Weathering of Chlorite in a Quartz-Chlorite Schist: I. Mineralogical and Chemical Changes

Takashi Murakami†, 1, Hiroshi Isobe2, Tsutomu Sato2 and Toshihiko Ohnuki2
1 Department of Earth Sciences, Faculty of Science, Ehime University, Matsuyama, Ehime 790, Japan
2 Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
Present address: Mineralogical Institute, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan.

Abstract: The weathering of chlorite, one of the major minerals of the host rock in the uranium ore deposit at Koongarra, Australia, was examined by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron microprobe analysis, and transmission electron microscopy (TEM). The conversion sequence of chlorite weathering is: (1) chlorite; (2) chlorite/vermiculite intergrade (showing XRD responses to various treatments intermediate between those of chlorite and vermiculite); (3) interstratified chlorite and vermiculite; (4) vermiculite; and (5) kaolinite. This sequence may be more simply expressed as chlorite → vermiculite → kaolinite. The weathering finally changed chlorite into sub-micrometer to micrometer sized Fe minerals and kaolinite. The transformation of chlorite to vermiculite is chemically characterized by an Fe and Mg loss with a slight decrease in the Al/Si ratio. Mg continues to be released throughout the weathering. Fe minerals formed through chlorite weathering are located between chlorite and vermiculite domains (a few µm in size) at first, and then accumulated between grain boundaries, occasionally forming veins. The distribution of Fe minerals is suggestive of preferential pathways of water movement. The time-dependent nature of mineral alteration demonstrated in the present study must be taken into account for the quantitative estimation of radionuclide migration.

Key Words: Alteration • Chlorite • Kaolinite • Uranium Ore Deposit • Vermiculite • Weathering

Clays and Clay Minerals; April 1996 v. 44; no. 2; p. 244-256; DOI: 10.1346/CCMN.1996.0440210
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)