Diagenetic K-feldspar overgrowths from the Permo-Triassic continental clastic sediments of the Elgin area in Scotland were examined using conventional petrographic techniques and transmission electron microscopy to investigate the mechanisms of feldspar authigenesis. The detrital K-feldspar in these rocks is tartan-twinned, whereas the diagenetic K-feldspar is untwinned. The overgrowths are optically homogeneous but are not in optical continuity with the detrital grains. Overgrowths are up to 50 mu m thick and comprise numerous rhombs, indicating that nucleation was a relatively easy process. Most overgrowths consist of a mosaic of micron-sized rhombs. Nucleation on different twin planes on the detrital grain surface resulted in different orientations of the diagenetic nuclei; the different orientations were propagated into the overgrowth as new nuclei formed on the surface of pre-existing diagenetic crystals. The detrital K-feldspar is triclinic maximum microcline. Diagenetic K-feldspar deviates only slightly from monoclinic orthoclase, even though the triclinic polymorph is the stable K-feldspar under the low temperature conditions of growth. Disordered crystals involve a simpler nucleation/growth mechanism than ordered crystals; consequently, disordered crystals grow faster than ordered crystals. Monoclinic feldspar has disordered Al-Si and triclinic feldspar has ordered Al-Si. Diagenetic feldspar has metastable low triclinicity, because this crystal structure could grow faster than the stable feldspar. Nanometer-scale growth-zones occur within individual rhombs, representing variations in Al-Si disorder. Such variations probably result from subtle variations in the state of supersaturation of the pore fluids and therefore in the rate of growth. The interface between the detrital grain and the overgrowth represents no profound break in the feldspar crystal lattice, despite the change in crystallography, but is marked by the termination of twin planes in the detrital grain and the presence of dislocations in the overgrowth. The termination of twins resulted in strain in the overgrowth which was relieved by the formation of dislocations. Small pores, representing fluid inclusions, and rare clay grains, also occur at the interface.