The trend of a plot by Foster (1960) of Mg vs (Fe2+ + Mn) vs R3+ (Al, Ti, Fe3+) in the octahedral sheet of biotite has led some workers to infer an inverse correlation between AlVI and Mg/Fe2+ ratio. This paper presents arguments and data which indicate that such a correlation does not exist over most of the biotite composition range and that the trend mentioned above is in part an accident of sample availability.

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