To understand compositional variation in igneous biotite, full analyses
of a suite of biotites of variable composition from volcanic and
xenolith parageneses have been completed. Major and minor elements were
determined by electron microprobe analysis, water was determined by
manometry and SIMS analysis, and Fe3-/Fe2+ was determined by microXANES
and Mossbauer spectroscopy. Our new data, together with previous biotite
analyses (total of 52 analyses), reveal correlation,, between O2-
(2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + Fe3+ + Cr.
This correlation allows estimation of either OH or Fe3+/Fe2+ as long as
one or the outer has been determined. The hydroxyl site in most mantle
micas contains at least 1.0 O-2 atoms per formula unit (apfu),
indicating that the oxy-component cannot be ignored. The large
oxy-component in melt inclusion micas from the martian meteorite
Chassigny does not necessarily indicate oxidized or hydrous magmatic
conditions because dehydrogenation may have occurred and/or because the
oxy-component may be stable at low oxygen fugacity. The large variation
in Ti, Al, and Fe2+ in natural igneous micas is most likely dependent
upon bulk compositional differences in each specific system such as
variation of a(FeO2) and a(Al2O3) in silicate melts.