OH substitution in garnets: X-ray and neutron diffraction,
infrared, and geometric-modeling studies
GEORGE A. LAGER
Department of Geology
University of Louisville, Louisville, Kentucky
40292, U.S.A.
THOMAS ARMBRUSTER
Laboratory for Chemical and Mineralogical Crystallography
University
of Bern, Freiestrasse 3, CH-30 12, Bern, Switzerland
FRANK J. ROTELLA
IPNS Division, Argonne National Laboratory, Argonne, Illinois 60439,
U.S.A.
GEORGE R. ROSSMAN
Division of Geological and Planetary Sciences
California Institute of
Technology, Pasadena, California 91125, U.S.A.
ABSTRACT
Tetrahedral-site occupations were determined for three titanian
andradites (San Benito County, California) and a synthetic deuterated
hibschite by using X-ray single-crystal and neutron powder data,
respectively. Site refinements reveal the presence of tetrahedral
vacancies (4-14%) in all three andradites. Infrared absorption spectra
measured for the same material used in the X-ray analysis indicate
structurally bound water (as OH-) in amounts of
0.8 to 5.7 wt% OH, which is in good agreement with water contents
derived from the refinements. These data confirm that the tetrahedral
site is not fully occupied and that charge balance can be achieved by
the substitution (O4H4)4-
= (SiO4)4-. The proton
position could not be determined because of the small amount of H
present.
Diffraction maxima in the neutron profile for hibschite exhibited
small, well-defined shoulders related to chemical inhomogeneity. The
data were fit using multiphase Rietveld techniques assuming four phases
with slightly different Si/D ratios. Structural parameters [a = 12.0105(3)
Å; O(x.y,z) = 0.03561 (14), 0.04653(12), 0.64957( 12); d site = 0.767(6)
Si) refined for the major phase [62(3)% mole fraction) were consistent
with X-ray refinements of natural hydrogrossu1ars. The deuterium atom
[0.0965(5), 0.0520(4), 0.6600(5)) was located (ΔF map)
outside the
tetrahedral volume near the position reported for the Si-free
end-member. A significant improvement in R factor was
obtained after
refinement of a split-atom model to describe the oxygen positional
disorder. The short O-D distance [0.744(6)] calculated for the ordered
(average) structure can be interpreted within the context of this model.
Distance-least-squares (DLS-76) calculations were used to simulate the
effect of the (O4H4)4-
= (SiO4)4-substitution on
the grossular
structure. If the tetrahedral d-O
distance, calculated from vacancy
concentration, is weighted heavily in the geometric refinement,
structural variations in the hydrogrossular series
[Ca3Al2(SiO4)3-Ca3Al2(O4H4)3)
can be predicted. Application of DLS to
other garnet structures suggests that mantle garnets (rich in pyrope
component) may contain only very limited amounts of water.