A Mössbauer spectral study is described of 15 potassic ferruginous 1M micas representing a wide range of chemical compositions and parageneses. Considerable precaution was taken to alleviate problems due to oxidation and preferred orientation of mica platelets during sample preparation. Biotite Mossbauer spectra were resolved into two Fe2+ and two Fe3+ doublets assigned to cations in the two cis-M2 and one trans-M1 positions. Deviations of peak areas from the site multiplicity 2:1 ratio demonstrated that Fe2+ ions are relatively enriched in cis-M2 positions and that this is accentuated in biotites from metamorphic rocks. Tetrahedral Fe3+ ions were confirmed in ferriphlogopite, ferriannites from banded iron formations, and some annites. Annite from the type locality in Massachusetts contains high concentrations of octahedral Fe2+ and Fe3+ cations in comparable amounts, in accordance with Dana’s (1868, A System of Mineralogy, 5th edition) original definition of this annite. Of the samples studied, the annite from Pikes Peak, Colorado, approximates most closely Winchell’s (1925, American Journal of Science, 5th ser., 9, 309-327) classification of annite as the Fe2+ endmember analogue of phlogopite.

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