Investigation of Titanium in Metamict Nb-Ta-Ti Oxides Using the Extended X-Ray Absorption Fine Structure Technique
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Werner. Lutze, editor INVESTIGATION OF TITANIUM IN METAMICT Nb-Ta-Ti OXIDES USING THE EXTENDED X-RAY ABSORPTION FINE STRUCTURE TECHNIQUE
R. B. GREEGOR AND F. W. LYTLE The Boeing Company, Seattle, Washington 98124 R. C. EWING AND R. F. HAAKER Department of Geology, University of New Mexico, Albuquerque,
NM
87131
INTRODUCTION Recent proposals have suggested that radioactive wastes can be isolated 1 titanate assemblage. One
as dilute solid solutions in a crystalline, titanate assemblage,
SYNROC,
consists of zirconolite (CaZrTi
(CaTiO3 ) and "hollandite" (BaAl There are two major problems in
2 Ti 6 0 1 6
2 0 7 ),
perovskite
) with additional accessory phases.
the evaluation of the long term stability of
any crystalline wasteform such as SYNROC:
1) it
is
difficult to assess the
long term stability of materials from short term laboratory experiments that are not necessarily valid simulations of complex geochemical processes, the corresponding titanate minerals are uncommon,
making it
and 2)
difficult to study
long term alteration and radiation effects on a significant number of specimens from different localities and geologic environments.
There has even been con-
siderable controversy concerning the stability of the reasonably common and simple phase, perovskite (CaTiO3 ). An important approach in the evaluation of crystalline wasteforms is
to
study the long term stabilities of closely related mineral species which have become metamict (radiation damaged)
and have been exposed to weathering
processes for geologic periods of time.
The documented metamictization and
alteration effects can then be used as "benchmarks" for comparison with the results of short term laboratory leaching and irradiation experiments which have been designed to simulate long term effects. oxide minerals, Th,
are natural analogues
forms.
The Nb-Ta-Ti complex
containing appreciable rare earth elements
(REE),
U, and
for phases in proposed titanate radioactive waste-
Because of the geochemical similarities among Ti, Nb and Ta,
a
study of the metamict state and alteration effects in complex Nb-Ta-Ti oxides may yield data that is
important in evaluating
the long term stability of
radioactive wasteforms. The investigation reported here is
concerned with the application of the
extended x-ray absorption fine structure (EXAFS) edge structure (XANES)
and x-ray absorption near
spectroscopy to the study of the structure of the
410 metamict state.
These techniques were used to obtain information on the
nearby atomic arrangement for the Ti site of the metamict Nb-Ta-Ti oxides of euxenite and blomstrandine.
X-ray absorption spectroscopy is
unique in
its
ability to provide such information about specific elements in these minerals which are often non-crystalline. DESCRIPTION OF SAMPLES AND EXPERIMENTAL DETAILS The metamict Nb-Ta-Ti oxides of the type formula A B 0 Mn, Na,
Ca,
Th,
U, Pb; B = Ti, Nb,
the following minerals: aeschynite, tantalite, tions.
Fe+2,
Fe+ ) are generally assumed to include
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