Effect of orthophosphate on the dissolution kinetics of biogenic magnesian calcites     

Lynn M. Walter  Jeffrey S. Hanor 《Geochimica et cosmochimica acta》1979,43(8):1377-1385

The dissolution behavior of two biogenic Mg-calcites, the echinoid, Tripneustes (12 mol% MgCO₃), and the red alga, Neogoniolithon (18 mol% MgCO₃ plus brucite), was studied using free-drift methods in distilled water and phosphate-spiked solutions at 25°C and P_CO2 = 1 atm. Small concentrations of phosphate strongly inhibit dissolution rates. Inhibition increases with increased phosphate levels and with approach toward saturation. Near saturation, dissolution rates are reduced by 10³ by the presence of 0.6 μmol adsorbed-P/m². The magnitude of phosphate inhibition is similar to that observed for low-Mg calcite, and like calcite, the mechanism of inhibition is probably by adsorption of P at surface kink sites. Phosphate appears to inhibit removal of Mg and Ca equally during Mg-calcite dissolution. Rates of brucite dissolution are unaffected by phosphate.Mg-calcites containing >8.5 mol% MgCO₃ should be thermodynamically unstable relative to aragonite in most natural waters. Recent work, however, shows that in contrast to its effect on the behavior of Mg-calcites. phosphate does not inhibit aragonite dissolution. The presence of phosphate might thus reverse the relative stability of these two minerals during diagenesis of shallow marine carbonate sediments.  相似文献   

Americium interaction with calcite and aragonite surfaces in seawater     

P.M. Shanbhag  John W. Morse 《Geochimica et cosmochimica acta》1982,46(2):241-246

Observations of the distribution of ²⁴¹Am in the marine environment indicate that Am has a high affinity for solid surfaces. The adsorption of Am onto calcite and aragonite surfaces from seawater and related solutions has been studied, in order to establish the interaction of Am with a major component of many marine sediments. Results indicate that Am is rapidly and strongly adsorbed. This occurs even when both dissolved Am concentrations and solid to solution ratios are low. The minimum value for $\text{K}{}_{\mathrm{D}}$ determined is 2 × 10⁵. Measurements of reaction kinetics established that Am is adsorbed from seawater at 40 times the rate per unit surface area on synthetic aragonite that it is on synthetic calcite. Approximately 15% of the difference is attributable to epitaxial influences, with the remainder being due to enhanced site competition by Mg on calcite relative to aragonite. The adsorption rate is first order with respect to Am concentration, but follows approximately the square root of the solid surface area to solution volume ratio.Adsorption rate of Am on biogenic aragonite and Mg-calcites are, within a given particle size range, close to equal. It is not possible to normalize these adsorption rates to surface area due to the differing microporous structure of biogenic carbonates. The Am adsorption rates on a shallow water calcium carbonate-rich sediment gave results which were predicted from, its mineralogie mixture of components.  相似文献   

Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica   总被引：3，自引：0，他引：3  

LYNTON S. LAND 《Sedimentology》1973,20(3):411-424

Wholesale removal of the unstable carbonate phases aragonite and Mg-calcite, and precipitation of calcite and dolomite is currently taking place where phreatic waters (the modern water table) invade 120,000-year-old Pleistocene biolithites (Falmouth Formation), North Jamaica. Pleistocene rocks presently in the vadose zone are relatively unaltered, and consist of mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a ‘water table cave’ is commonly encountered. Below the water table the rocks are invariably more highly altered than those above. Mg-calcites are very rare, and considerable dissolution of aragonite has commonly occurred. Dolomite occurs as 8–25 μm, subhedral to euhedral crystals replacing micrite, or precipitated as void linings. The isotopic composition of the dolomite (δO¹⁸=-1·0 %0, δC¹³=-8·4 %0), and its high strontium content (3000 p.p.m.) suggest precipitation as CO₂-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve Mg-calcites and Sr-rich aragonites, and de-gas. Because some dolomitized rocks are enriched in magnesium relative to unaltered biolithites, addition of magnesium to the system is necessitated, and is probably derived from sea water in the mixing zone. Phreatic meteoric diagenesis is thus demonstrated to be a rapid process, and to be capable of dolomitization.  相似文献   

Sm-Nd in marine carbonates and phosphates: Implications for Nd isotopes in seawater and crustal ages     

H.F Shaw  G.J Wasserburg 《Geochimica et cosmochimica acta》1985,49(2):503-518

This study explores the possibility of establishing Nd isotopic variations in seawater over geologic time. Calcite, aragonite and apatite are examined as possible phases recording seawater values of ?_Nd. Modern, biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have Nd concentrations of from 0.2 to 70 ppb, showing that primary marine CaCO₃ contains little REE and that Nd/Ca is not greatly enhanced relative to seawater during carbonate precipitation. Very young marine limestone and dolomite containing no continental detritus have ~200 ppb Nd. All the carbonates are LREE enriched ($\text{?0.16 ≤f}{}^{\mathrm{<mtext>Sm</mtext><mtext>Nd</mtext>}}\text{≤?0.45}$). Modern and very young Atlantic and Pacific carbonates have ?_Nd in the range of shallow Atlantic and Pacific seawater respectively, implying that they derive their REE from local seawater. The Nd in well preserved carbonate fossils is ≤4 × 10⁴ ppb, much greater than in their modern counterparts but like the high values found for carbonates in other studies. We believe the high REE contents (at the 500 ppb level) in some detritusfree carbonates are due to REE-rich Fe-hydroxide in/on the carbonate. In favorable cases, such material may record seawater ?_Nd values, however introduction of extraneous REE may obscure the original isotopic composition of pure CaCO₃ because of its very low intrinsic primary REE abundance.Modern biogenic apatite is also shown to have very low REE content (<150 ppb Nd) but appears to quickly scavenge REE from seawater. Inorganically precipitated apatite from phosphorites has high concentrations of seawater-derived REE. Young phosphorite apatite from the Atlantic and Pacific oceans has ?_Nd in the range of the seawater from these oceans. Older apatite samples of similar age from different localities bordering common oceans record similar values of ?_Nd(T). Sedimentary apatite has ?_Sr(T) values in good agreement with the curves for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ of seawater as a function of time. Individual conodonts from a single formation yield the same ?_Sr(T) and ?_Nd(T). Other workers have shown that sedimentary apatite preserves seawater REE patterns. These characteristics suggest that sedimentary apatite can be used to determine ?_Nd(T) in ancient seawater. The seawater values so inferred range between ?1.7 and ?8.9 over the last 700 my and lie in the range of modern seawater, showing no evidence for drastic changes. High values of seawater ?_Nd(T) in the Triassic and latest Precambrian may correlate with the breakup of large continental landmasses. The initial ?_Nd(T) =?15.0 of a 2 AE old phosphorite implies the presence of ~ 1.5 AE old continental crust at 2 AE ago. The approach outlined here can be used to constrain the age of the exposed crust as a function of time.  相似文献   

The effect of cobalt ion on nucleation of calcium-carbonate polymorphs     

David M. Barber  Philip G. Malone  Robert J. Larson 《Chemical Geology》1975,16(3):239-241

Cobalt, like Mg, may cause the precipitation of aragonite rather than calcite in aqueous solutions due to the adsorption and crystal poisoning of calcite by a hydrated ion. Solutions containing NaCl and CaCl₂, having the ionic strength and Ca content of seawater (35‰ salinity), were spiked with known amounts of CoCl₂. Calcium carbonate was precipitated by the addition of 0.7 ml of 1 M Na₂CO₃. All experimental runs were made at 25°C, and all products were examined by X-ray diffraction. At low concentrations of Co (< 5·^?4M) calcite and vaterite formed. At concentrations from 5·10^?4 M to 2·10^?3M, the products consisted of combinations of calcite and vaterite; aragonite and calcite; aragonite and vaterite; calcite, vaterite and aragonite. In solutions of 3·10^?3M CoCl₂, most precipitates were aragonite with only one sample containing a small amount of calcite. All precipitates from 5·10^?3M CoCl₂ solutions either contained aragonite or were amorphous. Solutions with concentrations of 1 · 10^?2M CoCl₂ produced only amorphous precipitates. All precipitates contained an amorphous violet phase, assumed to be basic cobaltous carbonate (2CoCO₃·Co(OH)₂·H₂O).  相似文献   

Salinity stress in harpacticoid copepods     

Colin M. Finney 《Estuaries and Coasts》1979,2(2):132-135

Three species of intertidal harpacticoid copepods,Tigriopus japonicus, Tachidius brevicornis andTisbe sp., were tested for their response to salinities ranging from 0‰ to 210‰. At 90‰Tigriopus became dormant, but could be reanimated if placed in 30‰ seawater within 18 hours.Tachidius became dormant at 60‰ but could also be revived if placed in 30‰ seawater.Tisbe died shortly after an exposure to seawater of 45‰. Death was brought about inTigriopus andTachidius by salinities of 150‰. Naupliar, copepodite, and adult stages ofTigriopus withstood the salinities equally well, while egg sacs could tolerate five times the length of exposure of these stages. The respiratory rate ofTigriopus was 3.76 (±1.32) μl 0₂ hr^?1 for active adults and 0.03 (±0.01) nl 0₂ hr^?1 mg dry wt^?1 for adults in the dormant state. The duration of dormancy, with successful revival, appears to be limited to the time before the loss of the vital water content of the tissues due to the high osmotic pressure. The ability to enter dormancy in times of stress may have high survival value to some intertidal copepods.  相似文献   

Uptake of dissolved Cd by biogenic and abiogenic aragonite: a comparison with sorption onto calcite   总被引：3，自引：0，他引：3  

Manuel Prieto  Pablo Cubillas 《Geochimica et cosmochimica acta》2003,67(20):3859-3869

The uptake of Cd²⁺ by aragonite and calcite is investigated by combining macroscopic measurements with some qualitative sorption experiments performed in a hydrogel medium. Both biogenic and abiogenic aragonites were studied in order to evaluate the process on materials with different textures. Assuming that sorption occurs by surface precipitation of metal-bearing solids, the gel produces a drastic decrease in the nucleation density, which allows for the precipitation of crystallites that are large enough to be analysed by scanning electron microscopy and characterized by glancing-incidence X-ray techniques. The macroscopic study reveals that aragonite is a powerful sorbent for cadmium in aqueous environments. Microscopic observations indicate that cadmium is sorbed onto aragonite by surface precipitation of (Cd, Ca)CO₃ solid solutions with a calcite-type structure. The precipitating individuals grow randomly oriented on the surface to reach sizes in the micrometre range. As a consequence, the concentration of cadmium in the aqueous solution decreases dramatically to values controlled by the low solubility of the cadmium-rich end member. This mechanism involves simultaneous dissolution-crystallization and is the same for both abiogenic and biogenic aragonites, the only difference being a result of the higher specific surface area of the biogenic starting material. Long-term uptake of cadmium by calcite occurs through a similar dissolution-crystallization mechanism, the final outcome being virtually the same, that is, surface precipitation of (Cd,Ca)CO₃ solid solutions. In this case, however, substrate and precipitate are isostructural and the process occurs by oriented overgrowth of thin lamellar crystallites, which spread to quickly cover the surface by a layer a few nanometers thick. This epitaxial layer armors the substrate from further dissolution, so that the process stops when only a small amount of cadmium has been removed from the fluid. As a result, the “sorption capacity” of calcite is considerably lower than that of aragonite. The study illustrates reaction pathways and “partial” equilibrium endpoints in surface-precipitation processes involving solid solutions.  相似文献   

Isotopic exchange in mineral-fluid systems. I. Theoretical evaluation of oxygen isotopic exchange accompanying surface reactions and diffusion     

David R. Cole  Hiroshi Ohmoto  Antonio C. Lasaga 《Geochimica et cosmochimica acta》1983,47(10):1681-1693

  相似文献   

Towards the establishment of a reliable proxy for the reactive surface area of smectite     

Volker Metz  Hadas Raanan  Dirk Bosbach 《Geochimica et cosmochimica acta》2005,69(10):2581-2591

Reactive surface area is one of the key parameters for studying the kinetics of mineral dissolution. The common practice in experimental kinetics is to normalize the dissolution rate to the surface area measured by the BET method. The relationship between BET surface area and the reactive surface area is not trivial in minerals such as smectites, which possess both internal and external surface areas, and in which the dissolution is controlled by the chemical attack on the edge surface. The present study examines two proxies for the reactive surface area of the Clay Mineral Society reference smectite SAz-1: BET surface area and the edge surface area measured using AFM.Since smectites are very microporous, their BET surface area is strongly influenced by the degassing procedure. It is demonstrated that outgassing the smectite powder at 135°C in a 15 mL min⁻¹ N₂ gas flow for at least 24 hours minimizes contribution from micropores to less than 11% of the BET surface area.Following dissolution experiments in solutions with a low electrolyte concentration, the BET surface area increased from 34 ± 2 m² g⁻¹ in raw SAz-1 to 127 ± 13 m² g⁻¹ in SAz-1 sample recovered from dissolution experiments. This increase in BET surface area is explained by a decrease in the average size of the smectite aggregates, and by an increase in microporosity due to the depletion in the major interlayer cation, i.e., Ca²⁺. As the BET surface area of the raw smectite sample includes considerably less microporosity compared to the BET surface area of the smectite recovered from dissolution experiments, the former is a better approximation of the external surface area of the dried sample powder.AFM measurements show that there is no correlation between the specific external surface area of the sample and its specific edge surface area. This observation is explained by the platy morphology of the smectite particle in which the specific external surface area depends linearly on the height reciprocal, whereas the specific edge surface area is independent of the particles height and depends linearly on the sum of the reciprocals of the length of the axes. Therefore, there is no reason to expect a correlation between the BET and the edge surface area. Our results show that the edge surface area (4.9 ± 0.7 m² g⁻¹) of the smectite particles cannot be predicted based on its external surface area (136 ± 20 m² g⁻¹). Therefore, the BET surface area cannot serve as a proxy for the reactive surface area. We suggest using AFM measurements of the specific edge surface area as an alternative proxy for the reactive surface area of smectite.  相似文献   

Kinetic and thermodynamic factors controlling the distribution of SO₃²⁻ and Na⁺ in calcites and selected aragonites     

Eurybiades Busenberg  L. Niel Plummer 《Geochimica et cosmochimica acta》1985,49(3):713-725

Significant amounts of SO₄^2?, Na⁺, and OH^? are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO₄^2?. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na⁺, but very low concentrations of SO₄^2?. The SO₄^2? content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na⁺ and SO₄^2? increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO₄^2? beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO₄^2? is the same as that of aragonite. Na⁺ appears to have very little effect on the solubility product of calcites. The amounts of Na⁺ and SO₄^2? incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ($\text{D}$) of SO₄^2? in calcite at 25.0°C and 0.50 molal NaCl is described by the equation $\text{D = k}{}_0\text{+ k}{}_1\text{R}$ where $\text{k}{}_0$ and $\text{k}{}_1$ are constants equal to $\text{6.16 × 10}{}^{\mathrm{?6}}$ and $\text{3.941 × 10}{}^{\mathrm{?6}}$, respectively, and $\text{R}$ is the rate of crystal growth of calcite in mg·min^?1·g^?1 of seed. The data on Na⁺ are consistent with the hypothesis that a significant amount of Na⁺ occupies interstitial positions in the calcite structure. The distribution of Na⁺ follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na⁺ distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na⁺ contents of calcites are not very accurate indicators of environmental salinities.  相似文献   

Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO₂ and “ocean acidification”: Role of high Mg-calcites     

John W. Morse  Andreas J. Andersson 《Geochimica et cosmochimica acta》2006,70(23):5814-5830

Carbonate-rich sediments at shoal to shelf depths (<200 m) represent a major CaCO₃ reservoir that can rapidly react to the decreasing saturation state of seawater with respect to carbonate minerals, produced by the increasing partial pressure of atmospheric carbon dioxide (pCO₂) and “acidification” of ocean waters. Aragonite is usually the most abundant carbonate mineral in these sediments. However, the second most abundant (typically ∼24 wt%) carbonate mineral is high Mg-calcite (Mg-calcite) whose solubility can exceed that of aragonite making it the “first responder” to the decreasing saturation state of seawater. For the naturally occurring biogenic Mg-calcites, dissolution experiments have been used to predict their “stoichiometric solubilities” as a function of mol% MgCO₃. The only valid relationship that one can provisionally use for the metastable stabilities for Mg-calcite based on composition is that for the synthetically produced phases where metastable equilibrium has been achieved from both under- and over-saturation. Biogenic Mg-calcites exhibit a large offset in solubility from that of abiotic Mg-calcite and can also exhibit a wide range of solubilities for biogenic Mg-calcites of similar Mg content. This indicates that factors other than the Mg content can influence the solubility of these mineral phases. Thus, it is necessary to turn to observations of natural sediments where changes in the saturation state of surrounding waters occur in order to determine their likely responses to the changing saturation state in upper oceanic waters brought on by increasing pCO₂. In the present study, we investigate the responses of Mg-calcites to rising pCO₂ and “ocean acidification” by means of a simple numerical model based on the experimental range of biogenic Mg-calcite solubilities as a function of Mg content in order to bracket the behavior of the most abundant Mg-calcite phases in the natural environment. In addition, observational data from Bermuda and the Great Bahama Bank are also presented in order to project future responses of these minerals. The numerical simulations suggest that Mg-calcite minerals will respond to rising pCO₂ by sequential dissolution according to mineral stability, progressively leading to removal of the more soluble phases until the least soluble phases remain. These results are confirmed by laboratory experiments and observations from Bermuda. As a consequence of continuous increases in atmospheric CO₂ from burning of fossil fuels, the average composition of contemporary carbonate sediments could change, i.e., the average Mg content in the sediments may slowly decrease. Furthermore, evidence from the Great Bahama Bank indicates that the amount of abiotic carbonate production is likely to decline as pCO₂ continues to rise.  相似文献   

Formation of low‐magnesium calcite at cold seeps in an aragonite sea     

Dong Feng  Harry H. Roberts  Samantha B. Joye  Ezat Heydari 《地学学报》2014,26(2):150-156

This study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The ¹³C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca²⁺‐enriched and Mg²⁺‐depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H₂S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.  相似文献   

Concentration of Mn and separation from Fe in sediments—I. Kinetics and stoichiometry of the reaction between birnessite and dissolved Fe(II) at 10°C     

Dieke Postma 《Geochimica et cosmochimica acta》1985,49(4):1023-1033

Redox reactions between Fe²⁺ in solution and Mn-oxides are proposed as a mechanism for concentration of Mn in sediments both during weathering and diagenesis in marine sediments, e.g. the formation of Mn-nodules.If such a mechanism is to be effective, then reaction rates between Fe²⁺ and Mn-oxides should be fast. The kinetics and stoichiometry of the reaction between dissolved Fe²⁺ and synthetically prepared birnessite (Mn₇O₁₃·5H₂O) were studied experimentally in the pH range 3–6.Results show a stoichiometry which at pH < 4 conforms to a simple reaction between Fe²⁺ and birnessite, releasing Mn²⁺ and Fe³⁺ to the solution. At pH > 4 FeOOH is precipitated and excess Fe²⁺ consumption compared to the theoretical stoichiometry is observed. The excess Fe²⁺ consumption is not due to a formation of a quantitative MnOOH layer but rather to adsorption.Reaction kinetics are very fast at pH < 4 and change at pH 4 to a slower mechanism. At pH > 4 the reaction is fast initially until 17% of the bimessite has dissolved and changes then to a slower stage. The later stage can be described by the equation: $\text{J = km}{}_0\text{(H}{}^{\mathrm{+}}\text{)}{}^{\mathrm{?0.45}}\text{[Fe}{}^{\mathrm{2+}}\text{]}{}^{\mathrm{\gamma }}\text{(}\text{m}\text{m}{}_0\text{)}{}^{\mathrm{\beta }}$ where J is the overall rate of Mn²⁺ release, m₀ and m the mass of birnessite at time t = 0 and t > 0, β = 6.76?0.94 pH and γ has values of 0.76 at pH 5 and 0.39 at pH 6. The rate constant k is 7.2·10^?7 moles s^?1 g^?1 (moles/1)^?0.31 at pH 5 and 9.6·10^?8 moles s^?1 g^?1 (moles/1)^0.06 at pH 6.Diffusion calculations show that the rate is controlled by surface reaction and it is tentatively proposed that the availability of vacancies in octahedral [MnO₆]sheets of the birnessite surface could be rate controlling. It is concluded that reactions between Fe(II) and birnessite, and probably other Mn-oxides, are fast enough to be important in natural environments at the earth surface.  相似文献   

Secular variations in seawater chemistry controlling dolomitization in shallow reflux systems: insights from reactive transport modelling          下载免费PDF全文

Tatyana Gabellone  Fiona Whitaker 《Sedimentology》2016,63(5):1233-1259

Dolomitization often plays a critical role in the pore network development of platform carbonates, with implications for reservoir quality distribution. Understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies. Here, the role of secular variations in seawater Mg/Ca as a control on dolomitization and early porosity modification was evaluated using one‐dimensional reactive transport models and fluids based on modern (aragonite sea), Mississippian and Aptian (calcite sea) seawaters. The sensitivity of dolomitization to a range of extrinsic controls (brine salinity, temperature, fluid flow rate and pCO₂) and to intrinsic reactivity of the sediments (effective reactive surface area) was also explored. Simulations suggest faster calcite replacement by dolomite for seawaters with higher Mg/Ca, indicating that dolomitization potential is determined more by Mg/Ca rather than saturation index. Increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density‐driven fluid flux. In addition to brine composition, effective surface area of precursor sediments and temperature exert a critical control on replacement rate, while secular variations of pH and carbonate alkalinity associated with changes in pCO₂ are only secondary controls. Above flow rates of 0·01 m yr^?1 replacive dolomitization is reaction‐limited rather than flux limited, favouring alteration of fine‐grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low. Post‐replacement dolomite cementation is flux dependent, and thus favoured in areas of high head gradient and high permeability sediments and, contrary to replacement, supersaturation is a more important driver than Mg/Ca. While uncertainties remain regarding low‐temperature dolomitization kinetics, the capability of numerical simulations to decouple individual controls provides new insights which can be used, in conjunction with traditional comparative sedimentology, to generate more rigorous conceptual models for individual reservoir settings.  相似文献   

Dissolution Kinetics of Calcite in NaCl–CaCl2–MgCl2 Brines at 25 °C and 1 bar pCO2     

Dwight K. Gledhill  John W. Morse 《Aquatic Geochemistry》2004,10(1-2):171-190

Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl₂–MgCl₂ solutions at 1 bar (0.1 MPa) pCO₂ and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca²⁺:Mg²⁺= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca²⁺:Mg²⁺= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 ? Omega;) ⁿ , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10^?2 mol m^?2 hr^?1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10^?2 mol m^?2 hr^?1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca²⁺:Mg²⁺ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO₂ sequestration.  相似文献   

Sedimentary iron monosulfides: Kinetics and mechanism of formation     

Albert J Pyzik  Sheldon E Sommer 《Geochimica et cosmochimica acta》1981,45(5):687-698

The reaction between hydrous iron oxides and aqueous sulfide species was studied at estuarine conditions of pH, total sulfide, and ionic strength to determine the kinetics and formation mechanism of the initial iron sulfide. Total, dissolved and acid extractable sulfide, thiosulfate, sulfate, and elemental sulfur were determined by spectrophotometric methods. Polysulfides, S₄^2? and S₅^2?, were determined from ultraviolet absorbance measurements and equilibrium calculations, while product hydroxyl ion was determined from pH measurements and solution buffer capacity.Elemental sulfur, as free and polysulfide sulfur, was 86% of the sulfide oxidation products; the remainder was thiosulfate. Rate expressions for the reduction and precipitation reactions were determined from analysis of electron balance and acid extractable iron monosulfide vs time, respectively, by the initial rate method. The rate of iron reduction in moles/liter/minute was given by $\text{d(}\text{reduction}\text{Fe)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^{0.5}\text{(J}{}^{\mathrm{+}}\text{)}{}^{0.5}\text{A}{}_{\mathrm{FeOOH}}{}^1$ where S_t was the total dissolved sulfide concentration, (H⁺) the hydrogen ion activity, both in moles/ liter; and A_FeOOH the goethite specific surface area in square meters/liter. The rate constant, k, was 0.017 ± 0.002m^?2 min^?1. The rate of reduction was apparently determined by the rate of dissolution of the surface layer of ferrous hydroxide. The rate expression for the precipitation reaction was $\text{d(FeS)}\text{dt}\text{= kS}{}_{\mathrm{t}}{}^1\text{(H}{}^{\mathrm{+}}\text{)}{}^1\text{A}{}_{\mathrm{FeOOH}}{}^1$ where $\text{d(FeS)}\text{dt}$ was the rate of precipitation of acid extractable iron monosulfide in moles/liter/minute, and k = 82 ± 18 mol^?1l²m^?2 min^?1.A model is proposed with the following steps: protonation of goethite surface layer; exchange of bisulfide for hydroxide in the mobile layer; reduction of surface ferric ions of goethite by dissolved bisulfide species which produces ferrous hydroxide surface layer elemental sulfur and thiosulfate; dissolution of surface layer of ferrous hydroxide; and precipitation of dissolved ferrous specie and aqueous bisulfide ion.  相似文献   

A fundamental equation for calcite dissolution kinetics   总被引：1，自引：0，他引：1  

E.L. Sjöberg 《Geochimica et cosmochimica acta》1976,40(4):441-447

A fundamental rate equation for the dissolution of calcite in a pure 0.7 M KC1 solution has been determined. Between pH 8.0 and 10.1 the kinetics of the dissolution reaction can be expressed by the equation , where d[Ca²⁺]/dt is the rate in mole cm^?3s^?1, k is the apparent rate constant in s^?1 cm^?2, A is the calcite surface area and C is the square root of the calcite solubility constant. The apparent rate constant at 20°C is 9.5 × 10^?6s^?1cm^?2. The apparent activation energy for the reaction between 5 and 50°C is 8.4 kcal mole^?1.The reaction rate is pH independent above pH = 7.5. At pH values less than 8, [CO₃^2?] becomes negligible, and the rate becomes fast and should be dependent on the calcite surface area alone, if there is no change in mechanism.The stirring coefficient between 2.8 and 11.1 rev s^?1 is 0.33. This, together with the relatively high activation energy, indicates that the reaction is mainly chemically controlled.Interpolation of the experimental results into seawater systems gives a computed rate several magnitudes greater than the observed rate, but considerably less than that calculated for a diffusion-controlled reaction.  相似文献   

Ca-Mg inter-diffusion in synthetic polycrystalline dolomite-calcite aggregate at elevated temperatures and pressure     

Wuu-Liang Huang  Teh-Ching Liu  Pouyan Shen  Allen Hsu 《Mineralogy and Petrology》2009,95(3-4):327-340

This study measures the reaction rate of dolomite and aragonite (calcite) into Mg-calcite at 800, 850, and 900°C and 1.6 GPa. The dry synthetic dolomite-aragonite aggregate transformed very rapidly into dolomite-calcite polycrystalline aggregate while Mg-calcites formed at a relatively slow rate, becoming progressively richer in Mg with run time. We modeled the reaction progress semi-empirically by the first-order rate law. The temperature dependence of the overall transport rate of MgCO₃ into calcite can be described by the kinetic parameters (E?=?231.7 kJ/mol and A _o ?=?22.69 h^?1). Extrapolation using the Arrhenius equation to the conditions during exhumation of UHPM rocks indicates that the reaction of dolomite with aragonite into Mg-saturated calcite can be completed as the P-T path enters the Mg-calcite stability field in a geologically short time period (<1 Ky). On the other hand, the extrapolation of the rate to prograde metamorphic conditions reveals that the Mg-calcite formed from dolomitic marble in the absence of metamorphic fluid may not reach Mg-saturation until temperatures corresponding to high-grade metamorphism (e.g., >340°C and >10 My). SEM-EDS analysis of individual calcite grains shows compositional gradients of Mg in the calcite grains. The Mg-Ca inter-diffusion coefficient at 850°C is around 1.68?×?10^?14 m²/sec if diffusion is the major control of the reaction. The calculated closure temperatures for Ca-Mg inter-diffusion as a function of cooling rate and grain size reveal that Ca/Mg resetting in calcite in a dry polycrystalline carbonate aggregate (with grain size around 1 mm) may not occur at temperatures below 480°C at a geological cooling rate around 10°C/My, unless other processes, such as short-circuit interdiffusion along grain boundaries and dislocations, are involved.  相似文献   

Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry   总被引：2，自引：0，他引：2  

JAMES P. HENDRY  NIGEL H. TREWIN†  ANTHONY E. FALLICK 《Sedimentology》1996,43(5):877-900

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.  相似文献   

Solubility of aragonite in seawater—II. Effect of pressure on the onset and maintenance of dissolution     

Robert C. Cooke  Paul E. Kepkay 《Geochimica et cosmochimica acta》1980,44(8):1077-1080

The apparent solubility product (K'_sp) of aragonite in a variety of seawater compositions has been determined at pressures from 0 to 1019 atm and a nomogram developed to allow the determination of the K'_sp when the apparent ion product (AIP) at one atmosphere and the collection depth of a water sample are known.This nomogram provides a basis from which the onset of aragonite dissolution can be determined for conditions representative of aragonite sedimentation through the changing water masses of the open ocean.  相似文献