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Low-temperature aqueous alteration of crystalline pyrochlore: correspondence between nature and experiment

Published online by Cambridge University Press:  05 July 2018

T. Geisler ,
J. Berndt ,
H.-W. Meyer ,
K. Pollok  and
A. Putnis
T. Geisler
Affiliation:
Institut für Mineralogie, University of Münster, Corrensstr. 24, 48149 Münster, Germany
J. Berndt
Affiliation:
Institut für Mineralogie, University of Münster, Corrensstr. 24, 48149 Münster, Germany
H.-W. Meyer
Affiliation:
Institut für Physikalische Chemie, University of Münster, Corrensstr. 30, 48149 Münster, Germany
K. Pollok
Affiliation:
Institut für Mineralogie, University of Münster, Corrensstr. 24, 48149 Münster, Germany
A. Putnis
Affiliation:
Institut für Mineralogie, University of Münster, Corrensstr. 24, 48149 Münster, Germany
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Abstract

Pyrochlore has been considered as a waste form to immobilize high-level nuclear waste such as excess weapons-grade plutonium. In order to study the aqueous stability of pyrochlore, we have carried out hydrothermal experiments with a natural microlite (Nb + Ta > 2Ti; Ta ⩾ Nb) at 175°C in a neutral and acidic solution for 14 days. The starting material and the experimental products were studied by electron microprobe, backscattered electron (BSE) imaging, powder infrared (IR) and micro-Raman spectroscopy, and powder X-ray diffractrometry (XRD). The microlite has small U (∼200 ppm) and Th (∼800 ppm) contents and is crystalline. The hydrothermal treatment in a solution containing l mol/l HCl and l mol/l CaCl2 causes the partial replacement of the microlite (up to ∼10 μm) by a new pyrochlore phase. This new phase is characterized by a larger unit-cell volume and contains a large number of vacancies at the A site (A = Ca, Na) as well as anion vacancies, molecular water, and possibly OH groups. Analyses of the experimental fluid further revealed that U was also selectively lost to the solution. Treatment in pure water did not produce reaction zones detectable by BSE imaging or powder XRD. However, significant spectral changes in powder IR spectra of the reaction product and the detection of Na and Ca in the solution indicate that the microlite has also reacted in pure water. The experimental alteration features bear a remarkable resemblance to those seen in naturally altered microlite samples, suggesting that short-term experimental results can be applied to natural systems and vice versa in order to evaluate the long-term stability of a pyrochlore waste form.

Keywords

pyrochloremicroliteaqueous alterationnuclear waste
Type
Research Article
Information
Mineralogical Magazine , Volume 68 , Issue 6 , December 2004 , pp. 905 - 922
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2004

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