Nano-Size
Hydrogarnet Clusters and Proton Ordering in Calcium Silicate Garnet:
Part I. The Quest to Understand the Nature of "Water" in Garnet
Continues.
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 and
their solid solutions Ca3(Alx,Fe3+1-x)2Si3O12, can
incorporate various amounts of structural OH-.
This has
important mineralogical, petrological, rheological and geochemical
consequences
and extensive experiment investigations have focused on the nature of
“water”
in these phases. However, it was not known how OH- was
incorporated
and this has seriously hampered the interpretation of many different
research results.
The IR single-crystal spectra of a number of calcium silicate garnets,
both
“end-member” and solid-solution compositions, were recorded at room
temperature
and 80 K between 3000 and 4000 cm-1. Five
synthetic hydrogarnets in
the system grossular-andradite-katoite (Ca3Al2H12O12)-Ca3Fe3+2H12O12 were also
measured via IR ATR powder methods. The various spectra are rich in
complexity
and show a number of different wavenumber OH-
stretching modes
between 3500 and 3700 cm-1. The data, together
with published
results, were analyzed and modes assigned by introducing
atomic-vibrational and
crystal-chemical models to explain the energy of the OH-
dipole and the
structural incorporation mechanism of OH-,
respectively. It is
argued that OH- is located in various local
microscopic- and
nano-size Ca3Al2H12O12-
and Ca3Fe3+2H12O12-like
clusters.
The basic substitution mechanism is the hydrogarnet one, where (H4O4)4-
Û (SiO4)4-,
and various local configurations containing different numbers of (H4O4)4-
groups define the cluster type. Some spectra also possibly indicate the
presence of tiny hydrous inclusion phases, as revealed by OH-
modes
at wavenumbers greater than about 3670 cm-1.
They were not
recognized in earlier studies. Published proposals invoking purely
hypothetical
“defect” and coupled-substitution mechanisms to account for OH-
in
garnet are not needed to interpret the IR spectra, at least for OH-
modes above about 3560 cm-1. Significant
mineralogical, petrological,
geochemical and rheological implications result from the analysis and
are
discussed in Part II of the investigation.