Stability Relations of the Ferruginous Biotite, Annite

Annite, KFe₃AISi₃O₁₀(OH)₂ a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo₂), and total pressure (P_totalPH₂O+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 propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo₂-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH₂0 and fo₂ (or fH₂0 and fH₂). At 2, 070 bars total pressureannite+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 morerestricted. Phase equilibria in the system KAlSiO₄–SiO₂–Fe–O₂–H₂have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo₂values in nature as well as mechanisms for changes in fo₂ areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo₂changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks.