Clay mineral facies and lateritization in basalts of the southeastern Parana Basin, Brazil

Seventeen samples from two lateritic profiles, each with five facies, were studied. These profiles occur on the old planation surface of the plateau b asalts of the southern part of Parana Basin, Brazil. Optical microscopy, X- ray diffraction, electron microprobe, Mossbauer spectroscopy and Fourier Tr ansform Infrared Spectra were used to obtain information about the nature a nd chemical composition of each weathering facies. In addition, scanning el ectron microscopy and analyses of clay minerals were performed to detect mi crocrystalline environmental changes. Both profiles have two major parts: a loose red-clay latosol separated from an underlying mottled clay and an al terite facies; a stone line may or may not be present between the latosol a nd the underlying units. In both profiles the latosol consists principally of kaolinite, hematite and goethite. Two alterite facies, shaped by differe ntial weathering, are also present in the lower profile: a halloysite-nontr onite clayey matrix with a well developed fissure system occurs in the argi llaceous alterite and a network of Al-goethite aggregates is typical of the highly porous cortex of the boulder alterite that is found in the stone li ne and below it. Gibbsite has crystallized in the large pores of porphyriti c boulder alterite but is absent in the small pores of the subaphyric bould er alterite. Clay minerals observed in fissures include halloysite associat ed with goethite and manganese oxides. The basalt has hydrothermal green-cl ays (mixed layers and trioctahedral smectites) that formed between primary plagioclase, pyroxene and Ti-magnetite crystals while fresh corestones of t he boulder alterite have cryptocrystalline iron-rich material. The study of these profiles shows one principal evolutionary trend for clay minerals. T his trend is from smectite and mixed layers that form green clays in altere d bedrock at the base of the profile to an intermediate association of nont ronite and halloysite in the argillaceous alterite to kaolinite in the mott led clay facies at the top of the profile. Then is also another trend; the presence of a cryptocrystalline material sequence in the weathering of the corestones. This study of two polycyclic profiles developed on basalts show s the presence of iron-rich laterites in the southern Parana Basin which ar e closely similar to the laterites in the northern part of the Parana Basin . (C) 1998 Elsevier Science Ltd. All rights reserved.