Three independent errors affect X-ray diffraction estimates of dolomite in pre-Cenozoic carbonates. When using calcite/dolomite [104] peak ratios, each 1% of excess calcium in the dolomite lattice causes approximately a 2% overestimate of the amount of dolomite. Use of the [113] peak ratios avoids the stoichiometry problem; however. if quartz is present, the interference between the [102] quartz peak ( d = 2.282 A) and the [113] calcite peak ( d = 2.285 A) can cause a serious underestimate of the dolomite proportion. In addition, error will occur if the crystallite size in the powdered standards used to prepare X-ray calibration curves differs significantly from the crystallite size in the sample unknowns. All these errors can be avoided or corrected; however, point count or visual estimation of stained thin sections is a simple, reliable technique and is preferable for routine analysis of visibly crystalline, lithified, sediment.

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