The formation of sulfides during alteration of biotite to chlorite-corrensite

Transmission electron microscopy/analytical electron microscopy (TEM/AEM) w ere utilized to study pyrite and sphalerite inclusions in chlorite or mixed -layer chlorite-corrensite from an analcimized ash bed in the Etalian stage (Middle Triassic), South Otago, New Zealand. These sulfide inclusions occu r as elongated crystals up to 1 x 15 mu m in size, within lens-shaped voids between separated, deformed (001) layers of (primarily) chlorite and mixed -layer chlorite-corrensite grains of typical detrital shape or chlorite pac kets in chlorite-mica stacks (intergrowths of chlorite and phengite packets ) up to 40 x 150 mu m in size. Relict biotite layers within chlorite, mixed -layer chlorite-corrensite and berthierine have textures implying replaceme nt of the former by the latter, whereas in other unaltered samples only fre sh biotite was observed. Anatase occurs in otherwise Ti-free chlorite, wher eas relict biotite contains significant Ti (0.3 moles per 22 oxygen atoms). No sulfide minerals have been found in fresh biotite and phengite. Mass balance considerations indicate that S and Zn were introduced via pore fluids and that the Fe was provided by the decomposition of biotite to sec ondary phyllosilicates. The alteration of biotite and the reaction of bioti te to form chlorite and pyrite is controlled by aH(+)/aK(+) as well as oxid ation of reduced S species or reduction of oxidized S species from solution . Simple calculations with the observed compositions of chlorite and biotit e suggest that some of the Fe in biotite was actually removed in solution r ather than precipitated in pyrite and chlorite. Similar textures are abunda nt in ferroan phyllosilicates elsewhere, implying that the mechanism may ap ply widely to precipitation of sulfides in phyllosilicates during early dia genesis of sediments.