Abstract
Mössbauer spectroscopy data for the Fe3+ contents of biotite from metapelitic rocks of northwestern Maine was accumulated in the context of metamorphic grade for Al-, Si-, and Ti-saturated limiting assemblages. Specimens from two areas contain graphite; those from a third area, magnetite.
The results show: (1) all biotites have 8 ± 3% of Fetot as [4]Fe3+; (2) biotite coexisting with graphite contains only 4% of Fetot as [6]Fe3+, whereas those coexisting with magnetite contain significant [6]Fe3+ (10–13% of Fetot); (3) Fe3+ contents are independent of metamorphic grade and silicate mineral assemblage; and (4) partitioning of Fe2+ into M2 vs. M1 is ca. 3/1, not the expected 2/1, and is independent of T. Whether Fe2+ is ordering directly or in response to the ordering of other cations is ambiguous.
The new Fe2+ data, plus discussions in the literature suggesting the possibility that interlayer site deficiencies involve replacement of alkalis by H3O+, taken in conjunction with petrologic constraints, support the basis for some site substitution models for biotite. Two such substitutions are: K+, Na+ ⇋ H3O+ and Ti4+ octahedral vacancies ⇋ 2(R2+).
Data now available for biotite from graphite-bearing rocks suggest a need for modification of biotite activity models. The data also imply some nonideal aspects of biotite substitutions.
It remains unclear if the Fe3+ content typical of biotite from graphite-bearing rocks significantly affects application of the various garnet-biotite geothermometers. However, the Fe3+ content typical of biotite from magnetite-bearing rocks must be considered if one expects reasonable results from garnet-biotite geothermometry.
