The decomposition of the iron mica, annite, to sanidine plus magnetite and vapor, KFe3AlSi3O10(OH)2 = KAlSi3O8 + Fe3O4 + H2, has been reexamined experimentally with the use of a variety of buffers coupled with fH2 sensors at 2 kbar and between 400 and 840 ??C. Various capsule configurations were used in this study to delineate the equilibrium constant for this reaction in conjunction with 57Fe Mo??ssbauer spectroscopy measurements to monitor the oxy-annite component in mica in selected experiments. Results at the most reducing and highest temperature conditions of this study extend the annite stability field to higher temperature and fo2 values than those defined in previous work. Lower temperature results indicate that the annite-sanidine-magnetite stability boundary does not intersect the hematite + magnetite + H2O buffer at 400??C as previously reported but rather is subparallel to the buffer curve at lower fo2 values. The equilibrium fH2 (in bars) for the assemblage annite + sanidine + magnetite + vapor at 2 kbar and between 400 and 840 ??C can be described by the relation log fH2 (??0.08) = 13.644 - (17368/T) + (5.168 ?? 106)/T2, where T is temperature in kelvins.