Abstract
Modal proportions of quartz, plagioclase, perthite, and/or orthoclase are determined in artificial mixtures (presumed to simulate granitoids) and in actual granitoids by X-ray diffraction. This procedure utilizes the ubiquitous presence of quartz in all samples as an internal standard. Selected peak ratios along with the total volume fraction of these minerals and the predetermined constants can be used to calculate the volume fraction of various minerals.
The percentage relative error for this technique, excluding counting statistics, has a standard deviation of 7%. X-ray diffraction analysis is capable of routine, rapid modal analysis, provided the preparation and analysis of the sample is carefully controlled. With minor modification, this method can be adapted to other rock types.
