Origin of anhedral Sr-rich calcite in deep-water mixed sediment, north-east Australia

At burial depths of 800-1000 m, within the epicontinental Queensland Trough of north-east Australia (ODP Site 823), microcrystalline inter- and intraskeletal mosaics of anhedral (loaf-shaped, rounded) calcite have Sr²⁺ values ranging from below microprobe detection limits (<150 ppm) to 8100 ppm. Host rocks are well lithified, fine-grained mixed sediment to clayey wackestone and packstone of Middle and Late Miocene age. Petrography demonstrates that calcite precipitation has spanned shallow to deep burial, overlapping formation of framboidal pyrite in the upper 50 m; shallow-burial dolomitization (<300 m); and dedolomitization during sediment consolidation and incipient chemical compaction at greater (>400–500 m) depths. Petrographic observations illustrate that the calcite microfabric formed through coalescing crystal growth resulting from one or a combination of displacive growth in clay, porphyroid neomorphism of aragonite/vaterite, and clay replacement by calcite. Sr²⁺ mean concentrations in calcite between depths of 800 and 1000 m are similar to an expected equilibrium pore-water concentration, using a D^sr of 0.06, and may indicate active calcite precipitation. However, Sr²⁺ variation (2000–5000 ppm) within and among crystals, and concentrations that range well above predicted equilibrium values for a given depth, illustrate either variable Sr²⁺ retention during recrystallization of shelf-derived aragonite (and authigenic local vaterite) or relative uptake of Sr²⁺ during calcite precipitation with burial. Within the context of calcite formation during burial to 1 km, diagenetic attributes that affect the latter process include increased concentrations of pore-water Sr²⁺ with depth associated with aragonite recrystallization/dissolution; upward migration of Sr-rich pore water; and increased D^Sr related to local variation in precipitation/recrystallization rates, differential crystal sector growth rates and/or microvariation in aragonite distribution.     

Strontium behavior in the aragonite-calcite transformation: An experimental study at 40–98°C

The behavior of strontium during the replacement of aragonite by calcite, in a closed system between 40°C and 98°C, has been experimentally investigated. The experiments were conducted in CaCl₂ solutions, with and without NaCl. The distribution coefficient of strontium in calcite (λ_Sr^2+C) was found to be affected only slightly by temperature changes, and almost insignificantly by the presence of NaCl. λ_Sr^2+C values at 0.01 m_Ca²⁺ (its concentration in normal sea water) are: 0.055 at 40°C and 0.058 at 98°C. These results indicate that the low (around 500 ppm) concentration of strontium in ancient limestones could have been brought about by aragonite-to-calcite transformation in a system open to sea water, and are not necessarily indicative of replacement in fresh waters.     

Uplift-related retrogression history of aragonite marbles in Western Crete (Greece)

In the low-grade, high-pressure (400°C, 10 kbar) metamorphic Phyllite-Quartzite Unit of Western Crete, widespread occurrences of aragonite marbles have been discovered recently. A sedimentary precursor is proved by relic structures (bedding, fossils). Partial or complete transformation of aragonite into calcite is ubiquitous. Compositional and microstructural features reflect the metamorphic history: (1) The high-pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of Sr and the lack of Mg. The most Sr-rich aragonite has about 9 wt% SrO (X _Sr ^arag =0.09). A compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcitearagonite transformation in the system CaCO₃-SrCO₃. Because the parent rocks probably were Sr-poor calcite limestones, one can speculate that strontium has been supplied from an external source under high-pressure conditions. (2) During uplift, calcite replacing aragonite did not equilibrate with unreplaced aragonite. Disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite. The calcite compositions range from that of the host aragonite (Sr-rich and Mg-free) to Mg-bearing and Sr-poor. (3) Calcite that recrystallized during retrogression is generally Sr-poor (mean value ofX _Sr<0.005), Mg-bearing (X _Mg0.010), and chemically homogeneous. Because practically no Sr remains in the calcite, an interaction with a fluid phase is indicated. In fine-grained calcite marbles rich in solid organic matter, microstructural features indicative of former aragonite may be present. (4) The last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules. This aragonite, which shows neither deformation nor retrogression, was probably formed metastably in a near-surface environment.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Adsorption and Desorption of Phosphate on Calcite and Aragonite in Seawater

The adsorption and desorption of phosphate on calcite and aragonite were investigated as a function of temperature (5–45 °C)and salinity (0–40) in seawater pre-equilibrated with CaCO₃. An increase in temperature increased the equilibrium adsorption; whereas an increase in salinity decreased the adsorption. Adsorption measurements made in NaCl were lower than the results in seawater. The higher values in seawater were due to the presence of Mg²⁺ and Ca²⁺ ions. The increase was 5 times greater for Ca²⁺ than Mg²⁺. The effects ofCa²⁺ and Mg²⁺ are diminished with the addition of SO₄ ^2- apparently due to the formation of MgSO₄ and CaSO₄ complexes in solution and/or SO₄ ^2- adsorption on the surface of CaCO₃. The adsorbed Ca²⁺ and Mg²⁺ on CaCO₃ (at carbonate sites) may act as bridges to PO₄ ^3- ions. The bridging effect of Ca²⁺is greater than Mg²⁺ apparently due to the stronger interactions of Ca²⁺ with PO₄ ^3-.The apparent effect of salinity on the adsorption of PO₄ was largely due to changes in the concentration of HCO₃ ^- in the solutions. An increase in the concentration of HCO₃ ^- caused the adsorption of phosphate to decrease, especially at low salinities. The adsorption at the same level of HCO₃ ^- (2 mM) was nearly independent of salinity. All of the adsorption measurements were modeled empirically using a Langmuir-type adsorption isotherm[ [PO₄]_ad = K_mC_m[PO₄]_T/(1 +K_m [PO₄]_T) , ]where [PO₄]_ad and [PO₄]_T are the adsorbed and total dissolved phosphate concentrations, respectively. The values of C_m (the maximum monolayer adsorption capacity, (mol/g) and K_m (the adsorption equilibrium constant, g/(mol) over the entire temperature (t, °C) and salinity (S) range were fitted to[ C_m = 17.067 + 0.1707t - 0.4693S + 0.0082S² ( = 0.7) ][ ln K_m = - 2.412 + 0.0165t - 0.0004St - 0.0008S² ( = 0.1) ]These empirical equations reproduce all of our measurements of[PO₄]_ad up to 14 mol/g and within ±0.7 mol/g.The kinetic data showed that the phosphate uptake on carbonate minerals appears to be a multi-step process. Both the adsorption and desorption were quite fast in the first stage (less than 30 min) followed by a much slower process (lasting more than 1 week). Our results indicate that within 24 hours aragonite has a higher sorption capacity than calcite. The differences between calcite and aragonite become smaller with time. Consequently, the mineral composition of the sediments may affect the short-term phosphate adsorption and desorption on calcium carbonate. Up to 80 % of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO₄ left on the CaCO₃ is close to the equilibrium adsorption. The release of PO₄ from calcite is faster than from aragonite. Measurements with Florida Bay sediments produced results between those for calcite and aragonite. Our results indicate that the calcium carbonate can be both a sink and source of phosphate in natural waters.     

Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg^?1 pH of 7.6 and p(CO₂) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg^?1, pH of 7.70 and p(CO₂) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO₂) up to 5.6 times the atmospheric P(CO₂).     

Dissolved carbon in pore waters from the carbonate barrier reef of Tahiti (French Polynesia) and its basalt basement

This paper deals with dissolved inorganic carbon (DIC) and organic carbon (DOC) in pore waters from a 150 m deep hole drilled through the carbonate barrier reef of Tahiti and its underlying basalt basement. Alkalinity-pH measurements were used to calculate the DIC species concentration, and DOC was analysed according to the high temperature catalytic oxidation technique. Salinity was used as a conservative tracer to help identify water origin and mixing within the hole. Water mixing, calcium carbonate dissolution and mineralization of organic carbon combined to form three distinct groups of pore water. In the deeper basalt layers, pore water with alkalinity of 1.4 meq kg^–1 pH of 7.6 and p(CO₂) of 1.2 mAtm was undersaturated with respect to both aragonite and calcite. In the intermediate carbonate layer, pore water with alkalinity of more than 2.0 meq kg^–1, pH of 7.70 and p(CO₂) of 1.4 mAtm was supersaturated with respect to both aragonite and calcite. The transition zone between those two groups extended between 80 and 100 m depth. The shift from aragonite undersaturation to supersaturation was mainly attributed to the mixing of undersaturated pore waters from the basalt basement with supersaturated pore waters from the overlaying limestone. In the top of the reef, inputs from a brackish water lens further increased p(CO₂) up to 5.6 times the atmospheric P(CO₂).     

The dolomitization of CaCO3: an experimental study at 252–295°C

The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCl₂-MgCl₂ aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole %) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr²⁺ and Mg²⁺ partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252°C, the total volume of these zones is in the range of 2 × 10^?5 to 2 × 10^?4 1., out of 5 × 10^?3 1., the volume of the bulk solution.The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively.Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized.Coprecipitation of Sr²⁺ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 × 10^?2 to 2.78 × 10^?2.     

Selected geochemical factors influencing diagenesis of eocene carbonate rocks,peninsular Florida,U.S.A.

The geochemical significance of three selected ions (Mg²⁺, Na⁺, and Sr²⁺) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg²⁺ to facilitate dolomitization ($\text{Mg}\text{Ca}\text{ratios 1}$). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity.Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr²⁺ on the average and calcite has 418 ppm Sr²⁺. The Sr²⁺ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo₃ and 195 ppm for the dolomite with 44–50 mole-% MgCO₃. The difference in the Sr²⁺ concentrations of the two dolomite groups indicates the higher mole-% MgCO₃ dolomite recrystallized in a less saline environment than the lower mole-% MgCO₃ dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment.The more nearly stoichiometric dolomite (44–50 mole-% MgCO₃) has less scatter when mole-% MgCO₃ is plotted against Sr²⁺ and Na⁺. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO₃ (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.     

Morphology and growth of aragonite crystals in hot-spring travertines at Lake Bogoria, Kenya Rift Valley

Pseudohexagonal aragonite crystals are common components in some hot-spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non-crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal. Today the hot (T>95 °C) Na-HCO₃-Cl spring waters at Chemurkeu have a salinity of 5–6 g L^?1 TDS, a pH of 8·1–9·1, Ca²⁺ concentrations of <2 mg L^?1 and Mg²⁺ concentrations of <0·7 mg L^?1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T～100 °C) and a deeper aquifer (T～170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca²⁺ than today, possibly under more humid conditions during the Holocene.     

Chemical equilibria in the thermal spring waters of Virginia

The composition of waters from 10 thermal springs located in western Virginia near the 38th parallel lineament have been analysed for major dissolved components and for Sr, Fe, Cu, Zn and Cd; from these analyses, free ion activities have been calculated. The temperatures of the springs range from 17° to 39°C, the heat apparently being derived simply from deep circulation along synclinal, Middle Ordovician limestones. More than 95 per cent of the dissolved solids consist of Ca²⁺, Mg²⁺ HCO₃^?, and SO₄^2?. The concentrations of these components, as well as the spring temperatures, have not changed appreciably in 140 yr in some springs. The waters that have temperatures below 25° are all undersaturated with respect to calcite and dolomite, possibly because they have been contaminated by shallow ground waters. The waters with temperatures above 25° are in equilibrium with calcite and dolomite. Furthermore, in this latter group, the calcium-sulfate activity product and the sulfate-carbonate activity ratio are nearly constant, even though the waters are under saturated with respect to gypsum, anhydrite, celestite and strontianite. This can be explained if CaSO⁴ is coprecipitated in a mineral such as aragonite. The waters have absorbed some dissolved oxygen near the surface, but at depth they may be anoxic with Eh controlled by the oxidation of pyrite to goethite. The extremely low chloride concentrations of these waters clearly distinguish them from the brines which deposited Mississippi Valley and Appalachian type epithermal ore deposits.     

The coprecipitation of Sr2+ with calcite at 25°C and 1 atm

The incorporation of Sr²⁺ into calcite at earth surface aqueous conditions is affected by the absolute concentration of Sr²⁺, the presence of Ba²⁺ and NaCl in the solution, and the rate of precipitation. At solution ratios (molar) of Sr²⁺ to Ca²⁺ in the low 10⁻³ range, which yield calcites with several hundred ppm Sr²⁺, K^sr_calcite typically assumes a value between 0.10 and 0.20. Above these concentrations the value of k^Sr_calcite drops to approximately 0.06. Furthermore, if minor amounts of Ba²⁺ or large amounts of Na⁺ (0.48 M) are added to a dilute Sr²⁺ solution, a value around 0.06 for k^Sr_calciteis found. This “strontium concentration effect” and the associated “competitive cation effect” suggest that small amounts of Sr²⁺ may be incorporated into a limited number of nonlattice sites in calcite. Incorporation of Sr²⁺ into these sites, presumably defects, noticeably affects k^Sr_calcite only at low Sr²⁺ concentrations and in the absence of competition from other large cations.An increase in k^Sr_calcite with rate of precipitation, qualitatively similar to that found in other studies, was observed only when precipitation times were decreased from days to hours.For many geologic settings a partition coefficient for Sr²⁺ into calcite of 0.06 appears appropriate, but there are situations—very low Sr²⁺ concentrations, the presence of Mg²⁺, and fast precipitation rates—in which a larger value might better approximate natural partitioning.     

Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.     

室温条件下青海湖水中镁离子浓度对沉淀碳酸钙同质多像类型的调控

青海湖是我国唯一报道过的现代湖底沉积物中白云石、方解石和文石等多种碳酸盐矿物共存的高原内陆咸水湖泊.以青海湖水和除菌青海湖水作为载体,以CaCl2和MgCl2·6 H2O作为反应原料,在实验室常温条件下采取控制变量法制备出不同浓度Mg2+参与下的钙质沉淀物,探讨Mg2+浓度对沉淀物类型的影响.仅添加CaCl2时,青海湖...     

Low-magnesian calcite limestones forming at the deep-sea floor, Tongue of the Ocean, Bahamas

Carbonate ooze in the deep troughs between the Bahama Banks is a mixture of pelagic and bank-derived material. It consists of aragonite, calcite and magnesium calcite in a ratio of about 3:2:1. Where exposed in erosional cuts at the sea floor, this ooze lithifies within 100,000 years and is transformed into calcite micrite of only 3.5-5 mol % MgCO₃. Where buried, the ooze maintains its original composition for at least 200,000-400,000 years and remains unlithified for tens of millions of years. Quite unexpectedly, the path of sea-floor diagenesis of peri-platform ooze was found to be the same as that of freshwater diagenesis. Most of the aragonite is leached, pteropod shells often leaving cement-lined moulds behind; magnesian calcite recrystallizes and loses magnesium; polyhedral calcite of 2-4 μm size appears as cement. The setting and the carbon-oxygen isotope ratios rule out any freshwater influence. Carbon isotope ratios remain heavy, oxygen ratios shift towards equilibrium with the cold bottom water. The calcite cement has 3.5-5 mol % MgCO₃ and can be interpreted as the least soluble form of calcite emerging from alteration at the sea floor or, alternatively, as a direct precipitate from cold sea water. The change in the composition of calcite cements with water depth supports the second interpretation. In the Bahamas and elsewhere in the world ocean, magnesium in calcite cements decreases from the warm surface waters down to 700-1200 m, i.e. the boundary between intermediate and cold deep-water masses. Below this level, calcite prevails and magnesian calcite and aragonite cements are restricted to semi-enclosed seas with exceptionally warm bottom waters.     

碳酸钙矿物低温化学合成及其同质多象转变矿物学机理研究

通过缓慢分解Ca²⁺-Mg²⁺-HCO₃^--Cl^--H₂O溶液和以菱锶矿(或碳钡矿、白铅矿)为晶种的附晶生长法,在0-90℃温度范围内定向合成了碳酸钙同质多象变体.矿物合成实验结果表明,随着温度升高,有利于亚稳态文石和不稳定六方方解石的生成;随着溶液中Mg²⁺离子浓度增大和Ca(HCO₃)₃溶液浓度减小,均有利于亚稳态文石的形成.以XRD和SEM技术为实验手段,详细研究了碳酸钙同质多象转变过程.结果显示:在流体参与的情况下,文石→方解石和六方方解石→方解石的同质多象转变速率很快,并且其转变的矿物学机理为溶解/再沉淀.     

The kinetics of calcite precipitation from a high salinity water

Calcium carbonate is one of the most common and important scale-forming minerals in oilfield produced water, but the kinetics of CaCO₃ precipitation has been ignored in most scale prediction models because of the lack of reliable precipitation rate model. There are none in the open literature for oilfield conditions (temperature > 100°C, pressure > 200 bar and salinity > 0.5 mol kg− 1). In this work the kinetics of calcite (CaCO₃) precipitation from high salinity waters (up to 2 mol kg⁻¹) have been studied by a pH-free-drift method in a closed water system. This method. is much easier to operate than the often used steady-state method. The experimental results indicate that the calcite precipitation rate is not only affected by the solution CaCO₃ saturation level, but also by the solution pH, ionic strength and the concentration ratios of Ca to HCO₃₋ ions (C_Ca²⁺/C_HCO3⁻). When the concentration ratios of Ca to HCO₃⁻ ions are close to their chemical stoichiometric ratio of 0.5, the calcite growth from a supersaturated solution is believed to be surface reaction controlled. However, at higher C_Ca²⁺C_HCO3⁻ ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.     

Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica

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.     

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca²⁺ (50%) > Si (22%) > Na⁺ (17%) > Mg²⁺ (6%) > K⁺ (5%). For major anions, the relative molar abundance is HCO₃⁻ (89%) > SO₄²⁻ (7%) > Cl⁻ (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca²⁺ and HCO₃⁻ mass-balance. Stream water Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7076 to 0.7097).Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca²⁺ and ∼30% more Sr²⁺ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr²⁺ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.     

Ocean Chemistry Revealed by Mineralogical and Geochemical Evidence at the Permian–Triassic Mass Extinction, Offshore the Persian Gulf, Iran

The Upper Permian Dalan Formation and the Lower Triassic Kangan Formation in the Persian Gulf area are mainly composed of shallow marine facies limestone and dolomite. Two subsurface-cored intervals were investigated in order to understand the original mineralogy and paleoceanic conditions. The decreasing trend of Sr concentration in these deposits shows that aragonite was precipitated during the Late Permian and then gradually changed to calcite toward the Permian–Triassic boundary (PTB). The dissolution rate of aragonite decreased from 60 m below the PTB toward the boundary, with the only exception at 10 m below the Permian-Triassic Boundary (PTB) due to the Permian–Triassic unconformity in this region. The increasing trend of Mg/Ca ratio in a global scale at the end-Permian time shows that the interpreted variation of mineralogy does not result from the change of this ratio. The increasing pCO2 and decreasing pH are considered to be the main controlling factors. The increase of Ca2+ at the end-Permian time due to the input of meteoric waters is too little to fully compensate this effect. A local maximum of the Si content just at the PTB confirms the input of runoff waters.