Authigenic K-feldspar is a ubiquitous feature of the Bromsgrove Sandstone Formation (Triassic) in Central England. It occurs mainly as partially to completely developed euhedral overgrowths on detrital K-feldspar cores. The authigenic K-feldspar is virtually pure KAlSi 3 O 8 (potassian low sanidine) and occurs as optically continuous overgrowths on detrital sanidine and optically discontinuous overgrowths on detrital orthoclase. Differences in structure and Al/Si disorder thus control the degree of optical continuity of the overgrowths. An important feature is the presence of optically and compositionally zoned overgrowths due to minor fluctuations in the BaO and Na 2 O content. The formation of the overgrowths was dependent upon intrastratal dissolution of detrital grains for a supply of potassium, aluminum, and silicon. Precipitation took place within the K 2 O--Na 2 O--Al 2 O 3 --SiO 2 --H 2 O system and in association with other phases in this system. The diagenetic sequence 1) illite 2) quartz 3) K-feldspar 4) illite 5) kaolinite is consistent with a progressive increase in K (super +) /H (super +) ratio or H 4 SiO 4 activity followed by a general decrease so that the later interstitial fluids were much depleted in K (super +) ions and kaolinite was the final phase to precipitate.

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