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1.
Hydrochemical investigations in the Kalambaina Formation have been initiated to determine potability and suitability of the shallow groundwater for domestic and agricultural uses. This limestone formation is an extensive aquifer supplying water to livestock and domestic wells in its outcrop areas. The aquifer is recharged by rainfall and discharges mainly into the Sokoto–Rima River system and lakes at Kware, Gwadabawa and Kalmalo in Nigeria. Because recharge to the aquifer is mainly from rainfall, the quality of the groundwater is controlled essentially by chemical processes in the vadose zone and locally by human activities. Water samples were taken at 11 sites comprising boreholes, dug wells and a spring and were chemically analysed for their major ion components. Hydrochemical results show water of fairly good quality. It is, however, hard and generally of moderate dissolved solids content. Concentration of the total dissolved solids is between 130 and 2,340 mg/l. Concentrations of ions vary widely but a high concentration of K+ is found in places. NO3− is on the higher side of the World Health Organization (WHO) permissible limits, indicating pollution in such areas. Groundwater chemistry is predominantly of two facies, namely the calcium–magnesium–bicarbonate and calcium–magnesium–sulphate–chloride facies. These facies probably evolved primarily as a result of dissolution of calcium and magnesium carbonates as well as some human/land-use activities.  相似文献   

2.
The source of fluoride toxicity in Muteh area,Isfahan, Iran   总被引：1，自引：0，他引：1  
Endemic dental fluorosis has been observed in most inhabitants of three villages of Muteh area, located in northwest of Isfahan province, with mottled enamel related to high levels of fluoride in drinking water (1.8–2.2 ppm). Forty-seven groundwater samples from six villages were collected and fluoride concentrations along with physico-chemical parameters were analyzed. Fluoride concentration in this area varies from 0.2 to 9.2 mg/l with highest fluoride level at Muteh gold mine (Chahkhatun mine). Fluoride concentration positively correlates with pH and HCO3 − indicating that alkaline pH provides a suitable condition for leaching of fluoride from surrounding rocks. The district is mainly covered by three lithological units, namely, metamorphic and granite rocks, alluvial sediments, and carbonate rocks. Factor analysis shows that parameters can be classified into four components: electrical conductivity (EC), total dissolved solids (TDS), Cl−, Na+ and K+, pH and F−, SO4 2−and Mg2+, HCO3 − and Ca2 +. The groundwaters from the three geological units were compared using Mann–Whitney U test. The order of median fluoride concentration is: metamorphic and granite rocks > alluvial sediments > carbonate rocks. Hence, the fluoride content is most probably related to fluoride-bearing minerals such as amphibole and mica group minerals in metamorphic and granitic rocks. The concentration of fluoride in drinking water wells located near the metamorphic complex in Muteh area is above 2 ppm.  相似文献   

3.
The aim of this study was to determine geochemical properties of groundwater and thermal water in the Misli Basin and to assess thermal water intrusion into shallow groundwater due to over-extraction. According to isotope and hydrochemical analyses results, sampled waters can be divided into three groups: cold, thermal, and mixed waters. Only a few waters reach water–rock chemical equilibrium. Thermal waters in the area are characterized by Na+–Cl−–HCO3−, while the cold waters by CaHCO3 facies. On the basis of isotope results, thermal waters in the Misli basin are meteoric origin. In particular, δ18O and δ2H values of shallow groundwater vary from −10.2 to −12.2‰ and −71.2 to −82‰, while those of thermal waters range from −7.8 to −10.1‰ and from −67 to −74‰, respectively. The tritium values of shallow groundwater having short circulation as young waters coming from wells that range from 30 to 70 m in depth vary from 10 to 14 TU. The average tritium activity of groundwater in depths more than 100 m is 1.59 ± 1.16, which indicates long circulation. The rapid infiltration of the precipitation, the recycling of the evaporated irrigation water, the influence of thermal fluids and the heterogeneity of the aquifer make it difficult to determine groundwater quality changes in the Misli Basin. Obtained results show that further lowering of the groundwater table by over-consumption will cause further intrusion of thermal water which resulted in high mineral content into the fresh groundwater aquifer. Because of this phenomenon, the concentrations of some chemical components which impairs water quality in terms of irrigation purposes in shallow groundwaters, such as Na+, B, and Cl−, are highy probably expected to increase in time.  相似文献   

4.
The hydrogeochemistry of 26 wells belonging to ten different aquifers in the county of Ensenada, Baja California, is studied. These wells are all used to supply the rural communities in the region, which comprise ~37,000 inhabitants, excluding the city of Ensenada. High total dissolved solids (TDS) concentrations (maximum 7.35 g l−1) indicate that salt is a ubiquitous contaminant in the aquifers due to seawater intrusion. The aquifers that support extensive agriculture activities (Maneadero, San Quintín, San Simón and El Rosario) are characterized by higher N–NO3 concentrations (maximum 20 mg l−1) derived from fertilizers. Fluoride concentrations exceed the 1.5 mg l−1 Mexican official limit in only four wells. The enrichments of F− in the southern aquifers are thought to be associated to water–rock interactions controlled mainly by Na–Ca equilibrium reactions with fluorite, as suggested from high dissolved Na concentrations in these waters. In the northern aquifer of Maneadero, no enrichment of Na is found and a geothermal source for F− is likely. Water is hard to moderately hard, with Ca/Mg ratios >1. Although drinking water directly from the tap is not a common practice in these localities, most sources have concentrations of major ions and TDS that exceed the Mexican official limits.  相似文献   

5.
The complex geochemical interactions in the groundwater of the industrial area of Šalek Valley (Slovenia) between natural and anthropogenic fluids were studied by means of major (Ca, Mg, Na, K, HCO3 −, Cl− and SO4 2−) and trace elements’ (As , Cd, Cu, Pb, Zn, Hg, Se and V) abundances, geochemical classification and statistical analysis of data. Cation abundances indicate mixing between a dolomitic end-member and an evaporitic or geothermal end-member. Anion abundances indicate mixing between bicarbonate waters and either sulphate-enriched waters (suggesting hydrothermalism) or chlorine-rich waters. Principal component analysis (PCA) allowed the extraction of seven factors, which describe, respectively: water–rock interaction mainly on dolomitic rocks; redox conditions of water; Cd–Zn enrichment in chlorine-rich waters (probably from industrial wastes); hydrothermal conditions in waters close to major faults; Pb and Cu pollution; V and K enrichments, indicating their common organic source; the role of partial pressure of CO2 dissolved in water, which is highest in three wells with bubbling gases. Average underground discharge rates of solutes from the Valley range between 0.09 t/a (V) and 1.8 × 104 t/a (HCO3 −) and indicate how natural fluids can significantly contribute to the levels of elements in the environment, in addition to the amount of elements released by human activities.  相似文献   

6.
Nitrate (NO3 −) is major pollutant in groundwater worldwide. Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources due to the rapid movement of water in their conduit networks. In this study, the isotopic compositions (δ15N–NO3 −, δ15N–NH4 +) and chemical compositions(e.g., NO3 −, NH4 +, NO2 −, K+) were measured in groundwater in the Zunyi area of Southwest China during summer and winter to identify the primary sources of contamination and characterize the processes affecting nitrate in the groundwater. It was found that nitrate was the dominant species of nitrogen in most of the water samples. In addition, the δ15N–NO3 − values of water samples collected in summer were lower than those collected in winter, suggesting that the groundwater received a significant contribution of NO3 − from agricultural fertilizer during the summer. Furthermore, the spatial variation in the concentration of nitrate and the δ15N–NO3 − value indicated that some of the urban groundwater was contaminated with pollution from point sources. In addition, the distribution of δ15N–NO3 − values and the relationship between ions in the groundwater indicated that synthetic and organic fertilizers (cattle manure) were the two primary sources of nitrate in the study area, except in a few cases where the water had been contaminated by urban anthropogenic inputs. Finally, the temporal and spatial variation of the water chemistry and isotopic data indicated that denitrification has no significant effect on the nitrogen isotopic values in Zunyi groundwater.  相似文献   

7.
We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)4(Be6Si12O36)(H2O)2(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm−1; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm−1, plus minor features at 4,000 and 3,228 cm−1 are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm−1 band is assigned to the ν3 stretching mode and the 1,620 cm−1 (associated with an overtone 2*ν2 at 3,230 cm−1) band to the ν2 bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm−1 is not associated with a corresponding ν2 bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm−1 feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.  相似文献   

8.
Groundwater is a critical resource in Deoria district, as it is the main source of drinking water and irrigation. The aquifer has deteriorated to a high degree, during the last two to three decades, in quality and quantity due to high population growth and environmental pollution. More than 90% of the population get their drinking water from subsurface waters. Fifteen wells were sampled in June 2006 to probe the hydrogeochemical components that influence the water quality. The results show that groundwater have EC, TDS, Na+, Mg2+, HCO3− and TH higher than the WHO, 1997 maximum desirable limits. A hydrogeochemical numerical model for carbonate minerals was constructed using the PHREEQC package. The regression analysis shows that there are three groups of elements which are significantly and positively correlated. The main hydrochemical facies of the aquifer (Ca + Mg–HCO3) represents 33.33% of the total wells. The geochemical modeling demonstrated that the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of salts, (2) precipitation of dolomite, (3) ion exchange. Solubility of dolomite, calcite, aragonite and gypsum were assessed in terms of the saturation index. The thermodynamic prerequisites for dolomite supersaturation reactions are satisfied by subsurface waters, since they are supersaturated with respect to dolomite, undersaturated (or in equilibrium) with respect to calcite, and undersaturated with respect to gypsum. The Ca2+ versus SO42− and Mg2+ versus SO42− trends are also compatible with homologous trends resulting from dolomite supersaturation.  相似文献   

9.
We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (εNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with εNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.  相似文献   

10.
The forest ecosystem in the Maolan karst forest, southwest China is the only concentrated, intact, and relatively stable karst forest ecosystem which has survived in the area at the same latitude in the world, and is a valuable karst forest plant resource as well. Groundwater samples from Maolan karst forest were collected from wells and springs during summer; and concentrations of major ions and dissolved inorganic carbon (DIC) isotopic compositions were measured. The pH values range from 7.2 to 8.3 results from the dissolution of carbonate, HCO3 − is the dominant species of DIC in groundwater. Calcium and HCO3 −, followed by Mg2+ and SO4 2− dominate the chemical composition of major ions in the groundwaters. Groundwater samples have δ13C values in the range from −8.1‰ to −16.6‰, which are lower than that of the other karst city groundwaters in the southwest China. Combining δ13CDIC ratios with measurements of HCO3 − and pH clearly distinguishes the principal processes underlying the geochemical evolution of groundwater in Carboniferous carbonate aquifers, where processes can be both degradation of organic matters in the soil and the carbonate dissolution.  相似文献   

11.
Hydrogeochemical investigations are carried out in and around Perumal Lake, Cuddalore district, South India in order to assess its suitability in relation to domestic and agricultural uses. The water samples (surface water = 16; groundwater = 12) were analyzed for various physicochemical attributes like pH, electrical conductivity (EC), sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), bicarbonate (HCO3 −), sulfate (SO4 2−), phosphate (PO4), silica (H4SiO4) and total dissolved solids (TDS). Major hydrochemical facies were identified using Piper trilinear diagram. Hydrogeochemical processes controlling the water chemistry are water–rock interaction rather than evaporation and precipitation. Interpretation of isotopic signatures reveals that groundwater samples recharged by meteoric water with few water–rock interactions. A comparison of water quality in relation to drinking water quality standard proves that the surface water samples are suitable for drinking purpose, whereas groundwater in some areas exceeds the permissible limit. Various determinants such as sodium absorption ratio (SAR), percent sodium (Na%), residual sodium carbonate (RSC) and permeability index (PI) revealed that most of the samples are suitable for irrigation.  相似文献   

12.
The Guadalupe Valley aquifer is the only water source for one of the most important wine industries in Mexico, and also the main public water supply for the nearby city of Ensenada. This groundwater is monitored for major ion, N-NO3, P-PO4, Fe, As, Se, Mo, Cd, Cu, Pb, Zn and Sb concentrations, as well as TDS, pH, dissolved oxygen and temperature. High concentrations of N-NO3 (26 mg l−1), Se (70 μg l−1), Mo (18 μg l−1) and Cu (4.3 μg l−1) suggest that groundwater is being polluted by the use of fertilizers only in the western section of the aquifer, known as El Porvenir graben. Unlike the sites located near the main recharge area to the East of the aquifer, the water in El Porvenir graben has low tritium concentrations (<1.9 TU), indicating a pre-modern age, and thus longer water residence time. No significant variations in water quality (generally <10%) were detected throughout 2001–2002 in the aquifer, suggesting that reduced rainfall and recharge during this dry period did not significantly affect water quality. However, the wells nearest to the main recharge area in the Eastern aquifer show a slight but constant increase in TDS with time, probably as a result of the high (∼200 L S−1) uninterrupted extraction of water at this specific recharge site. Relatively high As concentrations for the aquifer (10.5 μg l−1) are only found near the northern limit of the basin associated with a geological fault.  相似文献   

13.
In order to assess the extent of groundwater contamination by nitrate (NO3 −–N) and to provide information about the deterioration of the groundwater quality in Zhangye Oasis, Northwest China, a study was conducted in this area. The mean value of NO3 −–N concentrations in groundwater samples was 10.66 ± 0.19 mg l−1. NO3 −–N concentrations exceeding 10 mg l−1 (the threshold for drinking water set by the World Health Organization) were found in 32.4% of 71 wells, and were 13, 33.3, 52.4 and 50.0% in the groundwater samples from drinking wells, irrigation wells, hand-pumping wells and groundwater table observation wells, respectively. The result showed that the groundwater samples that had NO3 −–N concentrations exceeding the threshold for drinking water were mostly collected from a depth of less than 20 m. Groundwater NO3 −–N concentrations in areas used for the cultivation of vegetables, seed maize and intercropped maize were significantly higher than those in urban or paddy areas. NO3 −–N contamination of groundwater in areas with sandy soil was more severe than in those with loam soil.  相似文献   

14.
The concentrations of water and carbon dissolved in an icelandite glass quenched from 1400 °C and 10 kbar were measured using Fourier transform infra-red spectroscopy and elemental analyses of carbon and hydrogen. Only carbon dioxide and water were observed in the fluid phase as analysed after quenching with a qudrupole mass analyser. The mole fraction of carbon dioxide in the fluid phase ranged from 0.36 to 0.95. Carbon is dissolved as carbonate except at the highest CO2 fluid fugacity, where a small amount of molecular CO2 is observed. Dissolved carbon in the glasses, calculated as CO2, remained constant at approximately 1 wt %, in spite of the different CO2 fluid fugacities. Water was dissolved as molecular water and as hydroxyl groups, the hydroxyl concentration in the quenched glasses remaining almost constant over the whole interval, whereas the molecular water dissolves in accordance with Henry's law. Molecular water peaks at 5200␣cm−1 and 1630 cm−1, the hydroxyl peak at 4500␣cm−1, and the carbonate peaks at 1400 cm−1–1550 cm−1 have been calibrated using elemental analyses of C and H in the quenched glasses. As molecular water decreases in the melt the higher wavenumber carbonate peak is observed to move towards the molecular water peak at 1630 cm−1 causing a split of the carbonate peaks, ranging from 45 cm−1 to 100 cm−1. Received: 15 November 1995 / Accepted: 21 September 1996  相似文献   

15.
 Cordierite precursors were prepared by a sol-gel process using tetraethoxysilane, aluminum sec.-butoxide, and Mg metal flakes as starting materials. The precursors were treated by 15-h heating steps in intervals of 100 °C from 200 to 900 °C; they show a continuous decrease in the analytical water content with increasing preheating temperatures. The presence of H2O and (Si,Al)–OH combination modes in the FTIR powder spectra prove the presence of both H2O molecules and OH groups as structural components, with invariable OH concentrations up to preheating temperatures of 500 °C. The deconvolution of the absorptions in the (H2O,OH)-stretching vibrational region into four bands centred at 3584, 3415, 3216 and 3047 cm−1 reveals non-bridging and bridging H2O molecules and OH groups. The precursor powders remain X-ray amorphous up to preheating temperatures of 800 °C. Above this temperature the precursors crystallize to μ-cordierite; at 1000 °C the structure transforms to α-cordierite. Close similarities exist in the pattern of the 1400–400 cm−1 lattice vibrational region for precursors preheated up to 600 °C. Striking differences are evident at preheating temperatures of 800 °C, where the spectrum of the precursor powder corresponds to that of conventional cordierite glass. Bands centred in the “as-prepared” precursor at 1137 and 1020 cm−1 are assigned to Si–O-stretching vibrations. A weak absorption at 872 cm−1 is assigned to stretching modes of AlO4 tetrahedral units and the same assignment holds for a band at 783 cm−1 which appears in precursors preheated at 600 °C. With increasing temperatures, these bands show a significant shift to higher wavenumbers and the Al–O stretching modes display a strong increase in their intensities. (Si,Al)–O–(Si,Al)-bending modes occur at 710 cm−1 and the band at 572 cm−1 is assigned to stretching vibrations of AlO6 octahedral units. A strong band around 440 cm−1 is essentially attributed to Mg–O-stretching vibrations. The strongly increasing intensity of the 872 and 783 cm−1 bands demonstrates a clear preference of Al for a fourfold-coordinated structural position in the precursors preheated at high temperatures. The observed band shift is a strong indication for increasing tetrahedral network condensation along with changes in the Si–O and Al–O distances to tetrahedra dimensions similar to those occurring in crystalline cordierite. These structural changes are correlated to the dehydration process starting essentially above 500 °C, clearly demonstrating the inhibiting role of H2O molecules and especially of OH groups. Received: 1 March 2002 / Accepted: 26 June 2002  相似文献   

16.
The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered (“best”) samples of the Burzyan carbonates, which retain the 87Sr/86Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480 and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ18O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7 to 0‰, which is consistent with the assumption that δ18O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago. C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least altered carbonates of the Early Riphean, the δ18O variation range corresponds to 4.0–4.5‰.  相似文献   

17.
The assessment of groundwater quality and its environmental implications in the region of the abandoned Cunha Baixa uranium mine (Central Portugal) was carried out from 1995 to 2004. Shallow groundwater is the major water supply source for irrigation in the neighbourhood of Cunha Baixa village. Water samples from the mine site as well as from private wells were collected in order to identify the mining impact on water composition, the extent of contamination and the seasonal and temporal groundwater quality variations. Some of the sampled private wells contain waters having low pH (<4.5–5) and high values of EC, TDS, SO4, F, Ca, Mg, Al, Mn, Ni, U, Zn and 226Ra. The wells located through the ESE–WSE groundwater flow path (1 km down gradient of the mining site) display the most contaminated water. In the summer season, the levels of SO4, Al, Mn, and U were 50–120 times higher than those registered for uncontaminated waters and exceeded the quality limits for irrigation purposes, presenting soil degradation risks. Nevertheless, this study indicates that groundwater contamination suffered a small decrease from 1999 to 2004. The bioaccumulation of toxic metals such as Al, Mn, and U within the food chain may cause a serious health hazard to the Cunha Baixa village inhabitants.  相似文献   

18.
The crystal structure of Bi2Al4−x Fe x O9 compounds (x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi2M4O9 compounds (M = Al, Fe3+) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm−1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm−1 band group, which is assigned to Al(Fe3+)–O stretching vibrations of the corner-sharing MO4 tetrahedra. The Bi2Al4O9 and Bi2Fe4O9 endmembers display single bands with maxima centred at 922 and 812 cm−1, respectively. Intermediate Bi2Al4−x Fe x O9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm−1 range are assigned to low-energy stretching vibrations of the MO4 tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm−1 range are essentially determined by M–O stretching modes of the M cations in octahedral coordination.  相似文献   

19.
Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal EPR spectra at 295 K are as follows: g 1 = 2.02899(1), g 2 = 2.02011(2), g 3 = 2.00595(1); A 1/g e β e  = −0.881(1) mT, A 2/g e β e  = −0.951(1) mT, and A 3/g e β e  = −0.972(2) mT, with the orientations of the g 3- and A 3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g 1- and A 1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals.  相似文献   

20.
Biogeochemical processes occurring near the sediment-water interface of shallow (≈20 m) water sediments lying beneath the Mississippi River plume on the Louisiana shelf were studied using benthic chambers and sediment cores. Three sites were chosen with distinctly different characteristics. One was overlain by oxic water where aerobic respiration dominated organic matter remineralization. The second site was overlain by oxic water but organic matter remineralization was dominated by sulfate reduction. The third site was overlain by hypoxic water and aerobic remineralization was of minor significance. Major differences were observed in the fluxes of CO2(17–56 mmol m−2 d−1), O2(2–56 mmol m−2 d−1) and nutrients (e.g., NH4 +, 2.6–4.2 mmol m−2 d−1) across the sediment-water interface, and the relative importance of different electron acceptors, even though the sites were in close proximity and at nearly the same water depth. Large variations in the efficiency of organic-C burial (3%–51%) were also calculated based on a simplified model of the relationships between the fraction of organic matter remineralized by sulfate reduction and the fraction of sulfide produced that is buried as pyrite. These observations demonstrate the high degree of spatial heterogeneity of benthic biogeochemistry in this important near-deltaic environment.  相似文献   

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