Tonalite of the Lakeview Mountains pluton is extremely heterogeneous in modal composition at both locality and outcrop scales. We applied an X-ray diffraction (XRD) method of modal analysis to 335 whole-rock powders previously used to study variability in major-element chemistry. We developed sampling levels appropriate for XRD modal study, mapped compositional gradients, recognized contrasts between chemical and modal variability, and tested the XRD method's suitability for petrologic investigations. Modal variability was determined at differing scales ranging from drill-core samples and hand specimens to 610 m² subareas of the pluton. Compositing of three hand samples reduced variance within subareas of this size sufficiently for mapping modal composition; the equivalent number found by previous studies for major-element chemical composition was nine. For this pluton, the X-ray method permits a single machine analysis to substitute for point counting at least 18 standard thin sections in each subarea in order to map compositional gradients. Because potassium occurs significantly only in biotite, close similarity of trend surfaces for potassium and biotite provides confirmation of the efficacy of XRD modal analysis in petrologic studies.