Chlorite formation from biotite

Samples of the chlorite are relatively stable to heat, but the C horizon samples tend to collapse at 500° (K saturated) to 12.8 A, 11.6 A, and 10.0 A. The authors suggest that evidence exists for the formation of chlorite from biotite mica. 

On an AFM diagram (Fig. 16) all the minerals from iron-formations proper fall into a narrow and clearly defined field, while the rock-forming minerals of the associated iron-rich shales from BIF of Superior type (chamosite, ripidolite and other chlorites, garnets and some biotites) form another broader and less clear-cut field. These two fields do not overlap, which indicates the isochemical character of metamorphism and the limited mobility of the main rock-forming minerals, at least within individual layers and bands of the iron-formations and shales. The chemical composition of stilpnomelane occupies an intermediate position both in iron content and in alumina content. However, a high content of potassium (up to 2.2% K2O), an element not typical of most BIF, is necessary for the formation of this mica. 

The watershed is underlain by the Lower Pelitic Schist of the Wissa-hickon Formation. The minerals constituting the rock, in order of abundance, consist of quartz, muscovite, oligoclase (An24Ab7c), biotite, and staurolite with minor amounts of garnet, tourmaline, zircon, apatite, chlorite, and pyrite. The bulk chemical composition of the unweathered rock calculated from the modes is shown in Table I. The mantle of weathered rock and soil consists primarily of quartz and muscovite in the sand sized fraction and kaolinite and illite in the clay sized fraction. Gibbsite was observed in samples from the tops of the drainage divides. 

The sandstone is primarily composed of quartz grains some of which are well rounded and may have been derived from the Alexandra Formation of the Devonian Taylor Group. Grains of microcUne and sodic plagioclase occur throughout but deCTease in abundance toward the top of the formation. In addition, the sandstones contain lithic grains (schist and phyllite), biotite, muscovite, and pink garnet The matrix consists of sericite, chlorite, quartz, and calcite which are post-depositional. 

---