Geochemical evolution of pyrochlore during supergene alteration of CatalaoII ore deposit.

The crystallochemical evolution of pyrochlores in the course of the meteori c alteration has been followed, with classical techniques of mineralogical investigation (optical microscopy, SEM, electron microprobe SX50 CAMECA), o n drilling and trenches tallying the thick ferrallitic alteration cover in the Catalao II ore deposit (the Goias state, Brazil). An abrupt transformat ion of the primary pyrochlores (pyrochlore s.s.) into secondary species of the type bario or cerio-pyrochlore has thus been observable to the pedogene tic silcrete level of depth (30-40 m). This transformation shows the charac teristics of a pseudomorphose and results from the preferential washing of some cations (Na, Ca) and anions (F), partially replaced by Ba or REE and ( OH). (C) 2001 Academie des sciences / Editions scientifiques ct medicales E lsevier SAS.