A Petrogenetic Grid for Reactions Producing Biotite and other Al--Fe--Mg Silicates in the Greenschist Facies

This report presents an analysis of phase relations among biotite,muscovite, chlorite, stilpnomelane, actinolite and K-feldsparin a ten component system within the greenschist facies. Itis based on study of the chemical composition of these minerals,on calculations to balance chemical equationsamong them, andon their field distribution. A petrogenetic grid resulting fromthis treatment consists of a single invariant point and multipleunivariant lines, the number depending on what part of the assemblageset is held constant. The reactions which involve biotite aresimilar 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±actinolitereact to form biotite. A biotite-free reaction, not previouslyidentified, divides the chlorite zone into high pressure andlow pressure fields, characterized by the assemblages muscovite+stilpnomelane+actinoliteand chlorite+K-feldspar, respectively. In the blueschist facies,muscovite plus stilpnomelane and/or actinolite are stable insteadof biotite.