Chemistry of detrital biotites and their phyllosilicate intergrowths in sandstones.

Microprobe analyses of optically homogeneous detrital biotites from sandstones of the Visingso group and the Dala sandstone, Sweden, reveal a consistently low K content (generally <0.75 atom/formula unit) and variable Fe, Mg, Al and Si. Two types of interstratification occur, one consisting mainly of illite layers and the other apparently consisting of chlorite layers. The layer thicknesses are 0.5-3 mu m. Some thick illitic layers appear to have a phengitic composition, with the mica relatively rich in octahedral Fe and Mg. The chloritic layers are Fe-Mg rich and generally have octahedral totals of <6 atom/formula unit. These illite and chlorite layers were apparently formed by pseudomorphous replacement of detrital biotites, i.e. gradual replacement of one biotite layer by one layer of illite and/or chlorite during diagenesis.-R.A.H.