The atomic ratios of natural ferruginous biotites with reference to 'the stability relations of the ferruginous biotite, annite'

The atomic ratios given by Eugster and Wones (1962) for analyses of high FeO-low MgO micas cited by Foster (1960) are in error. The recalculated atomic ratios herein presented show that only three of the thirteen have low (OH+F), and only one has characteristics that may possibly be due to loss of water during determination. None of the analyses exhibit characteristics suggestive of oxidation of iron and loss of H, as in progression towards oxyannite.

The average recalculated octahedral occupancy is 2.69, not 2.91 as in the previous calculation.

The differences between atomic ratios calculated on the basis ot determined H₂O⁺ and F and those calculated on the basis of the theoretical O₁₀ (OH)₂ content are not great enough to change the essential compositional character of these micas as interpreted by Foster (1960). However, they do affect significantly the calculated octahedral occupancy.