Comptes Rendus. Géoscience (Apr 2022)
Morphology, chemical composition and origin of 2:1 phyllosilicates in Bw horizons of latosols of the Brazilian Central Plateau: contribution to the discussion of the microgranular structure origin
Abstract
Polished sections of two Bw horizons collected in latosols located on the South American Surface (LSAS) and the Velhas Surface (LVS) in the Brazilian Central Plateau were studied using backscattered scanning images (BESI) in scanning electron microscopy and energy dispersive spectroscopy (EDS). BESI showed areas with a strong microgranular structure associated with areas having a weak to moderate microgranular structure. BESI at high magnification also showed the presence of elongated particles 1 to $30~\mu \mathrm{m}$ long and 0.2 to $5~\mu \mathrm{m}$ wide, most of them exhibiting an internal layer organization of 2:1 phyllosilicate particles. Chemical composition determined using EDS showed that the core of the largest particles was consistent with the chemical composition of poorly weathered muscovite with a small proportion of Al in the interlayer space as hydroxy-Al. Results also showed that when the $\mathrm{K}_{2}\mathrm{O}$ content decreased, indicative of a stronger weathering of the primary minerals such as muscovite, the chemical analysis showed that the proportion of hydroxy-Al in the interlayer increased. From our results, it can be assumed that these particles correspond to weathered micas and hydroxy-Al interlayered vermiculites (HIVs) recognized earlier in the literature. They are interpreted as having been brought in the Bw horizons by the burrowing activity of termites and originating from deep horizons of the latosols studied or from the saprolite in which the latosol has developed. This explains their presence associated with minerals resulting from intense weathering of the mineral present in the parent material. In line with this interpretation, the strong microgranular structure of the Bw horizons results from fragmentation and transport activity while the weak to moderate microgranular structure corresponds to old portions of channels and cavity walls. Finally, our results show that some latosols should be considered as a biomantle in which the physical properties related to the microstructure development and chemical properties related to the presence of poorly to intensely weathered muscovites and of HIVs result from faunal activity.
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