Juergen,
Speaking of biotite tangents, the thermodynamic properties of
Ti, vacancy substitutions, and solid solution of muscovite also remain
unclear. One would expect that these problems would be exacerbated at
higher T. Below is what I wrote in a review paper (Essene, 2009).
"Determination of the Ti formula in a given biotite requires
measurement of both FeO and total Fe as well as H2O, and even then one
cannot distinguish Ti-1Si1 from Ti-1(Al)IV-2(Fe2+)1Si2 without
additional structural information. One cannot simply assume all Fe2+
in biotite even in graphitic schists because of the documented
significant (8-15%) Fe3+ found in Mössbauer in biotite by Guidotti and
Dyar (1991). A complex biotite composition means the major mechanism
for Ti is ambiguous because reactions can be written among postulated
biotite species, including the above Ti exchanges, as well as Al-oxy
or Fe3+-oxy and vacancy as dioctahedral solid solution, [ ]2R3+=3R2+
(Bohlen et al. 1980; Rancourt et al. 2001). The latter substitution
is fixed for Al vs. Mg + Fe2+ in both micas when muscovite coexists
with biotite. Patiño Douce (1993) postulated an octahedral site
vacancy substitution for Ti, [ ]Ti=2R2+, but there is no way to
distinguish it from a vacancy engendered by dioctahedral Al or Fe3+
substitution. The correlation of octahedral site vacancy with
increasing Ti (Patiño Douce 1993) is potentially flawed: a false
vacancy in Ti-oxy biotite is generated by a 22 O normalization.
Consider the formula of a Ti-oxy biotite,
K2TiFe4.5Al0.5Si5.5Al2.5O20(OH)2O2, which must be normalized to 23 O!
Erroneously normalizing to 22 O yields
K1.91Ti0.96Fe4.30Al0.12[ ]0.62Si5.26Al2.74O20(OH)4 on an anhydrous
basis, apparently closer to a dioctahedral than a trioctahedral mica,
but only because it was misnormalized."
"Electron microprobe data alone cannot be used to predict the VI
occupancy in biotite (Afifi and Essene 1988). The only reliable way
to normalize titanian biotite is to measure Fe3+/Fe2+ as well as H2O
and apply an anion normalization (Bohlen et al. 1980; Sassi et al.
2008). One cannot evaluate alternative substitutions in biotite or
muscovite with an electron microprobe analysis when also granting the
possibility of Fe3+. Atomic plots such as those shown by Sassi et al.
(2008) are not persuasive of a particular substitution even with
complete analyses of biotite because they represent projections from
multicomponent biotite space onto two dimensions. Thus, Ti-oxy
substitution cannot be resolved from Al- or Fe-oxy, nor Ti-vacancy
from Al- or Fe3+-vacancy substitutions. Structure refinements of 10
biotite grains from medium grade metapelites show 6-24% vacancies
ordered into the M1 octahedral site and 0-15% vacancies in the K site
of the biotite crystals, as well as 7-12% trioctahedral substitution
in four muscovite crystals that coexist with the refined biotite
grains (Brigatti et al. 2008). Reasonable crystal-chemical
assumptions were made in reaching this result, including placing all
Ti and AlVI in the M2 site of biotite. The data indicate limited but
significant mutual solid solution of biotite and muscovite in
peraluminous metapelites and granites (Brigatti et al. 2000, 2008)."
Drill core or quarry sampling is one way to test the problem.
One would think that if the biotite is oxidized late it would be by
the oxybiotite substitution, Fe2+OH / Fe3+O (barring partial
chloritization). In that case the biotite composition would remain
otherwise unchanged. It would be nice to compare nearby metamorphic
rocks at the same grade with different inferred oxidation states
(e.g., hematite quartzites vs. graphitic schists). We have found that
biotite from drill core samples of oxidized granites having hematite-
rich ilmenite has higher Fe3+ than from more reduced samples with
hematite-poor ilmenite. Yes, systematically comparing the valence of
biotite in a variety of graphitic schists is a good approach. I have
faith in petrographic observation but it needs testing.
cheers,
eric
Afifi AM, Essene EJ (1988) Minfile: a microcomputer program for
storage and manipulation of chemical data on minerals. Am Mineral
73:446-448
Bohlen SR, Peacor DR, Essene EJ (1980) Crystal chemistry of a
metamorphic biotite and its significance in water barometry. Am
Mineral 65:55-62
Brigatti MF, Frigieri P, Ghezzo C, Poppi l (2000) Crystal chemistry of
Al-rich biotites coexisting with muscovites in peraluminous granites.
Am Mineral 85:436-448
Brigatti MF, Guidotti CV, Malferrari A (2008) Single-crystal X-ray
studies of trioctahedral micas coexisting with dioctahedral micas in
metamorphic sequences from western Maine. Am Mineral 93:396-408
Essene EJ (2009) Thermobarometry gone astray. In: Physics and
Chemistry of Earth's Interior, Gupta AK, Dasgupta S, eds., Platinum
Jubilee, Indian Nat. Sci. Acad., Springer-Verlag, Chap. 6, 101-129.
Guidotti CV, Dyar MD (1991) Ferric iron in metamorphic biotite and its
petrologic and crystallochemical implications. Am Mineral 76:161-175
Patiño Douce AE (1993) Titanium substitution in biotite: an empirical
model with applications to thermometry, O2 and H2O barometries, and
consequences for biotite stability. Chem Geol 108:133-162
Rancourt DG, Mercier PHJ, Cherniak DJ, Desgreniers, S, Kodama H,
Robert JL, Murad E (2001) Mechanisms and crystal chemistry of
oxidation in annite: resolving the hydrogen-loss and vacancy
reactions. Clays Clay Minerals 49:455-491
Sassi R, Cruciani G, Mazzoli C, Nodari L, Craven J (2008) Multiple
titanium substitutions in biotites from high-grade metapelitic
xenoliths (Euganean Hills, Italy): complete crystal chemistry and
appraisal of petrologic control. Am Mineral 93:339-350
Hi everyone,
This is now going off on a tangent perhaps, but following up on Eric’s
remark, I am wondering where we are with the Fe3+ in biotite problem.
There seems to be some consensus that all biotite contains substantial
Fe3+, even under reducing conditions, based on quite a number of
published ferrous/ferric Fe determinations in biotites from a variety
of rocks which appear to confirm that. There are two comments I want
to make, just to play the devil’s advocate (and to provoke debate):
(1) I remember that many years ago Moessbauer analyses had been done
on drillcore samples from the KTB deep drilling project showing
variable Fe2+/Fe3+ ratios in biotite, but Fe3+ was not detected in all
biotite samples. If that is real, any Fe3+ measured may well be
secondary. I believe all these rock samples were graphite-bearing and
compositionally similar.
(2) Biotite is notoriously sensitive to secondary oxidation. Whether
it “looks fresh” in the microscope is not a particularly strong
argument to exclude any secondary alteration. It just means there is
no obvious visual evidence. So, how can we ever be sure that the Fe3+
contents actually measured, let alone those introduced via correction
procedures, are even roughly correct, in terms of primary biotite
composition? Most commonly, we pick our samples from surface outcrops,
and yet assume that minerals such as biotite remained isolated from
the influence of subsurface oxidizing fluids (before exposure) and
weathering agents. Isn’t that wishful thinking in order to keep our
lives simple?
Are there any experiments to shed some light on this?
Cheers,
Juergen
J. Reinhardt
School of Geological Sciences
University of KwaZulu-Natal
Durban, 4000
South Africa
