This report presents an analysis of phase relations among biotite, muscovite, chlorite, stilpnomelane, actinolite and K-feldspar in a ten component system within the greenschist facies. It is based on study of the chemical composition of these minerals, on calculations to balance chemical equations-among them, and on their field distribution. A petrogenetic grid resulting from this treatment consists of a single invariant point and multiple univariant lines, the number depending on what part of the assemblage set is held constant. The reactions which involve biotite are similar to previously proposed reactions for the biotite isograd. At high pressure, biotite is produced from muscovite+stilpnomelane+actinolite±K-feldspar. At low pressure, chlorite+K-feldspar±stilpnomelane±actinolite react to form biotite. A biotite-free reaction, not previously identified, divides the chlorite zone into high pressure and low pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinolite and chlorite+K-feldspar, respectively. In the blueschist facies, muscovite plus stilpnomelane and/or actinolite are stable instead of biotite.