| Summary: | The compositional evolution of the phyllosilicates in the lateritic profile of the Catalão I ultramafic alkaline-carbonatiticcomplex (GO) is proposed in this work based on petrographic and X-ray diffraction analysis, as well as, chemical compositionobtained by WDS electron microprobe. The micaceous minerals found in unweathered rocks are classified as phlogopites(with different parageneses) and tetraferriphlogopites (metasomatic products). In the levels of altered rocks and isalteriticsaprolite, the phyllosilicates derived from micas are vermiculite and regular interstratified phlogopite-vermiculite (whichformation and characteristics are similar to hydrobiotite), characterizing a main vermiculitization process in this profile.Subordinately, in the upper portions and punctual areas of isalteritic saprolite, the alteration of the vermiculitized phlogopitegenerates smectitic products, which are characterized mainly by the occurrence of expansible regular interstratified mineraland even smectite. Kaolinite occurs on the top of the isalteritic saprolite and on the alloteritic level. The evolution sequenceof the phyllosilicates in the weathering profile of the Catalão I complex presents the following formation order: phlogopite,interstratified phlogopite-vermiculite, vermiculite, smectitic products (expansible interstratified minerals of local occurrence), smectite and kaolinite, of which, the last two are probably associated to supergene processes. This sequence is compatible with the most recent theories for the formation of clay minerals in soils. The main differences between the phlogopites and their products ofvermiculitic alteration are related to a significant decrease of K+ and oxidation of Fe2+, characteristic changes of the vermiculitizationprocess. In the specific case of vermiculites, the interlayer site is filled mainly by Mg2+. In addition to that, the presence of interstratified minerals (intermediate alteration products) is a key indicator of relatively recent and low-altered weathering profiles.
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