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1.
The coprecipitation of Sr, Mg, Na and K with anhydrite during the dehydration of gypsum was studied in laboratory experiments. The partition coefficients of Mg and Sr between anhydrite and solution decrease with increasing temperature. The partition coefficients of the alkali-ions do not depend upon temperature, but are affected by the brine composition.The mechanism of the phase transformation gypsum → anhydrite occurs via dissolution and precipitation, when the coprecipitated-ions are repartitioned between the new phase and the solution. The partition coefficients established in this study are applicable also for primary anhydrite.During the dehydration of gypsum at elevated temperatures metastable bassanite may form as an intermediate stage. The amount of cations coprecipitated with bassanite is much larger than the amount coprecipitated with anhydrite or gypsum. This phenomenon may have an influence on the partitioning of cations during the dehydration of gypsum, particularly on Sr.The partition coefficients of seawater cations between anhydrite and the brine are similar to those between gypsum and the brine. For this reason the coprecipitated-ions are not expected to be good indicators to distinguish between primary and secondary calcium sulfate minerals.The temperature effect on the coprecipitation of Mg and Sr with anhydrite makes these ions possible indicators for the temperature at which the phase transformation occurred. This temperature corresponds to the depth of burial of the gypsum at the stage of dehydration.The coprecipitation of seawater cations with anhydrite in the natural environment was studied in two systems: A small Pleistocene evaporite lens from the Sinai Peninsula and the Triassic anhydrite of the Mohilla Formation, Israel. The coprecipitated-ion composition of these samples was used to derive the conditions under which the anhydrite was formed.  相似文献   

2.
The Dead Sea brine is supersaturated with respect to gypsum (Ω = 1.42). Laboratory experiments and evaluation of historical data show that gypsum nucleation and crystal growth kinetics from Dead Sea brine are both slower in comparison with solutions at a similar degree of supersaturation. The slow kinetics of gypsum precipitation in the Dead Sea brine is mainly attributed to the low solubility of gypsum which is due to the high Ca2+/SO42− molar ratio (115), high salinity (∼280 g/kg) and to Na+ inhibition.Experiments with various clay minerals (montmorillonite, kaolinite) indicate that these minerals do not serve as crystallization seeds. In contrast, calcite and aragonite which contain traces of gypsum impurities do prompt precipitation of gypsum but at a considerable slower rate than with pure gypsum. This implies that transportation inflow of clay minerals, calcite and local crystallization of minerals in the Dead Sea does not prompt significant heterogeneous precipitation of gypsum. Based on historical analyses of the Dead Sea, it is shown that over the last decades, as inflows to the lake decreased and its salinity increased, gypsum continuously precipitated from the brine. The increasing salinity and Ca2+/SO42− ratio, which results from the precipitation of gypsum, lead to even slower kinetics of nucleation and crystal growth, which resulted in an increasing degree of supersaturation with respect to gypsum. Therefore, we predict that as the salinity of the Dead Sea brine continues to increase (accompanied by Dead Sea water level decline), although gypsum will continuously precipitate, the degree of supersaturation will increase furthermore due to progressively slower kinetics.  相似文献   

3.
This study presents the results from precipitation experiments carried out to investigate the partitioning of the alkaline earth cations Mg2+, Ca2+, Sr2+, and Ba2+ between abiogenic aragonite and seawater as a function of temperature. Experiments were carried out at 5 to 75 °C, using the protocol of Kinsman and Holland [Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO3 IV. The coprecipitation of Sr2+ with aragonite between 16 and 96 °C. Geochim. Cosmochim. Acta33, 1-17.] The concentrations of Mg Sr and Ba were determined in the fluid from each experiment by inductively coupled plasma-mass spectrometry, and in individual aragonite grains by secondary ion mass spectrometry. The experimentally produced aragonite grains are enriched in trace components (“impurities”) relative to the concentrations expected from crystal-fluid equilibrium, indicating that kinetic processes are controlling element distribution. Our data are not consistent with fractionations produced kinetically in a boundary layer adjacent to the growing crystal because Sr2+, a compatible element, is enriched rather than depleted in the aragonite. Element compatibilities, and the systematic change in partitioning with temperature, can be explained by the process of surface entrapment proposed by Watson and Liang [Watson, E.B., Liang, Y., 1995. A simple model for sector zoning in slowly grown crystals: implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks. Am. Mineral.80, 1179-1187] and Watson [Watson, E.B., 1996. Surface enrichment and trace-element uptake during crystal growth. Geochim. Cosmochim. Acta60, 5013-5020; Watson, E.B., 2004. A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals. Geochim. Cosmochim. Acta68, 1473-1488]. This process is thought to operate in regimes where the competition between crystal growth rate and diffusivity in the near-surface region limits the extent to which the solid can achieve partitioning equilibrium with the fluid. A comparison of the skeletal composition of Diploria labyrinthiformis (brain coral) collected on Bermuda with results from precipitation calculations carried out using our experimentally determined partition coefficients indicate that the fluid from which coral skeleton precipitates has a Sr/Ca ratio comparable to that of seawater, but is depleted in Mg and Ba, and that there are seasonal fluctuations in the mass fraction of aragonite precipitated from the calcifying fluid (“precipitation efficiency”). The combined effects of surface entrapment during aragonite growth and seasonal fluctuations in “precipitation efficiency” likely forms the basis for the temperature information recorded in the aragonite skeletons of Scleractinian corals.  相似文献   

4.
Gold partitioning in melt-vapor-brine systems   总被引：5，自引：0，他引：5  
We used laser-ablation inductively coupled plasma mass spectrometry to measure the solubility of gold in synthetic sulfur-free vapor and brine fluid inclusions in a vapor + brine + haplogranite + magnetite + gold metal assemblage. Experiments were conducted at 800°C, oxygen fugacity buffered at Ni-NiO (NNO), and pressures ranging from 110 to 145 MPa. The wt% NaCl eq. of vapor increases from 2.3 to 19 and that of brine decreases from 57 to 35 with increasing pressure. The composition of the vapors and brines are dominated by NaCl + KCl + FeCl2 + H2O. Gold concentrations in vapor and brine decrease from 36 to 5 and 50 to 28 μg/g, respectively, and the calculated vapor:brine partition coefficients for gold decrease from 0.72 to 0.17 as pressure decreases from 145 to 110 MPa. These data are consistent with the thermodynamic boundary condition that the concentration of gold in the vapor and brine must approach a common value as the critical pressure is approached along the 800°C isotherm in the NaCl-KCl-FeCl2-HCl-H2O system.We use the equilibrium constant for gold dissolution as AuOH0, extrapolated from lower temperature and overlapping pressure range, to calculate expected concentrations of AuOH0 in our experimental vapors. These calculations suggest that a significant quantity of gold in our experimental vapors is present as a non-hydroxide species. Possible chloridogold(I) species are hypothesized based on the positively correlated gold and chloride concentrations in our experimental vapors. The absolute concentration of gold in our synthetic vapor, brine, and melt and calculated mass partition coefficients for gold between these physicochemically distinct magmatic phases suggests that gold solubility in aqueous fluids is a function of aqueous phase salinity, specifically total chloride concentration, at magmatic conditions. However, though we highlight here the effect of salinity, the combination of our data with data sets from lower temperatures evinces a significant decrease in gold solubility as temperature drops from 800°C to 600°C. This decrease in solubility has implications for gold deposition from ascending magmatic fluids.  相似文献   

5.
Some of the factors that affect the preferred positions of cations in ionic-solid solutions were investigated utilizing vibrational spectroscopy. Solid solutions of the sulfate and chromate ions codoped with La3+ and Ca2+ in a KBr host lattice were examined as a function of the polyvalent-cation concentration. The cation—anion pairing process was found to be random for Ca2+ whereas the formation of La3+SO42? ion-pairs with a C2v bonding geometry is highly preferential to any type of La3+CrO42? ion-pair formation. The relative populations of ion-pair site configurations are discussed in terms of an energy—entropy competition which can be applied to the partition of trace elements during magmatic processes.  相似文献   

6.
Gypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater-seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow. The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ18O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (?– 6%0), whereas measured brine δ18O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ?– 70%0, GWC are ?– 60 to – 65%0 and brine values are ?– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely. Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained.  相似文献   

7.
Oilfield brines (produced water) are produced as a waste product daily at the gathering centers (GCs) in Kuwait oilfields. The geochemical evolution of the water produced at the GC (fresh brine) to stagnant pit water (evaporate) has been investigated in the northern fields of Kuwait, and a model is presented showing time-dependent variations. Kuwait oilfield brines are globally similar to others in other large sedimentary basins (USA, Canada), but modifications have occurred due to seawater injection practices performed episodically during the oil extraction process. Brine water chemistry changes from generally average brine chemistry (based on cations and anions) to saturated mixture of seawater, oilfield brine, and anthropogenic chemical pollutants. The objective of this study was to harmonize the database of brine waters in terms of regional identity by comparison with oilfield brines elsewhere, identify water–rock interaction, and statistically treat daily recordings from the pits in order to identify injection peaks and troughs. Laboratory analysis of major and minor cations and anions from the Rawdatayn samples gave the following concentration ranges in parts per million (ppm): (Na+, 11,698–203,977), (Ca2+, 2,216–98,514), (Mg2+, 1,602–28,885), (K+, 1,528–16,573), (Sr2+, 70–502), (Ba2+, 0.01–18.04), (Fe2+, 0.01–8.93), (Li+, 0.09–6.48), (Si2+, 0.00–13.18), (B3+, 0.05–37.45), (SO 4 2+ , 330–3100). For the Sabriyah oilfield samples, the major and minor cations and anions concentration ranges in ppm are: (Na+, 9,807–274,947), (Ca2+, 2,555–77,992), (Mg2+, 1,415–28,183), (K+, 764–19,201), (Sr2+, 77.84–641), (Ba2+, 0.15–6.76), (Fe2+, 0.016–38.88), (Li+, 0.05–6.83), (Si2+, 0.0195–16.84), (B3+, 7.17–55.33), (SO 4 2+ , 44,812–135,264). The stable isotopic analysis of five samples indicates normal trends in oxygen and hydrogen isotopes that classify the waters as “connate” which follow an evaporation trend. Carbon isotopic signatures are normal for hydrocarbon fields and average out around GC15, δ18O‰?=?1.4, δD‰?=??10, δ13C‰?=??3.6; while for GC23, δ18O‰?=?2.3, δD‰?=??4, δ13C‰?=??2.5; for GC25, δ18O‰?=??2.0, δD‰?=??14, δ13C‰?=??4.6; for pit1, δ18O‰?=?2.3, δD‰?=??5, δ13C‰?=??18.3; and for pit 2, δ18O‰?=?2.5, δD‰?=??4, δ13C‰?=??17.8. Carbon isotope average values for all brine samples from the GCs is?=??56 which falls within normal hydrocarbon formation water category. Data spikes coincide with injection periods at the following times (A: May–Jun, 2006), (B: Sep–Oct, 2006), (C: Jan–Feb, 2007), (D: Mar, 2007), (E: May–Jun, 2007), (F: Feb, 2006), (G: Mar–Apr, 2006) and, subsequently the decay to “normal” brine occurs over a period of several weeks. The database was large enough to apply a principal component statistical analysis (PCA). PCA and geo-statistical techniques reveal several distinct population groups. The main chemical groups in the data are as follows: plateau, spike groups, and pit evaporation group. The spike periods correlate closely with seawater injection periods (Jan–Feb, Mar–Apr, May–Jun, and Sep–Oct). The pit chemistry reveals exceptionally high evaporation processes coinciding with summer peak temperature. PCA results show distinct groupings centered around the major elements reminiscent of other oilfields, but with the added evaporation trend strongly enhanced.  相似文献   

8.
Twenty-four brine samples from the Heletz-Kokhav oilfield, Israel, have been analyzed for chemical composition and Li isotope ratios. The chemical composition of the brines, together with geological evidence, suggests derivation from (Messinian) seawater by evaporation that proceeded well into the gypsum stability field but failed to reach the stage of halite crystallization. The present salinity of the samples (18-47 g Cl/L) was achieved by dilution of the original evaporitic brine by local fresh waters. Like brines from other sedimentary basins, the Li/Cl ratios in the Heletz-Kokhav samples show a prominent Li enrichment (five-fold to eight-fold) relative to modern seawater. The isotopic ratios of Li, expressed in the δ 6Li notation, vary from −26.3 to −17.9‰, all values being significantly higher than that of modern seawater (−32‰) irrespective of their corresponding Li concentration (1.0-2.3 mg/L). The isotopic composition of Li and the Li/Cl ratio in the oilfield brines were acquired in two stages: (a) The original evaporated seawater gained isotopically light Li during the diagenetic interaction between the interstitial Messinian brine and the basin sediments. A parent brine with an elevated Li/Cl ratio was formed. The brine was later diluted in the oilfields. (b) The δ 6Li values of the final brines were determined during epigenetic interaction with the Heletz-Kokhav aquifer rocks. At the same time, the Li/Cl ratio inherited from stage (a) remained largely unchanged. This work represents the first use of lithium isotopic composition to elucidate the origin and evolution of formation waters in sedimentary basins.  相似文献   

9.
The electron spin resonance (ESR) spectrum of Cr3+ in a synthetic single crystal of forsterite doped with Cr2O3 was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr3+ positions may be possible, but were not observed. The site occupancy numbers of Cr3+ at M1 and M2 are roughly 1.2×10?4 and 0.8×10?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.  相似文献   

10.
《Chemical Geology》2006,225(3-4):322-335
This work deals with the growth behaviour of calcite {101¯4} surfaces in contact with multicomponent aqueous solutions containing divalent cations (Ba2+, Sr2+, Mn2+, Cd2+, or Mg2+). The result is the formation of solid solutions, with calcite or aragonite as one of the end-members. In situ atomic force microscopy has revealed a wide variety of surface phenomena occurring during the formation of these solid solutions. Among them are: (1) the thickening of growth steps and the subsequent dissolution of surfaces followed by the nucleation of secondary three-dimensional nuclei on calcite surfaces, (2) the transition between growth mechanisms, (3) the formation of an epitaxial layer that armours the substrate from further dissolution and (4) the inhibitory effect of the newly formed surface on the subsequent growth (template effect). The two last phenomena can considerably limit coprecipitation as an effective mechanism for divalent metal uptake. All the phenomena described are a consequence of the interplay between thermodynamics, supersaturation of the aqueous solution with respect to the possible solid solutions and the crystallographic control of the surfaces on the cation incorporation, and indicates that there are many differences between the crystal growth of solid solutions and phases with fixed composition.  相似文献   

11.
Soil salinity and sodicity are escalating problems worldwide, especially in arid and semiarid regions. A laboratory experiment was conducted using soil column to investigate leaching of soluble cations during reclamation process of a calcareous saline–sodic soil (CaCO3?=?20.7%, electrical conductivity (EC)?=?19.8 dS m?1, sodium absorption ratio (SAR)?=?32.2[meq L?1]0.5). The amendments consisted of control, cattle manure (50 g kg?1), pistachio residue (50 g kg?1), gypsum (5.2 g kg?1; equivalent of gypsum requirement), manure + gypsum and pistachio residue + gypsum, in three replicates which were mixed thoroughly with the soil, while sulfuric acid as an amendment was added to irrigation water. To reflect natural conditions, after incubation period, an intermittent irrigation method was employed every 30 days. The results showed that EC, SAR, and soluble cations of leachate for the first irrigation step were significantly higher than those of the subsequent leaching runs. Moreover, the concentration of removed soluble cations was lower for the control and gypsum-treated soils. It was found that among applied amendments, treatments containing cattle manure showed higher concentrations of sodium, calcium, and magnesium in the leachate, while due to pistachio residue application, further amount of potassium was removed out of soil column. The addition of pistachio residue resulted in the highest reduction in soil salinity and sodicity since the final EC and exchangeable sodium percentage dropped to 18.0% and 11.6% of their respective initial values, respectively. In the calcareous soil, solubility of gypsum found to be limited, in contrast, when it was added in conjunction with organic amendments, greater amounts of sodium were leached.  相似文献   

12.
The results of experimental studies of ion exchange properties of Co-bearing ferromanganese crusts in the Magellan Seamounts (Pacific Ocean) are discussed. Maximum reactivity in reactions with the participation of manganese minerals (Fe-vernadite, vernadite) is typical of Na+, K+, and Ca2+ cations, whereas minimum activity is recorded for cations Pb2+ and Co2+. The exchange complex of ore minerals in crusts is composed of Na+, K+, Ca2+, Mg2+, and Mn2+ cations. The exchange capacity of manganese minerals increases from the alkali metal cations to rare and heavy metal cations. Peculiarities of the affiliation of Co2+, Mn2+, and Mg2+ cations in manganese minerals of crusts are discussed. In manganese minerals, Co occurs as Co2+ and Co3+ cations. Metal cations in manganese minerals occur in different chemical forms: sorbed (Na+, K+, Ca2+, Mn2+, Co2+, Cu2+, Zn2+, Cd2+, and Pb2+); sorbed and chemically bound (Mg2+, Ni2+, Y3+, La3+, and Mo6+); and only chemically bound (Co3+). It is shown that the age of crust, its preservation time in the air-dry state, and type of host substrate do not affect the ion exchange indicators of manganese minerals. It has been established that alkali metal cations are characterized by completely reversible equivalent sorption, whereas heavy metal cations are sorbed by a complex mechanism: equivalent ion exchange for all metal cations; superequivalent, partly reversible sorption for Ba2+, Pb2+, Co2+, and Cu2+ cations, relative to exchange cations of manganese minerals. The obtained results refine the role of ion exchange processes during the hydrogenic formation of Co-bearing ferromanganese crusts.  相似文献   

13.
Gypsum and celestine crystals coexisting with those of calcite and dolomite in marble geodes from the Apuan Alps (Italy) display oxygen-isotopic ratios of between +25.8 and +27.8‰ relative to SMOW, with an average value of +27.2‰.The δ 18O and δ 13C values of calcite-dolomite-marble assemblages fall in the range of marine limestone, the dolomites being enriched by about 1‰ both for oxygen and carbon isotopes.The homogeneity of gypsum and celestine δ 18O values seems to reflect an isotopic equilibrium of sulfate ions with environmental water, prior to their precipitation and at a temperature higher than 40°C.The calcite-dolomite pairs are not in isotopic equilibrium, and their oxygen-isotopic fractionations cannot be used as indicators of their deposition temperatures.  相似文献   

14.
Coprecipitation of barite with trace constituents was simulated with consideration of aqueous speciation and complexation, mixing properties for the binary solid solutions (Zhu, this issue), precipitation and dissolution kinetics, and advective-dispersive transport. Speciation-solubility modeling was used to reproduce BaSO4-RaSO4 coprecipitation experimental results, and to calculate CrO42− aqueous concentrations in equilibrium with a Ba(SO4,CrO4) solid solution. Kinetic reaction path modeling was used to simulate the coprecipitation of barite with RaSO4 to form an onion-like chemically zoned solid upon the cooling of oil field brine.A one-dimensional coupled reactive mass transport model shows a strikingly different transport pattern for the tracer Ra2+, when the dominant attenuation reaction is with solid solution (Ba, Ra) SO4 as compared to the case when it is controlled by pure RaSO4 and barite solids under local equilibrium conditions. A self-enrichment of Ra2+ in the groundwater and aquifer solid matrix—higher concentrations of Ra2+ downstream from the reaction front—results from the coprecipitation reaction and advective-dispersive transport. This self-enrichment process generates a secondary tracer source, which has tracer concentrations higher than that of the original source. On the other hand, coprecipitation reactions can reduce Ra2+ concentrations in groundwater to a much lower level (below ppb) than that of pure RaSO4(c) solubility (near ppm), which has been used to establish the Ra2+ concentration limits in groundwater, soil, and nuclear waste repositories.  相似文献   

15.
The total mineralization of solutions squeezed out of montmorillonite clay saturated in sea water was determined at different overburden pressures. The subsequent fractions of expelled solutions were also analysed for various anions (Cl?, SO2-4, HCO?3, F?) and cations (Na+, K+, Mg2+, Ca2+, B3+). The results indicate that the concentrations of squeezed-out solutions during the initial stages of compaction (at pressures up to 35 kg/cm2) are slightly higher than that of interstitial solution present initially. The concentration of squeezed-out solution goes through a maximum, or at least remains constant, before starting to decrease with increasing overburden pressure.  相似文献   

16.
《Applied Geochemistry》1999,14(4):433-445
Coprecipitation of radionuclides with secondary solids is currently neglected in safety assessments for radioactive waste repositories, although this process is thought to be important in limiting radionuclide solution concentrations. This paper provides a systematic review of laboratory data on metal coprecipitation with calcite, presented in the form of phenomenological partition coefficients. The aim of this investigation is to provide a consistent set of parameter values for the quantitative modelling of radionuclide coprecipitation with calcite, which will be the dominant alteration product in cementitious repositories accessed by carbonate-rich groundwater.From the data reviewed, empirical correlations have been derived that relate experimentally determined partition coefficients (λMe) to measurable chemical properties of the coprecipitated metals (ionic radii and sorption parameters of the incorporated trace metals, solubility products of the pure metal carbonates). These correlations have then been used to predict the partition coefficients of radionuclides for which no laboratory data exist.Such predictions indicate that the actinides will partition strongly into calcite under reducing conditions (λMe ∼200–1000 for trivalent, λMe ∼20–200 for tetravalent actinides). Nickel(II) incorporation will be moderate (λMe ∼1–10), while incorporation of ions like U(VI), Cs(I), Sr(II) and Ra(II) in calcite will be weak (λMe<1).In spite of substantial uncertainties, the estimated partition coefficients are sufficiently accurate to allow a semi-quantitative evaluation of the effect of radionuclide coprecipitation with calcite in limiting radionuclide solution concentrations in well characterised repository environments.  相似文献   

17.
In order to study the major ion chemistry and controls of groundwater, 65 groundwater samples were collected and their major ions measured from wells within Lhasa River Basin. Groundwater has the characteristics of slightly alkaline and moderate total dissolved solid (TDS). TDS concentration ranged from 122.0 to 489.9 mg/L with a median value of 271.2 mg/L. Almost all the groundwater samples suited for drinking and irrigation. The major cations of groundwater are Ca2+ and Mg2+, accounting for 59.6 and 31.3% of the cations, respectively. Meanwhile, HCO3? and SO42? constituted about 56.7 and 36.9% of the anions, respectively, in Lhasa River Basin. The hydrochemical type of groundwater is HCO3-SO4-Ca-Mg. The chemical composition of groundwater samples located in the middle of Gibbs model, which indicates that the major chemical process of groundwater is controlled by rock weathering. Carbonate weathering was the dominant hydro-geochemical process controlling the concentration of major ions in groundwater within Lhasa River Basin, but silicate weathering also plays an important role.  相似文献   

18.
The rate of silica removal from two montmorillonites (Chambers and Polkville) has been measured as a function of time, temperature, solution composition, and exchange ion on the clay. Silica removal rate increased with temperature from 200 to 350°C, decreased with time, and could be approximated initially by a parabolic rate law. Solution composition influenced silica removal rate by determining the exchange population of the clay; silica removal is most rapid when K-exchange ions are present. Thus increasing the concentration of K+ accelerated silica removal, whereas increasing the concentration of Na+, Ca2+, and Mg2+ inhibited silica removal. Activation energies for silica removal range from 5 to 10 kcal/mol. The largest values are associated with the largest concentrations of inhibitor ions in solution. Activation energies of this magnitude suggest that the rate-limiting step for silica removal is transport through a hydrated, expanded interlayer space. Application of experimental results to diagenesis in moderately to deeply buried sediments suggests that K+ uptake by montmorillonite may precede and accelerate illite formation.  相似文献   

19.
Sabkhas are ubiquitous geomorphic features in eastern Saudi Arabia. Seven brine samples were taken from Sabkha Jayb Uwayyid in eastern Saudi Arabia. Brine chemistry, saturation state with respect to carbonate and evaporate minerals, and evaporation-driven geochemical reaction paths were investigated to delineate the origin of brines and the evolution of both brine chemistry and sabkha mineralogy. The average total dissolved solids in the sabkha brines is 243 g/l. The order of cation dominance is Na+   >>  Mg2+ >>  Ca2+>K+, while anion dominance is Cl− >> SO4 2− >> HCO3 −. Based on the chemical divide principle and observed ion ratios, it was concluded that sabkha brines have evolved from deep groundwater rather than from direct rainfall, runoff from the surroundings, or inflow of shallow groundwater. Aqueous speciation simulations show that: (1) all seven brines are supersaturated with respect to calcite, dolomite, and magnesite and undersaturated with respect to halite; (2) three brines are undersaturated with respect to both gypsum and anhydrite, while three brines are supersaturated with respect to both minerals; (3) anhydrite is a more stable solid phase than gypsum in four brines. Evaporation factors required to bring the brines to the halite phase boundary ranged from 1.016 to 4.53. All reaction paths to the halite phase boundary follow the neutral path as CO2 is degassed and dolomite precipitates from the brines. On average, a sabkha brine containing 1 kg of H2O precipitates 7.6 g of minerals along the reaction path to the halite phase boundary, of which 52% is anhydrite, 35.3% is gypsum, and 12.7% is dolomite. Bicarbonate is the limiting factor of dolomite precipitation, and sulfate is the limiting factor of gypsum and anhydrite precipitation from sabkha brines.  相似文献   

20.
Partitioning of Ni2+, Co2+, Fe2+, Mn2+ and Mg2+ between olivine and silicate melts has been determined near the join (Mg0.5·-Fe0.5)2SiO4-K2O 4SiO2 and for seven different basaltic compositions. The experiments were made at 1 atm total pressure, 1500-1100°C, and under moderate to reducing oxygen fugacities. The concentration factor, defined as KMO = (MO)ol/(MO)liq (molar ratio), increases markedly for all the cations studied as the olivine component of the liquid decreases. Much of the increase in KMO is considered as due to the compositional effect of the coexisting liquid: the temperature effect on KMO is probably opposite to the compositional effect (KMO decreases as temperature decreases).The partition coefficient KMO-MgO = (MO/MgO)ol/(MO/MgO)liq for the reaction, Mol2+ + Mgliq2+ = Mliq2+ + Mgol2+. is relatively constant over a wide range of SiO2 content of the liquid, except in the case of Ni2+. The partition coefficients have similar ranges both in synthetic and natural rock systems: KNiO-MgO = 1.8–3.0, KCoO-MgO = 0.6–0.8, KFeO-MgO = 0.27–0.38, and KMnO-MgO = 0.23–0.32. There is a systematic variation in the partition coefficient KMO-MgO with the composition of liquid; KMO-MgO increases with increasing SiO2 content of melt. The partition coefficients for Co2+, Fe2+ and Mn2+ are useful to test the equilibration of olivine with magma of a wide compositional range.  相似文献   

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