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.Matrix (chemical analysis)
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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.
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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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