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Paramagnetic Defect Centers in Hydrothermal Kaolinite from an Altered Tuff in the Nopal Uranium Deposit, Chihuahua, Mexico

Published online by Cambridge University Press:  02 April 2024

Jean-Pierre Muller ,
Philippe Ildefonse  and
Georges Calas
Jean-Pierre Muller
Affiliation:
O.R.S.T.O.M., Département T.O.A., 75480 Paris Cedex 10, France Laboratoire de Minéralogie-Cristallographie, UA CNRS 09, Universités Paris 6 et 7, 4 Place Jussieu, 75252 Paris Cedex 05, France
Philippe Ildefonse
Affiliation:
U.F.R. Sciences Physiques de la Terre, Université Paris 7, 2 Place Jussieu, 75251 Paris Cedex 05, France Laboratoire de Minéralogie-Cristallographie, UA CNRS 09, Universités Paris 6 et 7, 4 Place Jussieu, 75252 Paris Cedex 05, France
Georges Calas
Affiliation:
Laboratoire de Minéralogie-Cristallographie, UA CNRS 09, Universités Paris 6 et 7, 4 Place Jussieu, 75252 Paris Cedex 05, France
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Abstract

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Point defect centers in hydrothermal kaolinite have been investigated using electron paramagnetic resonance (EPR). Kaolinite was sampled in petrographically well-defined materials coming from uranium-rich hydrothermally altered volcanic tuffs (Nopal I uranium deposit, Chihuahua, Mexico), which show extensive kaolinization and an intense redistribution of uranium. Several kaolinite parageneses were defined according to their origin (fissure fillings and feldspar pseudomorphs); their location relative to the U6+ mineralization at the scale of the deposit (mineralized breccia pipe vs. barren surrounding rhyolitic tuffs), and at the scale of mineral assemblages; and their crystal chemistry.

Two types of centers of axial symmetry were identified (A- and A′-centers) and represent positive holes trapped on apical oxygens (Si-O−-centers). A-centers were stable to 400°C, whereas A′-centers annealed at 350°C. A relation between defect-center concentration and U content demonstrates that natural irradiation was responsible for these centers. On the other hand, defect-center concentration was not directly linked to the origin (fissural or feldspar pseudomorph) or the crystal chemistry (structural order and substitutional Fe3+ content) of the kaolinite. According to petrographic data, and with respect to the relative thermal stability of A- and A′-centers, two successive irradiations of kaolinite were evidenced: (1) originally during crystallization of kaolinite from radioactive hydrothermal solutions, and (2) permanently when kaolinite was in contact with secondary U-silicates, which led to the formation of A′-centers.

Because of the short half-life of U, these two radiation-induced centers were created by short-lived elements of the U-decay series. As a consequence, variations of defect-center concentration possibly reflect variations in radioactive disequilibrium during the history of the alteration system. This provides a unique tool for tracing the dynamics of the transfer of radionuclides in the geosphere: kaolinite may be used as a sensitive in situ dosimeter, which may be useful in the fields of weathering petrology and nuclear waste management.

Keywords

Defect centersElectron paramagnetic resonanceHydrothermalKaoliniteNatural radiation
Type
Research Article
Information
Clays and Clay Minerals , Volume 38 , Issue 6 , December 1990 , pp. 600 - 608
Copyright
Copyright © 1990, The Clay Minerals Society

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