Quantitative analysis of calcite and Mg‐calcite by X‐ray diffraction: effect of grinding on peak height and peak area

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.

Identification and Quantification of Carbonate Species Using Rock-Eval Pyrolysis

“…Semi-quantitative determination of carbonates can commonly be done using classical optical methods (such as an optic microprobe on thin sections) or X-ray diffraction (Diebold et al, 1963;Gavish and Friedman, 1973;Weber, 1968;Roselle, 1982;Martinez and Plana, 1987;Hillier et al, 2001;Al Jaroudi et al, 2007) or by a group of different analytical methods (such as Scanning Electron Microscope, electron microprobe, infrared or Raman spectroscopy, chemical analysis, etc.). Nevertheless, the real quantification of carbonates, especially in the case of mixing of different carbonates in the same sample, is not such an easy task and, to date, only a few methods allowing one to directly quantify the carbonate content in samples of rocks or other solid materials are available.…”

Nature and genesis of silt-size carbonate sediments; Northern Red Sea, Egypt

“…The separation ofsilt and clay fractions was done by decantation using the settling tube according to Prokopovich (1958). Because of the interference of dispersant with the chemical analysis (Milliman et al 1993), and the effect of grinding on the peak height and peak area (Gavish and Friedman 1973), the samples were only ultrasonically dispersed. The carbonate mineralogy of the various silt fractions was analyzed by X-ray diffraction, scanning the interval from 25 0 to 33 0 2e employing Kn, Cu target, at 1.0 0 per minute.…”

Rietveld analysis of X‐ray powder diffraction patterns as a potential tool for the identification of impact‐deformed carbonate rocks

“…This is particularly true for calcite and dolomite from the central uplift of Sierra Madera. Several factors contribute to XRD peak profile variability including Mg content (Gavish and Friedman 1973), grinding time of powder (Burns and Bredig 1956;Gavish and Friedman 1973), grain size of powder (Klug and Alexander 1974;Langford et al 2000), and phase transformations (Burns and Bredig 1956;Merrill and Bassett 1975;Fiquet et al 1994). The above mentioned factors were considered when preparing samples for this study.…”