Summary: A rapid method for determining the mode of a four-component powder (microcline, orthoclase, plagioclase, and quartz) was sought to aid the interpretation of geochemical results. An accuracy of about ±5 % (absolute) was required. Three methods were tried : point counting proved too long as well as destroying the sample; infra-red spectroscopy was not sufficiently accurate for this particular type of rock powder; X-ray diffraction proved the most rapid and, although less accurate than point counting, was within acceptable limits and had the advantage of giving additional information about the composition of the feldspars.