Authigenic K-feldspar occurs in alteration profiles in uppermost Precambrian igneous and metamorphic basement rocks of the midcontinent. The K-feldspar is widespread and has been identified in six states. The profiles occur directly below the Cambrian-Precambrian unconformity and range from about <1 to 8 m in thickness. Authigenic K-feldspar occurs throughout the profile. The K-feldspar is monoclinic or triclinic by X-ray diffraction, of end-member composition, and may compose 63% of rock volume. Much of the K-feldspar formed by replacement of primary feldspar. A sample of wholly authigenic K-feldspar from altered basement in southern Illinois gives a K/Ar date of 549 ±18 Ma (Early Cambrian). This date is in agreement with Early Cambrian Rb/Sr dates for potassic alteration of uppermost Precambrian basement in Ohio. Dated authigenic K-feldspars from both Ohio and Illinois give identical δ18O values of 17.5, suggesting formation from a very similar fluid. Concordancy of both dates and δ18O values suggests that the K-feldspar formed during an episode of potassic alteration during Early Cambrian time that affected much of midcontinent North America. The dates and δ18O values for K-feldspar, when considered together, do not fit any of the hypotheses presented here.