Diagenetic replacement of detrital albite and K-feldspar by quartz is common in Proterozoic sandstones of the Visingso Group, the Dala Sandstone Formation (southern and central Sweden, respectively) and the Brottum Formation (southern Norway). Quartz has been precipitated as a few or numerous crystals along fractures and cleavage and twinning planes as well as in dissolution voids in the feldspar grains. Feldspars that are pervasively replaced by quartz appear as small relics among numerous euhedral quartz crystals. Authigenic clay minerals, mainly illite, are commonly associated with quartz in altered feldspar. The silica required for precipitation of quartz has been derived from 1) sources external to the feldspars (e.g., pressure solution of quartz grains and transformation of clay minerals), but which occur within the same sandstone sample, and 2) dissolution and/or replacement of the feldspars by diagenetic clay minerals.