Micro-
and Nano-Size Hydrogarnet Clusters in Calcium Silicate Garnet:
Part II. Mineralogical, Petrological
and Geochemical Aspects
Charles
A. Geiger1, George R. Rossman2
1Department of Chemistry and Physics of
Materials, Salzburg University, A-5020 Salzburg Austria
2Division
of Geological and Planetary
Sciences, California Institute of Technology, Pasadena, California
91125-2500, U.S.A.
ABSTRACT
The nominally anhydrous, calcium-silicate garnets,
grossular - Ca3Al2Si3O12,
andradite - Ca3Fe3+2Si3O12,
schorlomite - Ca3Ti4+2(Si,Fe3+2)O12, and their solid solutions can
incorporate various
amounts of structural OH-. The
IR single-crystal spectra
of a number of calcium silicate garnets were recorded at room
temperature
between 3000 and 4000 cm-1. Spectroscopic
results are also taken
from the literature. The spectra are analyzed and show various OH-
stretching modes with different wavenumbers between 3500 and 3700 cm-1.
Following the conclusions of Part I of this study, the garnets appear
to
contain various microscopic- and nano-size Ca3Al2H12O12-
and Ca3Fe3+2H12O12-like
clusters
dispersed throughout an anhydrous garnet “matrix”. Their sizes are
roughly
between 3 and 15 Å. The hydrogarnet cluster model can explain both IR
spectra
and also various other experimental results. Hypothetical “defect”
mechanisms are
not needed to account for OH- incorporation and
behavior in garnet. Key,
new understanding into published dehydration and H-species diffusion
results, as
well as H2O-concentration and IR
absorption-coefficient
determinations, is now possible for the first time at the atomic level.
IR
spectra with similar OH- bands are found between
end-member
synthetic and natural grossular crystals. Similar IR “OH-band patterns”
are also
found among different natural, calcium-silicate garnets possibly
indicating that
chemical equilibrium operated during their crystallization. Under this
assumption, the hydrogarnet-cluster types and their concentrations
could
potentially be used to decipher petrologic (i.e., P-T-X)
conditions under which a garnet crystal, and the rock in which
it occurs, formed. Schorlomites from phonolites contain very minor
amounts of H2O,
whereas Ti-bearing andradites from low-grade chlorite schists can
contain much
more H2O. Different hydrogarnet clusters and
concentrations can
occur in metamorphic grossulars from the well-known locality at
Asbestos,
Quebec, Canada. IR absorption coefficients for H2O
held in
hydrogrossular- and hydroandradite-like clusters must be different in
magnitude
and this work lays out how they can be best determined. Hydrogen
diffusion
behavior in garnet crystals at high temperatures is governed by the
temperature
stability of the various local hydrogarnet clusters.