Abstract: X-ray diffractometry (XRD) is well accepted as a definitive tool for mineral identification and quantitative analysis. Accurate quantification of clay-rich minerals has always been challenging. Three samples from the 9th biennial Reynolds Cup competition for quantitative phase analysis (QPA) were taken for the example. Application of the Rietveld method using X-Ray powder-diffraction data requires that all components are correctly identified. The weight percent (wt%) of the Rietveld refinement will be submitted and compared with the actual content. Results show that most kinds of the mineral were precisely calculated by Rietveld method and high content mineral phases were generally easy to identify. Non-clay minerals were less problematic and were quantified with a typical bias of better than 1.5% (wt%), with the exception of amorphous phases. Samples with clay-rich minerals are difficult to identify and quantified because of their complex structures. The average bias per non-clay mineral was just 1.0%~1.3%, but the average bias per clay mineral was three to four times higher at 2.8%~5.8%. The sum total bias of three samples was 156.7% (on placing 24, the sum total bias of the top three winners was 70.5%, 89.8%, 95.6% respectively). The bias is mainly due to inaccurate identification of minerals, especially clay-rich minerals. Under the condition of accurate mineral identification, the calculation deviation of the mineral can be greatly reduced (e.g., the total bias of RC9-3 can be reduced to 19.2%) if there are suitable crystal structure and fitting conditions. The results show that the Rietveld method can accurately quantify the content of most minerals in the sample, but also relies on researchers to make accurate identification of the mineral composition. In general, the QPA is reliable by XRD in our laboratory, which could be an efficient tool on tracing the source of marine sediments.