Annite, KFe3AISi3O10(OH)2 a member of the iron biotites and the ferrous analogue of phlogopite, has been synthesized and its phase relations have been determined as functions of temperature, fugacity of oxygen (fo2), and total pressure (Ptotal≈PH2O+PH2). A method for controlling fo2at high total pressures is described, and data for the ‘oxygen buffers’ used are given. Buffers range from quartz+iron+fayalite assemblages (low fo2) to magnetite-hematite assemblages (high fo2). Optical properties and unit-cell dimensions of synthetic annites depend on the conditions of synthesis. By recalculating published analyses of natural iron-rich biotites it can be shown that one cannot assume a constant hydrogen content for such biotites. Oxidation may have occurred by drying at 115°C. Octahedral occupancy therefore cannot be calculated from such data. Phase relations of annite are presented in 2,070 and 1,035 bar sections. Depending on fo2-T values annite was found to decompose to one of the following assemblages: hematite+ sanidine, magnetite+sanidine, fayalite+leucite+kalsilite, iron+sanidine. All decompositions are dehydration and redox reactions and are sensitive to changes in fH20 and fo2 (or fH20 and fH2). At 2, 070 bars total pressure annite+magnetite+sanidine can coexist between 425°C and 825° C, depending upon the magnitude of fo2. In the presence of quartz the stability field of annite is more restricted. Phase equilibria in the system KAlSiO4–SiO2–Fe–O2–H2 have been summarized schematically. Wherever possible, thermodynamic extrapolations are made to test the internal consistency of the data. Enthalpies of formation are calculated for both annite and phlogopite. Ranges of fo2 values in nature as well as mechanisms for changes in fo2 are investigated. It is useful to distinguish between assemblages which are internally buffered with respect to fo2changes and those which are not buffered. The applications of individual reactions involving annite to specific geologic problems are discussed with respect to igneous, metamorphic, and sedimentary rocks.
EUGSTER et al. (Mon,) studied this question.