Influence of the Mg-content on ESR-signals in synthetic calcium carbonate (original) (raw)
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Comparative Study on Esr Spectra of Carbonates�
2007
This paper is a comparative study on Electron Spin Resonance (ESR) spectra of carbonates such as mollusk shells collected from the Black Sea Coast (Romania), Mediterranean Coast (Cyprus), Tyrrhenian Sea Coast (Italy) and Atlantic Coast (Mauritania). Additionally we have been analyzed the ESR spectra of limestone, marbles, stalactites and corals. Some of the carbonate samples revealed the same characteristic Mn 2+ spectrum with an additional free radical peak in the centre of sextet, at a g value in the range of 2.0006-2.0062. The differences into the ESR spectra of carbonates have been observed depending on their species, geographical origin, harvesting year, geologically age and dose. The gamma-irradiated samples exhibited a complex spectrum consisting of 2 CO , − 3 3 CO , − 3 CO , − 2 SO − and 3 SO − species.
Synchrotron high-resolution XRD and thermal expansion of synthetic Mg calcites
Physics and Chemistry of Minerals, 2020
The structural parameters and the thermal behavior of a complete series of Ca-Mg carbonates synthesized at high pressure and temperature (1-1.5 GPa, 1273-1373 K) in the range 0-50 mol% MgCO3 has been investigated by in situ powder synchrotron high-resolution X-ray diffraction at ambient and up to 1073 K under self-controlled CO2 partial pressure. The crystal structures are disordered Mg calcite in the range 1-41 mol% MgCO3, and Ca dolomite at 49 mol% MgCO3. New calibration curves of the cell parameters for the Mg content and thermal expansion from ambient to 1073 K are given. Short range structural effects of cation substitution and ordering and their thermal behavior as a function of Mg content were identified from three sets of data: the peak broadening, the cell parameter strains and the Raman band enlargements. Both intra-and inter-crystalline levels of compositional heterogeneity are identified and allow splitting the Mg calcites into two groups: low and high-Mg calcites. The low Mg calcites (up to 22 mol% MgCO3) are homogeneous in Mg content with short range ordering. High Mg calcite (up to 41 mol% MgCO3) display domains with different local ordering configurations and similar or slightly different Mg contents, and to which is added a compositional variation between crystals, as determined by EMP, of the order of ± 0.8 mol% MgCO3. The cation ordering in Ca-rich dolomites similarly occurs in high-Mg synthetic calcites. The role of (CO3) 2group ordering is shown to be an important factor in the formation of Ca-Mg carbonates.
Speciation of Mg in biogenic calcium carbonates
Journal of Physics: Conference Series, 2009
A selection of marine biominerals, mostly aragonitic coral skeletons were probed at the Mg K-edge by XANES spectroscopy coupled to μXRF methods and compared to an extensive set of relevant model compounds (silicates, carbonates, oxides and organic). Extensive methodologies are required to better describe the speciation of Mg in those minerals. A combination of ab-initio XANES calculations for defective clusters around Mg in aragonite together with wavelets analyzes of the XANES region are required to robustly interpret the spectra. When using those methodologies, the speciation of Mg ranges from a magnesite-type environment in some scleractinian corals to an organic-type environment. In all environments, the Mg-domains probed appear to be less than 1 nm in size.
Mg/Ca Ratios in Synthetic Low-Magnesium Calcite: An Experimental Investigation
2021
The work presented sought to determine the effects of Mg/Ca ratios in solution have on Mg partitioning (KMg) between precipitated abiotic low-Mg calcite and solution. Experiments were set up so that Mg/Ca in precipitated abiotic calcite would match the Mg/Ca in planktonic foraminifera. This research intended to investigate the effect of Mg/Ca(Fluid) on KMg when the molar value of Mg/Ca(Fluid) was below 0.5, which is below the previously reported Mg/Ca range. The values of pH, salinity, and aqueous Mg/Ca were monitored during calcite precipitation, and Mg/Ca of calcite was determined at the end of experiments. Partition coefficients of Mg were evaluated as a ratio of Mg/Ca in calcite to the averaged ratio of aqueous Mg/Ca for each experiment.
Rate-controlled calcium isotope fractionation in synthetic calcite
Geochimica et Cosmochimica Acta, 2004
The isotopic composition of Ca (⌬ 44 Ca/ 40 Ca) in calcite crystals has been determined relative to that in the parent solutions by TIMS using a double spike. Solutions were exposed to an atmosphere of NH 3 and CO 2 , provided by the decomposition of (NH 4 ) 2 CO 3 , following the procedure developed by previous workers. Alkalinity, pH and concentrations of CO 3 2Ϫ , HCO 3 Ϫ , and CO 2 in solution were determined. The procedures permitted us to determine ⌬( 44 Ca/ 40 Ca) over a range of pH conditions, with the associated ranges of alkalinity. Two solutions with greatly different Ca concentrations were used, but, in all cases, the condition [Ca 2ϩ ]ϾϾ[CO 3 2Ϫ ] was met. A wide range in ⌬( 44 Ca/ 40 Ca) was found for the calcite crystals, extending from 0.04 Ϯ 0.13‰ to Ϫ1.34 Ϯ 0.15‰, generally anti-correlating with the amount of Ca removed from the solution. The results show that ⌬( 44 Ca/ 40 Ca) is a linear function of the saturation state of the solution with respect to calcite (⍀). The two parameters are very well correlated over a wide range in ⍀ for each solution with a given [Ca]. The linear correlation extended from ⌬( 44 Ca/ 40 Ca) ϭ Ϫ1.34 Ϯ 0.15‰ to 0.04 Ϯ 0.13‰, with the slopes directly dependent on [Ca]. Solutions, which were vigorously stirred, showed a much smaller range in ⌬( 44 Ca/ 40 Ca) and gave values of Ϫ0.42 Ϯ 0.14‰, with the largest effect at low ⍀. It is concluded that the diffusive flow of CO 3 2Ϫ into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca 2ϩ is not a significant factor. The data are simply explained by the assumptions that: a) the immediate interface of the crystal and the solution is at equilibrium with ⌬( 44 Ca/ 40 Ca) ϳ Ϫ1.5 Ϯ 0.25‰; and b) diffusive inflow of CO 3 2Ϫ causes supersaturation, thus precipitating Ca from the regions exterior to the narrow zone of equilibrium. The result is that ⌬( 44 Ca/ 40 Ca) is a monotonically increasing (from negative values to zero) function of ⍀. We consider this model to be a plausible explanation of most of the available data reported in the literature. The well-resolved but small and regular isotope fractionation shifts in Ca are thus not related to the diffusion of very large hydrated Ca complexes, but rather due to the ready availability of Ca in the general neighborhood of the crystal-solution interface. The largest isotopic shift which occurs as a small equilibrium effect is then subdued by supersaturation precipitation for solutions where [Ca 2ϩ ]ϾϾ[CO 3 2Ϫ ] ϩ [HCO 3 Ϫ ]. It is shown that there is a clear temperature dependence of the net isotopic shifts that is simply due to changes in ⍀ due to the equilibrium "constants" dependence on temperature, which changes the degree of saturation and hence the amount of isotopically unequilibrated Ca precipitated. The effects that are found in natural samples, therefore, will be dependent on the degree of diffusive inflow of carbonate species at or around the crystal-liquid interface in the particular precipitating system, thus limiting the equilibrium effect.
Contributions to Mineralogy and Petrology, 1986
The influence of different degrees of disorder of dolomites on the solubility of MgCO3 in calcite has been studied under isothermal and isobaric conditions. At 900 ~ C, 4kb and 1000 ~ C, 5 and 7 kb, VCatclte varied ~'MgCO3 smoothly as function of the particular structural and cationic disorder of coexisting dolomite. Higher degrees of disorder of dolomite, estimated by the doo.6-, dll.0-values and the peak height ratio I01.5/I00.6, lead to greater solubility of MgCO3 in calcite. The run time for all experiments was 96 h, much longer than in previous work. The influence "3~rCalcite appears to be larger of disorder of dolomite on ~Mgco3 than that of temperature, as shown by the large range of XMgCO3(0.12-0.30) in calcite at 900~ 4 kb, found in this study. The state of order of dolomite seems to control the solubility limits in this system, and may explain discrepancies found in previous experimental work.
DESALINATION AND WATER TREATMENT, 2017
Scale formation is a serious problem often observed in heat transfer equipments. Barite, calcite and aragonite are some of the major components of scale. Control of scale formation is usually achieved through usage of scale inhibitors/chelating agents. Here we report the effect of ethylene glycol-O,O'-bis(2-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA) on the morphology and polymorphism of CaCO 3 when synthesized from CaCl 2 solution using Na 2 CO 3 at three different temperatures, 60, 80 and 100°C. The samples were characterized using XRD, FTIR and SEM techniques. The details revealed that EGTA stabilizes aragonite more efficiently at 60°C and with raise in temperature formation of calcite is predominant. The study revealed an unusual and novel datura pod like morphology for calcite.