首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Molecular quantum mechanical calculations have been performed on high-spin ferrous iron tetrahedrally coordinated to sulfur and oxygen, respectively. The molecular orbital energies obtained from the calculations are compared with experimental optical and X-ray emission spectra. Good agreement was found between calculated and experimental spectral transition energies for the optical absorption spectra of Fe2+ in sphalerite, of Fe2+ in FeAl2O4, staurolite and (Zn, Fe)O, and for the FeKβ X-ray emission spectra of FeCr2O4. This both clarified interpretation of the spectra and established the validity of the calculations. Distinct differences occur in the molecular orbital structures of the sulfide and oxide clusters. In the sulfide, the crystal field type (mainly Fe 3d) molecular orbitals lie within the nonbonding (mainly S 3p) orbitais in energy, whereas in the oxide, they lie well above the 02p nonbonding orbitals. This also results in a wider valence band in the oxide than in the sulfide. The crystal field type (Fe 3d) molecular orbitais have more ligand character in the sulfide than the oxide and the chalcophilic properties of iron are partly attributed to this observation.  相似文献   

2.
The different Fe2+ lattice sites in iron-rich chlorites have been characterized by Mössbauer spectroscopy and molecular orbital calculations in local density approximation. The Mössbauer measurements were recorded at 77?K within a small velocity range (±3.5?mm?s?1) to provide high energy resolution. Additionally, measurements were recorded in a wider velocity range (±10.5?mm?s?1) at temperatures of 140, 200, and 250?K in an applied field (7?T) parallel to the γ-beam. The zero-field spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe2+ sites M1, M2, and M3 and with a quadrupole distribution for Fe3+ sites. Such a procedure is justified by the results obtained from MO calculations, which reveal that different anion (OH?) distributions in the first coordination sphere of M1, M2, and M3 positions have more influence on the Fe2+ quadrupole splitting than cationic disorder. The spectra recorded in applied field were analyzed in the spin-Hamiltonian approximation, yielding a negative sign for the electric field gradient (efg) of Fe2+ in the M1, M2, and M3 positions. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (ΔE Q ), their temperature dependence and in the isomer shifts (δ) of Fe2+ in M1, M2, and M3 positions can theoretically by justified. Therefore, the combined Mössbauer and MO investigation shows that the three Fe2+ lattice sites in the chlorites investigated here can be discriminated according to their ΔE Q -δ parameter pairs. With the calculated average iron-oxygen bond strength, the MO study provides an explanation for the observed trend that the population of the three lattice sites by Fe2+ increases according to the relation M1?相似文献   

3.
《Applied Geochemistry》2001,16(13):1499-1512
The oxidative dissolution rate of metacinnabar by dissolved O2 was measured at pH ∼5 in batch and column reactors. In the batch reactors, the dissolution rate varied from 3.15 (±0.40) to 5.87 (±0.39) × 10−2 μmol/m2/day (I=0.01 M, 23°C) and increased with stirring speed, a characteristic normally associated with a transport-controlled reaction. However, theoretical calculations, a measured activation energy of 77 (±8) kJ/mol (I=0.01 M), and the mineral dissolution literature indicate reaction rates this slow are unlikely to be transport controlled. This phenomenon was attributed to the tendency of the hydrophobic source powder to aggregate and minimize the effective outer surface area. However, in a column experiment, the steady-state dissolution rate ranged from 1.34 (±0.11) to 2.27 (±0.11) x 10−2 μmol/m2/day (I=0.01 M, 23°C) and was also influenced by flow rate, suggesting hydrodynamic conditions may influence weathering rates observed in the field. The rate of Hg release to solution, under a range of hydrogeochemical conditions that more closely approximated those in the subsurface, was 1 to 3 orders of magnitude lower than the dissolution rate due to the adsorption of released Hg(II) to the metacinnabar surface. The measured dissolution rates under all conditions were slow compared to the dissolution rates of minerals typically considered stable in the environment, and the adsorption of Hg(II) to the metacinnabar surface further lowered the Hg release rate.  相似文献   

4.
Sumner DY  Grotzinger JP 《Geology》1996,24(2):119-122
Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds.  相似文献   

5.
  相似文献   

6.
We recorded the photoelectron spectra of various crystalline calcium silicate hydrates (C–S–Hs) and have examined their O 1 s photoelectron spectra. The spectra are asymmetric, with contributions assigned primarily to bridging and non-bridging oxygen species. There is an increased contribution due to the presence of non-bonding oxygen atoms with increasing calcium:silicon ratio. Additionally, there are slight changes in theO 1s-binding energies with changes in calcium:silicon ratio. These changes are explained in terms of bonding and silicate structure.  相似文献   

7.
The pre-Cenozoic geology at Candelaria, Nevada comprises four main lithologic units: the basement consists of Ordovician cherts of the Palmetto complex; this is overlain unconformably by Permo-Triassic marine clastic sediments (Diablo and Candelaria Formations); these are structurally overlain by a serpentinitehosted tectonic mélange (Pickhandle/Golconda allochthon); all these units are cut by three Mesozoic felsic dike systems. Bulk-mineable silver-base metal ores occur as stratabound sheets of vein stockwork/disseminated sulphide mineralisation within structurally favourable zones along the base of the Pickhandle allochthon (i.e. Pickhandle thrust and overlying ultramafics/mafics) and within the fissile, calcareous and phosphatic black shales at the base of the Candelaria Formation (lower Candelaria shear). The most prominent felsic dike system — a suite of Early Jurassic granodiorite porphyries — exhibits close spatial, alteration and geochemical associations with the silver mineralisation. Disseminated pyrites from the bulk-mineable ores exhibit a 34S range from — 0.3 to + 12.1 (mean 34S = +6.4 ± 3.5, 1, n = 17) and two sphalerites have 34S of + 5.9 and + 8.7 These data support a felsic magmatic source for sulphur in the ores, consistent with their proximal position in relation to the porphyries. However, a minor contribution of sulphur from diagenetic pyrite in the host Candelaria sediments (mean 34S = — 14.0) cannot be ruled out. Sulphur in late, localised barite veins ( 34S = + 17.3 and + 17.7) probably originated from a sedimentary/seawater source, in the form of bedded barite within the Palmetto basement ( 34S = + 18.9). Quartz veins from the ores have mean 18O = + 15.9 ± 0.8 (1, n = 10), which is consistent, over the best estimate temperature range of the mineralisation (360°–460°C), with deposition from 18O-enriched magmatic-hydrothermal fluids (calculated 18O fluid = + 9.4 to + 13.9). Such enrichment probably occurred through isotopic exchange with the basement cherts during fluid ascent from a source pluton. Whole rock data for a propylitised porphyry ( 18O = + 14.2, D = — 65) support a magmatic fluid source. However, D results for fluid inclusions from several vein samples (mean = — 108 ± 14, 1, n = 6) and for other dike and sediment whole rocks (mean = — 110 ± 13, 1, n = 5) reveal the influence of meteoric waters. The timing of meteoric fluid incursion is unresolved, but possibilities include late-mineralisation groundwater flooding during cooling of the Early Jurassic progenitor porphyry system and/or meteoric fluid circulation driven by Late Cretaceous plutonism.  相似文献   

8.
This paper reports an oxygen isotopic study of corundums and associated minerals from the Yogo lamprophyres (Montana, US), plagioclase-corundum inclusions in alkali basalt (Tunkin depression, Russia), and from modern alluvium of Podgelbanochnyi alkaline basaltic volcano (Primorye, Russia). It is shown that all sapphires genetically related to mafic magmatic rocks have a similar oxygen isotopic composition (the variations of δ18O are within 2.5‰) with most values plotting between +4.5 and +7.0‰ SMOW. The oxygen isotopic ratios in the associated minerals (olivines, pyroxenes, mica, and others) and host rocks are plotted in the same interval. This indicates that the sapphire crystallized during evolution of the parental mafic magma. However, there are xenogenic corundums, which were only transported by basaltic magma to the Earth’s surface. They have a sharply distinct oxygen isotopic composition, which suggests their disequilibrium with the host lavas, and, correspondingly, a different genetic nature.  相似文献   

9.
Gem corundum, a minor but persistent megacryst in east Australian basalt fields, is mined from some placer concentrations. Laser ablation, inductively coupled plasma mass spectrometry analyses and O isotope determinations on a colour range of corundum from different fields, show that chromophore (Fe, Cr, Ti, V) and genetic indicator (Ga, Mg, δ18O) values can distinguish corundum sources (magmatic, metamorphic and metasomatic) before basalt incorporation. They also characterise corundum groups from different fields. This identified two metamorphic groups, one carrying ruby at Barrington Tops, and a magmatic group distinct from those from other gem fields (lower Fe, northeast Tasmania; higher Fe, Yarrowitch). Ruby-bearing groups show clear provincial characteristics and include lower temperature spinel-facies groups (Barrington, Yarrowitch) and higher temperature garnet-facies groups (Cudgegong–Macquarie River). High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. The corundum derived from diverse fold-belt and felsic sources in underlying lithosphere forms a dataset for comparing corundum from other basalt fields.  相似文献   

10.
  相似文献   

11.
Hydrogen‐ and oxygen‐isotope analyses of biotite (19), sericite (8), chlorite (2), quartz (27), and total rocks (37) from the Panguna porphyry‐copper deposit on Bougainville Island, place important constraints on the origin of the hydrothermal fluids responsible for mineralization and alteration in the mine region. Early high‐temperature amphibole‐magnetite alteration resulted from magmatic‐hydrothermal fluids. Several lines of evidence indicate 500°C as a realistic average temperature for mineralization, development of quartz veins, and biotitization processes. On the basis of mineral isotope data, responsible fluids could represent either 18O‐shifted ground‐waters or magmatic‐hydrothermal fluids at submagmatic temperatures. Independent evidence, as well as total‐rock 18O data, support the magmatic‐hydrothermal model.

Late‐stage sericitization processes probably resulted from fluids produced by 18O shifting of groundwaters during the evolution of the propylitic zone. Outermost quartz veins and biotitization conceivably resulted from fluids similar to those that caused sericitization, indicating that some interaction between relatively cool, 18O‐poor meteoric waters and the ore fluids occurred near the margins of the deposit. The origin of the chlorite‐sericite alteration cannot be resolved solely by isotope studies.  相似文献   

12.
A number of samples from a deep peat bog in Tierra del Fuego were analyzed using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in order to extract parameters that might be used to interpret the peat chemistry in terms of vegetation change, anaerobic and aerobic decomposition, and fire incidence. The choice of parameters was based on factor analysis of 177 pyrolysis products, quantified for 13 samples, separated into extract and residue, as well as the total samples. Factor analysis of extracts, residues and total samples yielded similar classifications in terms of vegetation and decomposition. Pyrolysis products and ratios that most clearly differentiated samples were used to interpret the depth profile. Although interpretation was not always straightforward, indications of parameters to describe vegetation shifts, aerobic and anaerobic decomposition, and fire largely coincided. These parameters will be used in a forthcoming study for a more complete interpretation of the peat profile.  相似文献   

13.
 116-year record of coral skeletal δ18O is presented from a colony of Porites lutea from Ningaloo Reef, western Australia. Interannual variability of sea-surface temperatures (SST) inferred from skeletal δ18O is dominated by a 9.5-year period, and may constitute a characteristic signal of the Leeuwin Current. On long-terms coral skeletal δ18O indicates a near-continuous increase of SST at Ningaloo Reef over one century. The skeletal δ18O time series was checked for the presence of seasonal cooling events resulting from major volcanic eruptions. An ∼1  °C cooling is evident following the eruption of Pinatubo in 1991, which reproduces the results of previous investigations. However, only weak or no signals can be related to the eruptions of Krakatau (1883) and Agung (1963). Received: 9 November 1998 / Accepted: 12 April 1999  相似文献   

14.
The aim of this study is to investigate the nature of the solutions and the temperature during the bentonite formation by using δ18O and δ2H values and the relationship between chemical and isotopic composition of smectites from Cabo de Gata region. Therefore, 36 bentonite samples had been selected. For analyses the <2 μm fraction were separated. Most of the samples contain smectite above 90 wt%. The stable oxygen and hydrogen isotopic composition of the analysed smectites is close to the “montmorillonite line”. However, from isotopic composition two types of bentonite can be distinguished. The Sierra de Gata (SG) and Serrata de Nijar (SN) types show δ18O and δ2H values above and below this line, respectively. Temperatures obtained from geothermometer equation are similar to the temperatures values obtained from the equation of Savin and Lee (1988). Estimated formation temperatures of SG type are 25 and 49 °C higher compared to those of the SN type (6 and 25 °C).Correlations between structural cations versus isotopic composition show no statistical significance concerning all samples. However, separating the samples of SG (Sierra de Gata) from SN (Serrata of Nijar) a statistical significance for the individual areas is obtained. In both cases the effect is similar for 18O and for 2H. This could be due to the different composition of the solutions forming these bentonites.  相似文献   

15.
The Langdu skarn copper deposit in the Zhongdian area, Yunnan Province, China, has an average Cu grade of 6.49 %. The deposit is related to a porphyry intrusion(*216 Ma), which was emplaced in the Upper Triassic sedimentary rocks of the Tumugou and Qugasi Formations.At the Langdu skarn copper deposit, carbon and oxygen isotope ratios of fresh limestones(d18O = 3.0–5.6 % relative to V-SMOW; d13 C = 24.5–25.7 % relative to PDB)and partly altered limestones(d18O = 27–7.2 to-1.9 %;d13C = 11.8–15.2 %) indicated that the deposit was a typical marine carbonate source. Oxygen and carbon isotope values for calcites formed at different hydrothermal stages are-9.1 to 0.2 and 10.1–16.3 %, respectively. Moreover,the carbon–oxygen isotopic composition of an ore-forming fluid(d18O = 5.0–9.5 %, d13 C =-7.3 to-5.3 %) suggested the presence of magmatic water, which most likely came from the differentiation or melting of a homologous magma chamber. The deposition of Calcite I may arise from metasomatism in an open system with a progressively decreasing temperature.Later, the minerals chalcopyrite, pyrrhotite, quartz and Calcite II were precipitated due to immiscibility. Water–rock interaction could potentially be responsible for Calcite III precipitation in the post-ore stage.  相似文献   

16.
Increased interest in paleoenvironmental studies is a result of climatic changes occurring at present and predicted for the future. Such studies could be done using the stable isotope compositions (δ2H and δ18O) of kaolins, which provide knowledge on the paleoenvironmental conditions prevailing during the time of kaolinisation. In this study, the stable isotopic compositions of clay-size fraction of kaolins occurring in Cretaceous and Tertiary Formations of the Douala Sub-Basin in Cameroon are presented, with the aim of reconstructing the paleoenvironmental conditions of the Sub-Basin. To achieve this, the clay-size fraction (< 2 μm fraction) of 8 kaolinite-rich samples were analysed for their δ2H and δ18O compositions, and results were reported as part per mil (‰) relative to the SMOW standard. The δ18O values of kaolins found in the Cretaceous–Tertiary Formations of the Douala Sub-Basin varied between +18.2 and +21.0‰, whereas the δ2H values varied between –69 and –53‰. Nine of the eleven samples plotted on the right of the supergene–hypogene line. Five of these nine samples plotted very close to the kaolinite line, which represents the composition of kaolinite in equilibrium with meteoric water at 20 °C; suggesting a supergene weathering origin of these kaolins. The determination of the temperature of kaolinisation yielded mean formation temperatures of 22 ± 2 °C and 27 ± 6 °C for Cretaceous and Tertiary kaolins, respectively. Excluding the two samples falling in the hypogene field, averages of kaolinisation temperatures were 20 and 25 °C during the Cretaceous and Tertiary periods, respectively. These temperatures are slightly below the present mean annual temperature in Douala (27 °C), thereby suggesting that the climate was becoming warmer from the Cretaceous to the Present. Therefore, Douala had a cooler and rainy climate during the Cretaceous, and the climate is gradually becoming hotter and more humid, favouring the refinement of existing kaolins and the kaolinisation of kaolin-forming minerals in the Sub-Basin.  相似文献   

17.
  相似文献   

18.
The first detailed isotope-geochemical study of carbonate deposits has been performed in the Lower Famennian stratotype section of the northwestern Kuznetsk Basin (Kosoy Utyos), which was localized in the middle latitudes of the Northern Hemisphere in the Late Devonian. The δ13Ccarb, δ13Corg, and δ18O variation curves were constructed for the section deposits. Geochemical and petrographic studies of carbonates allowed allocation of samples that underwent postsedimentation alteration and exclude them from further interpretation. Compared with coeval sections in the other world's regions, the Kosoy Utyos section is characterized by higher δ13Ccarb values, up to 5.4‰, whereas the maximum value in subequatorial area sections is 4‰. The isotope shift amplitude of the studied section reaches 4.6‰, which is 1.5‰ higher than those in other regions. The δ18O values are 3‰ lower than the ones of the world's coeval sections. The results obtained show that δ13C and δ18O variation trends differ from those of coeval subequatorial sections. The high shift amplitude and maximum δ13Ccarb values in the Kosoy Utyos section are due to the shallow-water carbonate sedimentation environments on the Siberian continental shelf and, probably, the lower temperatures of waters in the middle latitudes as compared with the subequatorial areas.  相似文献   

19.
Low molecular weight organic acids （LMWOAs） are important components of root exudates. They play an important role in immobilizing and remobilizing contaminants in rhizospheric soil. Effects of four LMWOAs on the sorption and desorption behavior of p-chlorophenol by yellow earth was studied in batch mode. The results showed that the previous application of LMWOAs to enhanced adsorption capacity of p-chlorophenol of the soil in the order of maleic acid〉malic acid〉oxalic acid〉citric acid. However, when LMWOAs were applied to soil where p-chlorophenol had been previously adsorbed, substantial p-chlorophenol was desorbed from soil by oxalic acid, whereas citric acid, malic acid and maleic acid didn＇t desorb as much p-chlorophenol from soil as deionized water.  相似文献   

20.
Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~?5.3‰) and are apparently at odds with the mantle-like εNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like εHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (>?90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号