Although thermodynamic calculations indicate a narrow range of conditions for coprecipitation of kaolinite and K-feldspar, coexistence of authigenic kaolinite and K-feldspar is common in Jurassic sandstones of the North Sea. This paper shows that the coexistence of authigenic K-feldspar and kaolinite may indicate that this narrow range of conditions did prevail and that the system behaved as a closed system. These conditions are presence of muscovite in the sediment, pore waters supersaturated with respect to quartz, and temperatures lower than 50 degrees C. At higher temperatures, the authigenic kaolinite and (authigenic) K-feldspar may react and form authigenic illite ("muscovite") and quartz cement. The direction of the isochemical reaction is thermodynamically determined by the degree of supersaturation with respect to quartz in the formation water. Published data on the degree of supersaturation with respect to quartz in formation waters suggest that the illitization of kaolinite may take place at temperatures as low as 50 degrees C--much lower than previously believed.