Piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks, northeastern Abukuma Plateau, was studied in terms of bulk chemical composition, mineral assemblage and mineral chemistry.
The Pm-schist occurs as intercalating member of the albite-porphyroblastic muscovite-quartz schist and epidote-amphibolite. Petrographic features suggest that the primary mineral assemblage is quartz+muscovite+albite+chlorite+piemontite+tourmaline+hematite+rutile. Apatite, epidote and carbonate also occur as minor minerals. Veins formed of euhedral quartz and anhedral K-feldspar intruded into microgranoblastic quartz aggregation.
The compositions of piemontite range between Pm₁₁Ps₂₂Cz₆₇ and Pm₁₇Ps₁₈Cz₅₅. Muscovite is phengitic (Si 6.54-6.64 atoms per 22 oxygens) and contains moderate amount of hydromuscovite component. Chlorites are clinochlore with Fe/(Mg+Fe)=0.02 and pycnochlorite with Fe/(Mg+Fe)=0.38. Hematite contains Mn₂O₃ component.
The bulk chemical composition of the Pm-schist is characterized by the high silica content, 82.41 wt.%, and high oxidation ratio, (Fe³⁺×100)/(Fe³⁺+Fe²⁺)=93. MnO content is very low at 0.02 wt.%.
Appearance of piemontite, Mg-chlorite and hematite containing Mn₂O₃ component suggests that f_O2 condition during the metamorphism was very high, which is consistent with the extremely high oxidation ratio of the bulk composition. The extremely high f_O2 condition during metamor-phism may be explained by the dissociation equilibrium of H₂O with H₂ and O₂.