Determination by X-ray diffraction of the relative proportions of component minerals in sediments and rocks involves comparison of the integrated intensity of selected reflections from component minerals in the unknown mixture to standard calibration curves. Although the analysis may be rapid and precise, a number of factors contribute to poor accuracy. The role of one of these, variation in the chemical composition of the component minerals, is not generally appreciated. Calculations of the relative integrated intensities of the two principal reflections from calcite, magnesite, siderite, rhodochrosite, dolomite, ankerite, and kutnahorite show that the strongest reflection, {211} ({1014} referred to hexagonal axes), is highly sensitive to isomorphous substitution of the cations. It is a poor reflection to use for analysis if there is any significant variation in the composition of the minerals in the mixture. One important example of compositional variation is the substitution of magnesium for calcium in the biogenic calcite of Recent sediments. The {210} ({1123} referred to hexagonal axes) reflection is the second strongest for most compositions of the rhombohedral carbonates. It is practically independent of solid-solution variation and makes an ideal choice for the analysis of mixtures. Calibration curves involving an internal standard show that if a component mineral comprises more than about eight weight percent of a sample, the {210} reflection will be strong enough to utilize. Solid-solution variation is common and extensive in carbonate sediments and rocks. An investigator should be aware of its effect on intensities before beginning X-ray analysis. The use of {210} is recommended for most natural mixtures, with cerric oxide as an internal standard.

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