Stability relations of the ferruginous biotite, annite

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 (P_total≈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.