Quantitative analysis of calcite and Mg‐calcite by X‐ray diffraction: effect of grinding on peak height and peak area
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ABSTRACT Mineralogical analysis of calcite and Mg‐calcite by X‐ray diffraction requires that the samples be ground to a powder. Such grinding determines the particle size of the powder and the structural damage of the minerals. Both of these in turn affect the peak intensities recorded by the X‐ray machine. Most carbonate sediments are inhomogeneous; they contain both calcite and Mg‐calcite which are affected differently by grinding. Such differences cause quantitative analytical results to be inconsistent with the true mineralogical abundance. The two acceptable methods of analysis—(1) measurement of peak height from the base and (2) measurement of the area under the peak—were compared to determine if sample preparation affects the quantitative results. In samples with variable and relatively small amounts of calcite and Mg‐calcite the measurement of peak height yields more reproducible results than does the measurement of peak areas. Different proportions of particle size of the mineralogical components in a sample powder, affect proportionally more the peak areas than the peak heights. Extensive grinding causes structural damage of the component minerals which affects much more the peak areas than the peak heights. Thus for quantitative analyses of calcite and Mg‐calcite in inhomogeneous carbonate samples which require differing grinding times and have greatly variable amounts of calcite and Mg‐calcite, the peak height measurement seems to be a better method than peak area measurement.Analyses of biopolymer/calcium carbonate composites grown on inorganic abiotic substrates implanted between the shell and the shell-secreting epithelium of live red abalones (Haliotis rufescens) provide detailed spatial and temporal data on the in vivo assembly process that generates the shell. X-ray diffraction and scanning electron microscopy analyses of the growth of these flat pearl composites reveal that biomineralization is initiated by the deposition of an organic sheet on the implanted substrate, followed by the growth of a calcite layer with preferred {10.4} orientation and, finally, by the growth of nacreous aragonite. The calcite layer is structurally similar to the green organic/calcite heterolayer of native shell nacre. It comprises 0.2−2.0-μm-diameter elongated crystallites of typical geological habits in various aggregate arrangements. The shell also contains an external layer of (00.1)-oriented prismatic calcite, which is deposited on one edge of a flat pearl and has a morphology similar to that of the {10.4}-oriented calcite layer. The transition from {10.4}-oriented calcite to aragonite in both the shell and the flat pearl is abrupt. In vitro calcium carbonate growth experiments reveal that a similar calcite-to-aragonite transition is induced by the addition of soluble proteins isolated from the aragonitic nacre. The growth of flat pearls is highly sensitive to physical and chemical properties of the abiotic substrate. Either roughened or hydrophobic substrates result in abnormal arrangements of the basal calcite layer, which are corrected for by a reinitiation of the biomineralization process, beginning with the deposition of an organic sheet. Insertion of flat pearls as substrates, however, results in continued nacre growth without the deposition of an organic sheet and a calcite layer.
Pinctada fucata
Amorphous calcium carbonate
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In the present study,the crystals of CaCO3 were obtained by the slow diffusion of NH3 and CO2 into the CaCl2 solutions.Experimental results showed that both 3-mercaptopropionic acid self-assembled monolayers(SAMs) and solution temperature have significant influence on the crystalline forms and morphologies of calcium carbonates crystals.The precipitations of calcium carbonate were easily obtained from solutions at 25 ℃ but with polymorphism including calcite,vaterite and aragonite,while on the self-assembled monolayers there exist calcite crystals only.The morphologies of calcite crystals can be mediated by temperature.For example,the calcite precipitated at 25 ℃ has smooth surface while that precipitated at 60 ℃ has porous structure even though they were obtained on the same SAMs.XRD analysis showed that the growth of calcite on SAMs were along(104) and(006) planes.
Vaterite
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This research project was conducted to study the possibility of using calcite powder as a calcium supplement for livestock feeding. Samples of calcite powder procured from different parts of India were analyzed for dry matter (DM), total ash, acid insoluble ash and major and trace minerals using Atomic Absorption Spectrophotometer followed by determination of ruminal solubility of calcium carbonate, calcite, dolomite, lime stone powder and di-calcium phosphate powder as source of calcium. The results showed that samples of calcite powder procured from different parts of India, varied in Ca content (%) from 39.0 to 41.3 with an average of 40.03 ± 0.27%. These values were comparable to the Ca content of calcium carbonate samples (39.17 ± 0.3%). However, P and Mg contents of calcite powder were higher than that of calcium carbonate. Mn content of both the calcium sources was similar, but Cu and Zn contents of calcite powder were higher than in calcium carbonate. Levels of toxic minerals like lead and cadmium were lower in calcite powder than in calcium carbonate but the fluorine content in the later was lower than in calcite powder indicating that calcite powder has the potential to be used as a source of calcium in the animal ration. Solubility of the calcium sources studied was low at pH 7 and ranged from 1.74 ± 1.36 in Dolomite to 2.94 ± 0.95 percent in Dicalcium phosphate (DCP). Reducing the pH of the ruminal buffer at 6 increased their solubility and the pattern was almost similar to that recorded at pH 7. Further reduction of pH of ruminal buffer to 2.5, increased their solubility significantly (up to 72.63%), however, Ca solubility of calcium carbonate and dolomite was lower than other calcium supplements. It was concluded that calcite and lime stone powders may be good source of Ca under the conditions when ruminal pH is towards lower side.
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Vaterite
Carbonate Ion
Amorphous calcium carbonate
Halite
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Matrix (chemical analysis)
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ABSTRACT Cultures of known species of fungi placed on crystals of Iceland spar calcite resulted in extensive dissolution of the calcite. This organically mediated dissolution produced large patches of spiky calcite within a period of 253 days. The dissolution of the calcite occurred via surface-reaction-controlled kinetic processes that were mediated by the fungi. This occurred despite the lack of vast quantities of fluids undersaturated with respect to calcite. Locally, at least 10 µm of calcite was removed from the original crystal surface.
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SUMMARY Solutions of calcium bicarbonate were allowed to lose carbon dioxide and evaporate to dryness under controlled temperature conditions. With filtered solutions prepared from spar calcite, precipitates were 100% calcite in the 2° to 100°C temperature range. When, in analogous experiments, coralline aragonite was the starting material, the precipitates were 100% calcite. Essentially the same was true when carbonate rocks from karst areas were used to prepare the experimental solutions. An artificially prepared mixture (maximum crystal size of about 7 u) of 70% aragonite and 30% calcite was also used in the study. The precipitates from this starting material were apparently affected by seed nuclei which passed through the filter. The stability of calcium carbonate seed nuclei appears to vary with temperature. Natural calcium bicarbonate solutions from caves yielded only calcite at 25°C. Calcite should be the dominant or only polymorph of CaCO 3 formed by the loss of carbon dioxide and evaporation of natural calcium bicarbonate solutions if temperature is the controlling factor. Since appreciable amounts of aragonite are found in many cave deposits, factors other than temperature must influence the polymorphs formed. POBEGUIN (1955) proposed that rapid evaporation and slow diffusion of solutions favor aragonite. If so, layers of aragonite and calcite in speleothems may reprsent alternate wet and dry paleoclimates. During these periods, rate of introduction of solution and rate of evaporation would change markedly.
Bicarbonate
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