A chemical approach to the preferred orientation of mica
The mineralogical and chemical changes accompanying diagenesis in Gulf Coast argillaceous sediments provide a good working model for such changes in similar sediments elsewhere. Knowledge of these reactions allows plausible explanations for mica fabric formation in mildly metamorphosed sediments to be developed. Holeywell and Tullis have identified three preferred orientations of mica across the strain gradient in the Martinsburg Formation at Lehigh Gap, Pennsylvania. These are muscovite in the bedding plane, chlorite at 20° to the bedding plane, and a slaty cleavage defined first by muscovite and with increased strain by chlorite, both at 90° to the bedding plane. The muscovite orientation in the bedding plane is attributed to sedimentary deposition of muscovite and an illite-smectite mixed-layer phase, followed by a replacement of the smectite component by illite during diagenesis. The chlorite is probably diagenetic, and its orientation may reflect a 20° rotation of the bedding plane from the plane normal to λ 3 . The orientation of muscovite and chlorite in the slaty cleavage is consistent with a pressure solution mechanism causing first the pressure solution of quartz and feldspars and the precipitation of muscovite, and later, with increased strain, the recrystallization of the earlier formed muscovite and chlorite.