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1.
Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1 M NaCl, pH = 9), and to solution with a supersaturation (−Δμ/kT) of 5.5 (pH = 9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca2+ rich (CaxMg2 − x(CO3)2, 1.7 > x > 1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite-water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions.  相似文献   

2.
In situ Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) studies on dolomite (101?4) were performed during exposure to supersaturated aqueous solutions (supersaturated in dolomite, calcite, aragonite, vaterite, huntite and magnesite) at pH = 9 at various Ca2+/Mg2+ aqueous ion activity ratios. At high saturation ratios, rapid growth of a single layer (∼3 Å thick) of a carbonate followed by much slower growth of a second layer was observed. Growth of the second layer was highly inhibited, suggesting that the first layer was essentially self-limited, and inhibited further layer-by-layer growth. The growth of the first layer was observed over a wide range of Ca2+/Mg2+ ratios, suggesting that the dolomite surface is favorable to formation of a range of Ca-Mg carbonates. LFM data revealed contrast in the tip-surface frictional forces on the first grown layer, but this contrast was only observed in layers grown from middle to high Ca2+/Mg2+ solutions. Thus, LFM may have detected or responded to differences in the structure and/or composition between the first layer relative and the dolomite substrate. Dissolution of the first layer occurred from significantly supersaturated solutions relative to ordered stoichiometric dolomite permitting an estimate of the excess interfacial strain energy of up to 10 mJ/m2.  相似文献   

3.
Calcium-poor dolomite from the sabkhas of Kuwait   总被引：1，自引：0，他引：1  
Small quantities of a rare Ca-deficient dolomite have been found coexisting in thin layers and alternating with the normal Ca-rich variety in the Holocene sabkhas of southern Kuwait. The Ca-poor dolomite has a molar composition of Ca46–49, whereas the Ca-rich type is Ca51–56. The former type has been found only in two localities of the supratidal zone that are not subjected to tidal flooding today, and mostly within the fluctuating ground-water table in these zones. The thin layers are either carbonate mud-rich or pellet-rich, and the amount of Ca-poor dolomite is never more than 4 to 6 per cent by weight of the total carbonate fraction. The dolomites vary in size between 2 μm and 5 μm and exhibit characteristic rhombic crystal morphologies.  相似文献   

4.
Hydrothermal experiments with H2O-CO2 fluids at Pfluid = 6 kbar yielded the following quilibrium conditions for reactions important in metamorphosed siliceous dolomites (T = °C; X = Xco2): (3) dolomite + 2 quartz = diopside + 2 CO2T = 620 ± 8X = 0.73 ± 0.03 (5) 5 dolomite + 8 quartz + H2O = tremolite + 3 calcite + 7 CO2T = 600 ± 5 550 ±5 540±5 500±5X = 0.66 ± 0.03 0.21 ± 0.03 0.21 ± 0.04 0.06 ± 0.02 (7) 3 dolomite + 4 quartz + H2O = talc + 3 calcite + 3 CO2T = 550±5 500±5 450 ±5X = 0.25 ± 0.05 0.07 ± 0.02 0.03 ± 0.02 (8) 2 dolomite + talc + 4 quartz = tremolite + 4 CO2T = 550 ± 5 540 ±5 500 ± 5X = 0.22 ± 0.03 0.21 ± 0.02 0.06 ± 0.02 A thermodynamically self-consistent 6 kbar T-XCO2, topology results by extrapolating equilibria from experimental brackets using a modified Redlich-Kwong equation for activities in H2O-CO2 mixtures. This topology restricts the assemblage talc + calcite to a narrow stability band in T-XCO2 space at XCO2 < 0.55 and T < 590°C. Accordingly, the occurrence of talc + calcite in pure siliceous dolomites metamorphosed at Pfluid = 6 kbar implies correspondingly water-rich fluids.  相似文献   

5.
The Upper Cretaceous Phosphate Formation in the Western Desert of Egypt displays a characteristic facies association that includes marine phosphorites interbedded with black shales and glauconitic sandstones. The upper part of the formation is characterized by the presence of thin phosphatic beds, which are filled-extensively-with disordered and non stoichiometric (mean MgCO3 = 41.4 ± 0.34 mol%) authigenic dolomite cement. SEM and the back scattered images of these coarse crystalline dolomite cements reveal that they display planar euhedral crystal boundaries, polymodal crystal size distribution and variable inclusion pattern. The relatively low and wide ranged δ18O (− 0.87 to − 4.15‰ VPDB) values of the dolomite cements coupled with their depleted Sr (mean = 187 ± 26 ppm) and high iron and manganese values (mean = 6851 ± 554 ppm and 11599 ± 229 ppm respectively) invoke that they were formed from mixed hypo-saline fluids within a mixing marine-meteoric zone probably during a low stand period at the vicinity of the Maastrichtian/Early Tertiary unconformity. Meanwhile, their negative δ13C (− 1.31 to − 3.56‰ VPDB) values argue for a possible involvement of isotopically light carbon, derived from degradation of organic matter, during their precipitation.  相似文献   

6.
Two strains of moderately halophilic bacteria were grown in aerobic culture experiments containing gel medium to determine the Sr partition coefficient between dolomite and the medium from which it precipitates at 15 to 45 °C. The results demonstrate that Sr incorporation in dolomite does occur not by the substitution of Ca, but rather by Mg. They also suggest that Sr partitioning between the culture medium and the minerals is better described by the Nernst equation (DSrdol = Srdol/Srbmi), instead of the Henderson and Kracek equation (DSrdol = (Sr/Ca)dol/(Sr/Ca)solution. The maximum value for DSrdol occurs at 15 °C in cultures with and without sulfate, while the minimum values occur at 35 °C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 °C, we observed that DSrdol values are greater in cultures with sulfate than in cultures without sulfate, whereas DSrdol values are smaller in cultures with sulfate than in cultures without sulfate at 15 °C.Together, our observations suggest that DSrdol is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the DSrdol. These results have implications for the interpretation of depositional environments of ancient dolomite. The results of our culture experiments show that higher Sr concentrations in ancient dolomite could reflect microbial mediated primary precipitation. In contrast, previous interpretations concluded that high Sr concentrations in ancient dolomites are an indication of secondary replacement of aragonite, which incorporates high Sr concentrations in its crystal lattice, reflecting a diagenetic process.  相似文献   

7.
Synchrotron single-crystal X-ray diffraction experiments at high-pressure and high-temperature conditions were performed up to 20 GPa and 573.0(2) K on a fully ordered stoichiometric dolomite and a partially disordered stoichiometric dolomite [order parameter, s = 0.26(6)]. The ordered dolomite was found to be stable up to approximately 14 GPa at ambient temperature and up to approximately 17 GPa at T = 573.0(2) K. The P–V data from the ambient temperature experiments were analysed by a second-order Birch–Murnaghan equation-of-state giving K 0 = 92.7(9) GPa for the ordered dolomite and K 0 = 92.5(8) GPa for the disordered dolomite. The high-temperature data, collected for the ordered sample, were fitted by a third-order Birch–Murnaghan equation-of-state resulting in K 0 = 95(6) GPa and K′ = 2.6(7). In order to compare the three experiments results, a third-order Birch–Murnaghan equation-of-state was also calculated for the ambient temperature experiments giving K 0 = 93(3) GPa, K′ = 3.9(6) for the ordered dolomite and K 0 = 92(3) GPa, K′ = 4.0(4) for the disordered dolomite. The derived axial moduli show that dolomite compresses very anisotropically, being the c-axis approximately three times more compressible than the a-axis. The axial compressibility increases as T increases, and the a-axis is the most temperature-influenced axis. On the contrary, axial compressibility is not influenced by disordering. Structural refinements at different pressures show that Ca and Mg octahedra are almost equally compressible in the ordered dolomite with K(CaO6) = 109(4) GPa and K(MgO6) = 103(3) GPa. On the contrary, CaO6 compressibility is reduced and MgO6 compressibility is increased in the disordered crystal structure where K(CaO6) = 139(4) GPa and K(MgO6) = 89(4) GPa. Disordering is found to increase CaO6 and to decrease MgO6 bond strengths, thus making stiffer the Ca octahedron and softer the Mg octahedron. Cation polyhedra are distorted in both ordered and disordered dolomites and they increase in regularity as P increases. Ordered dolomite approaches regularity at approximately 14 GPa. The increase in regularity of octahedra in the disordered dolomite is strongly affected by the very slow regularization of MgO6 with respect to CaO6. The phase transition to the high-pressure polymorph of dolomite (dolomite-II), which is driven by a significant increase in the regularity of both cations polyhedra and mineral crystal structure, occurs in the ordered dolomite at ambient temperature at approximately 14 GPa; whereas no clear evidences of phase transition were observed as regards the disordered crystal structure.  相似文献   

8.
Examination with scanning electron microscopy (SEM) and scanning force microscopy (SFM) revealed etch pits, layers and islands on dolomite crystal faces synthesized from calcite in Ca‐Mg‐Cl solutions at 200 °C and a wide variety of natural dolomites. Layers are broad, flat structures bounded by steps less than 100 nm high and greater than 1 μm wide. Islands are rounded topographic highs <20 nm high and <200 nm wide. The nanotopography of synthetic dolomite changed from islands throughout most of the reaction to layers at 100% dolomite. Island nanotopography formed on both Ca‐rich and near‐stoichiometric dolomite. Analyses of reaction products from dolomite synthesis indicates that there are no SFM‐detectable products formed in <10 h. SEM‐detectable products formed in 15 h. X‐ray diffraction (XRD)‐detectable products formed in ≈18 h, and the reaction went to completion in ≈40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for ≈20% of the total reaction time necessary to dolomitize CaCO3 completely under the experimental conditions used here. Island nano‐ topography is inferred to occur at higher degrees of supersaturation than layer nanotopography for three reasons. First, island nanotopography on synthetic calcite and gypsum forms at higher supersaturations than layer nanotopography. Secondly, island nanotopography formed in solutions with higher degrees of supersaturation with respect to dolomite. Thirdly, the greater surface roughness of a crystal face composed of islands compared with layers indicates that island surfaces have higher surface energy than layer surfaces. Therefore, the stability of island surfaces requires a higher degree of supersaturation. Because islands and layers form under a wide range of conditions, their presence provides broadly applicable criteria for evaluating relative degrees of supersaturation under which ancient dolomite formed. Comparison of synthetic dolomites with natural dolomites demonstrates (1) similar nanotopography on natural and synthetic dolomites and (2) both natural planar and non‐planar dolomite may have island nanotopography.  相似文献   

9.
One of the main effects of supergene alteration of ore-bearing hydrothermal dolomite in areas surrounding secondary zinc orebodies (Calamine-type nonsulfides) in southwestern Sardinia (Italy) is the formation of a broad halo of Zn dolomite. The characteristics of supergene Zn dolomite have been investigated using scanning electron microscopy and qualitative energy-dispersive X-ray spectroscopy, thermodifferential analysis, and stable isotope geochemistry. The supergene Zn dolomite is characterized by variable amounts of Zn, and low contents of Pb and Cd in the crystal lattice. It is generally depleted in Fe and Mn relative to precursor hydrothermal dolomite (Dolomia Geodica), which occurs in two phases (stoichiometric dolomite followed by Fe-Mn-Zn-rich dolomite), well distinct in geochemistry. Mg-rich smithsonite is commonly associated to Zn dolomite. Characterization of Zn-bearing dolomite using differential thermal analysis shows a drop in temperature of the first endothermic reaction of dolomite decomposition with increasing Zn contents in dolomite. The supergene Zn dolomites have higher δ18O but lower δ13C values than hydrothermal dolomite. In comparison with smithsonite-hydrozincite, the supergene Zn dolomites have higher δ18O, but comparable δ13C values. Formation of Zn dolomite from meteoric waters is indicated by low δ13C values, suggesting the influence of soil-gas CO2 in near-surface environments. The replacement of the dolomite host by supergene Zn dolomite is interpreted as part of a multistep process, starting with a progressive “zincitization” of the dolomite crystals, followed by a patchy dedolomitization s.s. and potentially concluded by the complete replacement of dolomite by smithsonite.  相似文献   

10.
Reaction rims of dolomite (CaMg[CO3]2) were produced by solid-state reactions at the contacts of oriented calcite (CaCO3) and magnesite (MgCO3) single crystals at 400 MPa pressure, 750–850 °C temperature, and 3–146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgCO3 interface with length ranging from about 6 to 41 µm. At the same time, a 5–71 µm wide rim of equiaxed granular dolomite grew at the contact with CaCO3. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12–80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the $[11\bar{2}0]$ equivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO are E a (CaO) = 192 ± 54 kJ/mol and E a (MgO) = 198 ± 44 kJ/mol, respectively.  相似文献   

11.
The dissolution kinetics of three stoichiometric dolomite specimens (hydrothermal single crystal, microcrystalline sedimentary rock, coarse-grained marble) were studied in aqueous carbonate solutions. Hydrodynamic conditions were controlled through use of a rotating dolomite disk in which one face was exposed to solution and fluid flow regime was defined by spinning rate. The resulting mass transfer properties were uniform across the disk surface. The dissolution experiments were begun at an initially undersaturated condition set by CO2 at ~ 1 atm dissolved in deionized water. The reaction was followed by measuring concentrations of Ca2+, Mg2+, HCO3?, and pH over time in a free-drift type of experiment at 0, 15, and 25°C.Dissolution rates for all three samples were similar in form and value; grain size effects were insignificant. Ca/Mg was constant throughout each run at 0.81–0.96. From initial conditions, the dissolution rate decreased as the solution became more saturated. At solution conditions still far from equilibrium (ion activity product = 10?19), rate dropped off sharply to a very low value. Surface morphology, determined by SEM, showed deep narrow holes in the single crystal, while the rocks dissolved along grain boundaries. These features suggested preferential dissolution of energetically favored sites and surface reaction rate control. Initial rates were used to calculate an apparent activation energy of 32 kJ mol?1 (sedimentary dolomite) and 27 kJ mol?1 (single crystal).Initial dissolution rates at 25°C and pH ~ 4 for all samples varied with spinning speed and ranged from 1–3 μmol m?2 s?1 for laminar flow conditions to almost 3–6 μmol m?2 s?1 as the transition to turbulence began. At lower temperatures, the rate was lower, and increasing spinning velocity had less effect. The strongest spinning rate dependence occurred far from equilibrium, and it became a less important factor as the saturation state increased.  相似文献   

12.
Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay‐fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na‐SO4‐dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 mm , respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 mm ). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 (ca 3‰) contrasts the formation of isotopically light (organically derived), hard‐lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C‐rich sediments.  相似文献   

13.
Diagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha-related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.  相似文献   

14.
A semi-automatic, on-line method was developed to determine the δ13C and δ18O values of coexisting calcite and dolomite. An isotopic mass balance is used to calculate the compositions of dolomite after having measured that of calcite and of the “bulk” sample. The limit of validity of this method is established by performing isotopic measurements of artificial mixtures made of precisely weighted and isotopically-characterised dolomite and calcite. The accuracy and repeatability of the calculation of dolomite δ13C and δ18O are statistically determined with a Monte-Carlo procedure of error propagation. Stable isotope ratios are determined by using an automated MultiPrep™ system on-line with an isotope-ratio mass-spectrometer (IRMS). The reaction time and the temperature of reaction were optimised by comparing the results with the isotopic composition of known mixtures. The best results were obtained by phosphoric acid digestion after 20 min at 40 °C for calcite and 45 min at 90 °C for dolomite. This procedure allows an accurate determination of the isotopic ratios from small samples (300 μg). Application of this protocol to natural mixtures of calcite and dolomite requires the accurate determination of the relative abundance of calcite and dolomite by combining Mélières manocalcimetry (MMC) and X-ray diffractometry (XRD).  相似文献   

15.
Dolomite is a common mineral in the rock record but rare in recent superficial environments. Where it does occur, it is related to anoxic, sulfate-rich environments and microbial activity. The occurrence of some dolomite deposits in caves, however, indicates that dolomite formation is also possible in oxic, non-sulfate settings. Dolomite is forming at 17 °C and in oxic-vadose conditions on the host rocks and aragonite speleothems of the Castañar Cave, Cáceres, Spain. It appears as spheroids and dumbbells 50–300 μm in diameter that internally consist of micron-sized rhombic to rounded crystals. Initially this dolomite is Ca-rich, non-stoichiometric and poorly ordered. Mg-rich solutions allow the precipitation of metastable Mg-rich carbonates, such as huntite. This soon transforms into this Ca-rich dolomite, which later “ages” to form a more stoichiometric dolomite. These dolomites show similarities to those grown under anoxic, sulfate-rich conditions and their presence in caves provides a different setting that may contribute to the understanding of the “dolomite problem”, including their initial formation and later recrystallization processes.  相似文献   

16.
This work is aimed at investigating the weathering processes of the granodiorites cropping out in a small catchment of the Sila Massif. The mineral constituents in this granodiorite are plagioclase, often zoned with a Ca-rich core and a Na-rich rim, quartz, chlorite, K-feldspar, white mica and epidote. During this study, dolomite was discovered in local stream sediments, as separate monomineralic grains, probably resulting from erosion of veins cutting the crystalline rocks. Prevailing dissolution of foreign dolomite and a Ca-rich plagioclase is suggested by the Ca–Mg–HCO3 chemical composition of local groundwaters and stream waters, which is rather unexpected for waters interacting with granitoid rocks. These qualitative observations are quantitatively confirmed by reaction path modelling of the weathering processes occurring in the study area, which was carried out using the EQ3/6 software package, version 8.0, and the Double Solid Reactant Method. Indeed, it was possible to ascertain that the release of both major dissolved constituents and several trace elements (Ba, Co, Cr, Fe, Mn, Ni, Pb, Sr, V and Zn), from rocks to waters, is chiefly controlled by the dissolution of foreign dolomite and the Ca-rich core of zoned plagioclases.  相似文献   

17.
The Miocene Port Campbell Limestone in the Otway Basin (Port Campbell Embayment), south-eastern Australia, is a shallowly buried (<350 m), temperate carbonate grainstone which consists primarily of benthonic foraminifera, bryozoans, brachiopods, echinoids and planktonic foraminifera. Volumetrically insignificant calcite cements include scalenohedral, blocky and syntaxial overgrowths. Dolomite is present in variable amounts (1–25%), scattered throughout the unit as euhedral rhombs, usually comprising <2% of the whole rock volume. The dolomite post-dates the calcite cements and is mainly an interparticle cement with crystal size varying between 10 and 150 μm (mean 50 μm). Under cathodoluminescence the dolomite rhombs have a dull core and a nonluminescent outer rim. The dolomite is nonstoichiometric, Ca-rich (Ca54–62Mg38–46), with high trace element concentrations. The Mn concentrations range from 0 to 310 p.p.m. in the crystal cores (mean 140 p.p.m.) and 80–240 p.p.m. in the crystal rims (mean 140 p.p.m.). The Fe concentrations increase from the crystal cores (range 640–5690 p.p.m.; mean 2030 p.p.m.) to the crystal rims (range 2840–9440 p.p.m.; mean 6040 p.p.m.), whereas the Sr concentrations decrease from the crystal cores (range 690–1510 p.p.m.; mean 1280 p.p.m.) to the crystal rims (620–1240 p.p.m.; mean 930 p.p.m.). The δ13CPDB values of the dolomite range between +2.5 and +2.6%, whereas the δ18OPDB values range from +0.3% to+0.6%. This dolomite occurrence supports the idea that marine or near-marine dolomite can form not only syndepositionally, but also in the shallow subsurface of temperate units, soon after sediment deposition, under reducing conditions. The fine-grained, low-permeability nature of the Port Campbell Limestone contributed to the reducing conditions at shallow depth, the high trace element concentrations of the dolomite (especially in Fe) and the near marine composition of the dolomitizing fluids, as large volumes of meteoric water were inhibited.  相似文献   

18.
The possibility of recrystallization is a long‐standing barrier to deciphering the genetic origin of dolomites. There is often uncertainty regarding whether or not characteristics of ancient dolomites are primary or the consequence of later recrystallization unrelated to the original dolomitization event. Results from 65 new high‐temperature dolomite synthesis experiments (1 m , 1·0 Mg/Ca ratio solutions at 218°C) demonstrate dolomite recrystallization affecting stoichiometry, cation ordering and nanometre‐scale surface texture. The data support a model of dolomitization that proceeds by a series of four unique phases of replacement and recrystallization, which occur by various dissolution–precipitation reactions. During the first phase (induction period), no dolomite forms despite favourable conditions. The second phase (replacement period) occurs when Ca‐rich dolomite products, with a low degree of cation ordering, rapidly replace calcite reactants. During the replacement period, dolomite stoichiometry and the degree of cation ordering remain constant, and all dolomite crystal surfaces are covered by nanometre‐scale growth mounds. The third phase (primary recrystallization period), which occurs in the experiments between 97% and 100% dolomite, is characterized by a reduced replacement rate but concurrent increases in dolomite stoichiometry and cation ordering. The end of the primary recrystallization period is marked by dolomite crystal growth surfaces that are covered by flat, laterally extensive layers. The fourth phase of the reaction (secondary recrystallization period) occurs when all calcite is consumed and is characterized by stoichiometric dolomite with layers as well as a continued increase in the degree of cation ordering with time. Inferences of recrystallization, in natural dolomite, based on cation order or stoichiometry of dolomite, usually depend on assumptions about the precursor dolomite subjected to recrystallization. If it is assumed that the experimental evidence presented here is applicable to natural, low‐temperature dolomites, then the presence of mounds is direct evidence of a lack of recrystallization and the presence of layers is direct evidence of recrystallization.  相似文献   

19.
The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surfaces; (2) mass transport of ions into solution from rock surfaces via diffusion; and (3) the conversion reaction of CO2 into H+ and HCO 3 − . Generally, it is the slowest of these three processes that limits the dissolution rate of carbonate rock. However, from experiment and theoretical analysis under similar conditions not only were the initial dissolution rates of dolomite lower by a factor of 3–60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, for limestone under the condition of CO2 partial pressures dissolution rates increased significantly by a factor of about ten after addition of carbonic anhydrase (CA) into solution, which catalysed the conversation reaction of CO2, whereas CA had little influence on dolomite dissolution. For dolomite, the increase of dissolution rate after addition of CA into solution appeared at Moreover, the enhancement factor of CA on dolomite dissolution rate was much lower (by a factor of about 3). In addition, when dissolution of both limestone and dolomite was determined by hydrodynamics (rotation speed or flow speed), especially under the dissolution of limestone was more sensitive to hydrodynamic change than that of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resource and environment in dolomite and limestone areas.  相似文献   

20.
The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution-reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite (An0) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An20 is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An30. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content (An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An80 and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, ΔG dependence, and formation of altered surface layers in the framework of the interfacial dissolution-reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution-reprecipitation mechanism governs the dissolution of plagioclase feldspars.  相似文献   

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