Detrital feldspars most commonly undergo dissolution or alteration and replacement in subsurface settings, yet in many sandstones one also can find examples of authigenic feldspar cements. Such cements, in almost all cases, are either albite (the sodic end-member feldspar) or orthoclase (the K-spar end member)—see Chapter 2 for more details. Such feldspar cements commonly occur as thin or irregular overgrowths (not always in optical continuity with their detrital host grain) or as fills of microfractures within feldspar grains; however, they can form major pore-occluding cements in some clastic terrigenous rocks. The overgrowths can be monocrystalline or polycrystalline (sometimes even consisting of a mosaic of micron-sized rhombs—Worden and Rushton, 1992; De Ros et al., 1994). Despite their generally minor role in cementation of sandstones, the recognition of authigenic feldspars is important in understanding burial-related pore-fluid chemistry variations through time, especially because such overgrowths (K-feldspar overgrowths in particular) can be radiometrically dated using K–Ar and 40Ar/39Ar methods (e.g., Hagen et al., 2001; Mark et al., 2005 and 2008). In this regard especially, it should be noted that feldspar compositional zonation and overgrowth can be inherited from igneous, metamorphic or sedimentary source rocks, and such “overgrowths” must be carefully distinguished from authigenic cement precipitation during burial (just as recycled quartz overgrowths must be distinguished from ones produced in situ during burial).