Petrographic studies of subsurface cores of the Fountain Formation (Pennsylvanian) in the Denver region, Colorado, show that detrital grains of potassium feldspar have been pervasively and complexly albitized during burial diagenesis. In some strata, all detrital grains of potassium feldspar have been convened to albite. In some instances, the conversion has taken place directly; in others it has involved prior replacement of the potassium feldspar by other minerals, particularly anhydrite, calcite, and dolomite, which in turn are replaced by albite. The albitized grains display fabrics such as perthitic intergrowths and chessboard twinning that are strikingly similar to replacement fabrics produced in feldspars by igneous and metamorphic processes. If successive stages of the conversion to albite were not preserved, the diagenetically albitized grains might easily be interpreted erroneously as detrital grains of plagioclase, perthite, or antiperthite derived from igneous or metamorphic source rocks. The sodium required for the albitization is presumed to have been derived from intraformational sources, particularly from diagenetically altered detrital grains of sodium-rich plagioclase, which have been extensively replaced by sodium-free clay minerals. Ions released in the course of the albitization have provided important intrastratal sources of potassium, aluminum, and silicon, which have been reprecipitated locally in authigenic minerals, particularly albite, illitic clays, and adularia.