Uncorrected x-ray diffraction data can be used to quantitatively determine the composition of a mixture of crystalline phases. As long as the samples are carefully prepared, the integrated intensity of a Bragg reflection from a sample is directly proportional to the concentration of the crystalline phase in the sample, and for most sedimentary mineral phases, the coefficient of proportionality is near unity. Because sample preparation is critical to the success of this approach, the sample preparation techniques used in this laboratory are presented in detail. The argument to substantiate the premise of this paper is in the form of five sets of data collected on suites of samples submitted by colleagues. The data represent a variety of analytical problems and demonstrate that the percent total integrated intensity (%TII) values accurately estimate the true composition of the samples. 12 figures.