Abstract
Macroscopically intergrown chlorite, wonesite (sodium trioctahedral mica), and potassium biotite from the Post Pond Volcanics, Vermont, have been studied with transmission electron microscopy and diffraction. The chlorite occurs in intergrown 1-layer and disordered polytypes, the wonesite occurs in 1-layer, 2-layer, 3-layer, and disordered forms, and the biotite occurs as 1-layer and 2-layer polytypes with minor stacking disorder. In addition, both chlorite and wonesite are twinned on (001) by a law involving layer rotations of 30° or angles very close to 30°. IN wonesite, this constitutes a sort of “chemical twinning,” with twin planes containing fewer interlayer sites than normal mica interlayers. Some of the wonesite also exhibits turbostratic stacking.
Low-angle grain boundaries occur within chlorite and wonesite and also between pairs of the sheet silicate phases. The structures at these grain boundaries can be discontinuous, or the structural layers can be continuous across them, depending on the grain boundary orientation.
Parts of the chlorite exhibit perfect alternation of the talc-like and brucite-like layers, but in other places, there are extra or missing brucite-like layers. Likewise, occasional brucite-like layers are found in most of the wonesite. In both chlorite and wonesite, these extra layers are in some cases observed to terminate. In some places, the chlorite and mica structures intergrow freely, either in almost total disorder or as slabs of chlorite and mica up to several hundred ångströms thick. Thin slabs of kaolinite, talc, and potassium biotite also intergrow with the chlorite and wonesite. The mixed layering phenomena in this specimen demonstrate that chlorite that appears normal in thin section can, at least in places, deviate substantially from ideal chlorite stoichiometry.
