacancy satellite was found at ～710.7 eV, and is presumed to originate from an LIIIMII,III initial state. The intensity of these satellites for the ferrous silicates and other iron compounds, and corresponding Fe LII/LIII intensity ratios are correlated with differences in band gap magnitudes and gap structure. Fe LII/LIII intensity ratios are not well correlated with iron oxidation state. 相似文献
7.
Islam Md Bayzidul Firoz A. B. M. Foglia Laura Marandi Andres Khan Abidur Rahman Schüth Christoph Ribbe Lars 《Hydrogeology Journal》2017,25(3):617-637
The water resources that supply most of the megacities in the world are under increased pressure because of land transformation, population growth, rapid urbanization, and climate-change impacts. Dhaka, in Bangladesh, is one of the largest of 22 growing megacities in the world, and it depends on mainly groundwater for all kinds of water needs. The regional groundwater-flow model MODFLOW-2005 was used to simulate the interaction between aquifers and rivers in steady-state and transient conditions during the period 1981–2013, to assess the impact of development and climate change on the regional groundwater resources. Detailed hydro-stratigraphic units are described according to 150 lithology logs, and a three-dimensional model of the upper 400 m of the Greater Dhaka area was constructed. The results explain how the total abstraction (2.9 million m3/d) in the Dhaka megacity, which has caused regional cones of depression, is balanced by recharge and induced river leakage. The simulated outcome shows the general trend of groundwater flow in the sedimentary Holocene aquifers under a variety of hydrogeological conditions, which will assist in the future development of a rational and sustainable management approach.
相似文献8.
The diffraction, n.q.r. and optical data on plagioclase feldspars are used to derive kinetic interpretations of structural changes induced by laboratory heat treatment and by geological processes. For anorthite, the Si, Al configuration is essentially ordered except for unusual transient processes. Cooperation between Ca atoms, and random nucleation, produces a domain texture in the primitive structure which is highly sensitive to temperature. The rapid inversion from the primitive to the body-centered structure is explained by increasingly rapid “rattling” of the Ca ions in the interstices of the semi-flexible alumino-silicate framework. The weakening of “b” reflections at higher temperatures is ascribed to incipient Si, Al disorder associated with irregular vibration of the alumino-silicate framework and the Ca atoms. Quenching phenomena are explained by variation of the domain boundary texture inherited from disorder at high temperature. For albite, the Si, Al configuration changes sluggishly from an ordered to a disordered pattern, and vice versa. Kinetic data are reinterpreted using a model in which the cell dimensions depend on local rather than distant order: the major change in distant order is deduced to occur at 450–600° C. Sodic plagioclase grown at high temperature shows distant disorder of the atoms, but cell dimensions suggest development of strong local order for calcic compositions. Low-entropy plagioclases of intermediate composition show complex intergrowths and domain structures because of kinetic barriers to atomic diffusion. X-ray diffraction data for slowly-cooled specimens are consistent with nucleation of albite- and anorthite-like regions from a high-temperature disordered phase. Electrostatic energy calculations show that Na and Ca atoms, although they face smaller energy barriers for diffusion, cannot form domains until the Si and Al atoms have moved jointly. The Si, Al ordering patterns of low albite and anorthite are topologically incompatible in a continuous framework if oxygen is not to be bonded to two Al. Therefore domains of low-albite and anorthite must be separated by disordered boundaries. For intermediate compositions, An15-An75, domains remain small. The anorthite-like domains probably form at higher temperatures than the albite-like domains. The latter tend to be about the same size for all bulk compositions. The atomic positions are influenced by neighboring atoms. Upon heating rapidly, Si and Al atoms remain in position and provide a memory for reformation of an identical structure upon cooling. The framework changes shape, and some Na, Ca atoms inter-diffuse to yield a quasi-homogeneous structure with a diffraction pattern which qualitatively approaches that of high albite. Upon prolonged heating at high temperature, Si, Al atoms inter-diffuse producing nonquenchable changes to the high-albite structure. At Na-rich bulk compositions, some domains of low albite grow into large lamellae while others remain small in contact with anorthite domains producing alternate lamellae of intermediate structure type; hence the peristerite intergrowth. A similar but opposite process could cause an intergrowth of lamellae of anorthite structure interposed with an intermediate type structure. A unique low plagioclase series is not expected. Plagioclases of intermediate composition trend towards slightly different endproducts depending on the details of the cooling history. Breaks and bends in plots of physical properties, and intergrowths for certain specimens, depend on special compositional, growth and annealing factors. The intergrowth responsible for iridescence of intermediate plagioclase is ascribed to Na, K segregation prior to development of the complex domain structure. Prolonged annealing at high temperature in a dry environment is suggested. It is futile to attempt to describe low entropy plagioclases in terms of classical thermodynamics: only a kinetic interpretation based on atomic and sub-microscopic textural factors can be viable. 相似文献
9.
Conceptual modelling to assess the influence of hydro-climatic variability on runoff processes in data scarce semi-arid Andean catchments 总被引：2，自引：2，他引：0
Using hydro-meteorological time series of 50 years and in situ measurements, the dominant runoff processes in perennial Andean headwater catchments in Chile were determined using the hydrological model HBV light. First, cluster analysis was used to identify dry, wet and intermediate years. From these, sub-periods were identified with contrasting seasonal climatic influences on streamflow. By calibrating the model across different periods, impacts on model performance, parameter sensitivity and identifiability were investigated, providing insights into differences in hydrological processes. The modelling approach suggested that, independently of a dry or wet period of calibration, the streamflow response is mostly consistent with flux from groundwater storage, while only a small fraction comes from direct routing of snowmelt. The variation of model parameters, such as the groundwater rate coefficient, was found to be consistent with differing recharge in wet and dry years. The resulting snowmelt–groundwater model is a realistic hypothesis of the hydrological operation of such complex, data scarce and semi-arid Andean catchments. This model may also be a useful tool for predictions of seasonal water availability and a basis for further field studies. 相似文献
10.
The climate along the subtropical east coast of Australia is changing significantly. Rainfall has decreased by about 50 mm per decade and temperature increased by about 0.1 °C per decade during the last 50 years. These changes are likely to impact upon episodes of hypersalinity and the persistence of inverse circulations, which are often characteristic features of the coastal zone in the subtropics and are controlled by the balance between evaporation, precipitation, and freshwater discharge. In this study, observations and results from a general ocean circulation model are used to investigate how current climate trends have impacted upon the physical characteristics of the Hervey Bay, Australia. During the last two decades, mean precipitation in Hervey Bay deviates by 13% from the climatology (1941–2000). In the same time, the river discharge is reduced by 23%. In direct consequence, the frequency of hypersaline and inverse conditions has increased. Moreover, the salinity flux out of the bay has increased and the evaporation induced residual circulation has accelerated. Contrary to the drying trend, the occurrence of severe rainfalls, associated with floods, leads to short-term fluctuations in the salinity. These freshwater discharge events are used to estimate a typical response time for the bay. 相似文献