Measurements of natural variations in isotopic abundance ratios of silicon are reported. A maximum natural range of 5.3 per mil in Si/sup 30/ is obsenved to date. Coexisting biotite, quartz, and feldspars from some specimens of igneous rock from the Yosemite region differ by as much as 3 per mil; in other grossly similar igneous rocks from the same region the same minerals differ by less than 0.3 per mil. In those rocks with large differences between the minerals, the ratio of STASi/sup 30/!/ STASi/sup 28/ + Si/sup 29/! increases in the order biotite, quartz, feldspar. All granitic rocks studied are enriched in Si/sup 30/ with respect to meteorites and basic rocks. Measurements on some sediments and biologic samples covered a range of approximately 4 per mil. A group of tektites covered a range of less than 0.7 per mil, with isotopic abundances centrally distributed within the observed normal terrestrial range. The results are interpreted, and applications of such studies to problems of geothermometry and formation of igneous rocks are discussed. (auth)