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Pedological studies using X-ray diffraction (XRD), X-ray fluorescence (XRF), optical microscopy, and scanning electron microscopy (SEM-EDS) showed a Xanthic Ferralsol formed from the degradation of bauxite on a slope in the Caparaó region, in southeastern Brazil. We found a decrease in the number and size of bauxite fragments toward the top of the profiles, bauxite fragments that were more degraded at the top of the profiles, transformation of gibbsite into kaolinite, and absolute enrichment in silicon in the mass balance. These indicators suggest that resilication could be the major process responsible for formation of the soil; detailed studies are needed to verify the origin of the silica. The reintroduction of silica into the system occurs by the biogeochemical cycling of vegetation and, in some cases, water table fluctuations, highlighting the role of resilication as a soil-forming process in bauxite-derived soils.

The pedological studies carried out so far in Trindade Island (TI) have obtained patchy evidences of allophane, without detailed mineralogical and micromorphological studies to confirm the occurrence of Andosols in TI. Therefore, in this study, the mineralogical, micromorphological, physical and chemical characterization of four soil profiles from Vulcão do Paredão (P1) and Morro Vermelho formations (P2, P3, and P4) were carried on the latest volcanic events in Brazil from Trindade Island (TI) with the aim of to evaluate the presence of Andosols in this oceanic island. Profiles P1 and P2 are developed on pyroclastic bombs, and show, respectively, A-Bi-C and decapitated A-C horizons, whereas P3 and P4 are developed on lapillitic and bomb pyroclasts, show A-C horizons. The soil profiles have a reddish and brownish clayey matrix, are highly friable and show a plastic consistency. Their microstructures are granular, single grain and intergrain microaggregate, in which aggregates display an undifferentiated b-fabric. The mineralogical constituents of the bulk fraction are biotite, hematite, magnetite, ilmenite, pyroxene, olivine, halloysite, goethite, anatase, and rutile. The clay fraction is marked by the presence of halloysite, ferrihydrite, and little amounts of allophane. All soils presented andic properties and can be classified as non-allophanic Andosols. In addition, micromorphological features closely resemble those reported in Andosols from other volcanic islands from elsewhere. The predominance of halloysite in the clay fraction formed by alteration of sideromelane, suggests that allophane would be an intermediate phase of this rapid transformation favored by the wet climate conditions of the highest parts of TI.

This paper reports the composition and alteration products of pyroclasts in the Holocene Paredão volcano (pyroclast 1) and Late Quaternary Morro Vermelho Formation (pyroclasts 2 and 3) of Trindade Island, South Atlantic, Brazil using combined macromorphological, micromorphological, mineralogical and geochemical techniques.The pyroclasts 1 and 2 are interpreted as volcanic tuff breccia deposits, whereas pyroclast 3 is a lapilli deposit. They are dark gray in color with some altered reddish regions and show vesicles and amygdales structures with small greenish crystals of 2.0 mm scattered throughout the matrix. The eruptions can be regarded as Strombolian-type by producing pyroclastic deposits with coarse fragments with high vesicularity and fluidal shape that indicate magmatic degassing and fragmentation. Petrologic and XRD data revealed a mixture of biotite, goethite, ilmenite, anatase, magnetite, hematite, pyroxene, zeolites, and olivine as their main mineral components. Optical microscopy analysis confirms the vesicular and amygdaloid structures, with a hypocrystalline texture and a pale brown stained vitreous mass classified as sideromelane, due to its basaltic composition. The sideromelane changes to a reddish brown and yellowishbrown staining material identified as palagonite, clearly indicating a hydrovolcanic eruption that occurs when the ascending magma comes into contact with water. Infrared analyses in the palagonitized regions revealed the presence of halloysite, suggesting alteration of sideromelane to tubular clay minerals. Amygdales and microfractures are partially or totally filled with zeolites, which are formed by the percolation of water that reacts with the palagonite and precipitation of chemical elements of hydrothermal fluid.Reddish dark brown iddingsite and anhedral crystals of titaniferous magnetites occur in the fractures and edges of the olivine. These crystals are also dispersed in the matrix while some of them are zoned with Cr-rich core and Cr-poor edge, suggesting a deep mantle origin of the magma. The high trace elements content can be related to clinopyroxene (diopside) that include these elements. Geochemical data show that the pyroclasts are undersaturated in silica, plotting in the ultrabasic and foidites fields on the TAS classification diagram.

A origem das depressões fechadas associadas à cobertura laterítica aluminosa nos topos dos morros e ao longo das vertentes na região de Espera Feliz, MG, divisa com Espírito Santo, foi pesquisada através de um levantamento geofísico da cobertura pedológica associado às análises morfológicas (macro e micro) de perfis de solos. O levantamento geofísico por GPR, realizado em 2 seções ao longo  de uma vertente, abrangeu duas depressões consecutivas e foi seguido  do levantamento sistemático da cobertura pedológica por meio da abertura de 5 perfis (N1, L1, L2, N2 e L3). Foram coletadas amostras deformadas para análises físicas, mineralógicas e químicas e amostras indeformadas para análise micromorfológica. No interior das depressões fechadas foi identificado o Neossolo Litólico húmico típico e entre as depressões ocorre o Latossolo Vermelho Amarelo distrófico húmico. A análise lateral realizada pelo GPR e a interpretação conjunta dos atributos da cobertura revelam um sistema pedológico contínuo do topo a base da vertente. As diferenças morfológicas que levam a existência de distintas classes de solos traduzem, neste contexto, a evolução pedogeomorfológica da vertente e consequente gênese das depressões, o que teria ocorrido de maneira distinta ao modelo de subsidência por lixiviação geoquímica, mais amplamente conhecido. Dois momentos são reconhecidos: i) a origem de uma cobertura latossólica relacionada ao avanço vertical da degradação da bauxita e à bioturbação sob condições climáticas úmidas e ii) remoção de partes dessa cobertura em condições climáticas (possivelmente áridas) posteriores.