CHEMISTRY OF DETRITAL BIOTITES AND THEIR PHYLLOSILICATE INTERGROWTHS IN SANDSTONES

Abstract-- Microprobe analyses of optically homogeneous detrital biotites from sandstones ofthe Visings6 Group and the Dala Sandstone, Sweden, revealed a consistently low K content (generally <0.75 atom/ formula unit) and variable amounts ofFe, Mg, A1, and Si. Electron probe profiles of some biotite grains indicated two major types of interstratification, one consisting of mainly illite layers and the other apparently consisting of chlorite layers. The layer thicknesses commonly ranged between 0.5 and 3 gm. Microprobe analyses of some thick (- 5 gin) illitic layers indicated a phengitic composition, wherein the mica was relatively rich in octahederal Fe and Mg. The chloritic layers appeared to be Fe-Mg-rich and generally had octahederal totals of < 6 atom/formula unit. Variations in the chemical composition ofthe biotite and some of the illite and chlorite were probably due to an uneven distribution ofsmall amounts of the interstratified phases. The illite and chlorite layers were apparently formed by pseudomorphic replacement ofdetrital biotites, i.e., gradual replacement of one biotite layer by one layer(s) of illite and/ or chlorite during diagenesis