César Augusto Chicarino Varajão

Microporosity of BIF hosted massive hematite ore, Iron Quadrangle, Brazil

Massive hematite ore (MHO) is a special high-grade iron ore, used as lump ore in the process of obtainingdirect reduction iron (DRI). The influence of porosity on the reducibility of MHO from the Capitao do MatoMine (Iron Quadrangle, Brazil) was investigated using optical and scanning electron microscopes on drillcore and open pit samples.Hematite is the main component of the samples and occurs as granular crystals

A comparison of properties of clay minerals in isalteritic and in degraded facies

Abstract The mineralogical, geochemical and micromorphological features of an isalteritic clay facies, which originated from weathering of an anorthosite, were compared to those of clay facies derived from the degradation of a bauxite developed from the same rock. The isalteritic clay was formed by the hydrolytic alteration of plagioclase, whereas the degraded clays were formed by decomposition of gibbsite and neoformation of kaolinite. This resilification process resulted from the reintroduction of silica via the oscillation of the phreatic level and/or the decomposition of organic matter on the surface. The degradation process was gradual and yielded two different facies: (a) degraded clays with almost total decomposition of gibbsite, and (b) degraded clays with gibbsite nodules. Morphologically, the isalteritic clays differ from the degraded clays because they contain larger hexagonal and pseudo-hexagonal crystals. The degraded clays have more irregular crystal shapes, ranging from laths to anhedral shapes.

Petrografia do minério hematita compacta da Mina do Tamanduá (Quadrilátero Ferrífero, MG)

The study of the mineralogy, macro and microstructures and anisotropy of magnetic susceptibility of hard hematite ore type of the Tamandua Mine enabled to identify three litostructural domains. The investigation was based on fieldwork, sampling (in situ and boreholes) and on the study of polished sections using optic microscope. Ore samples were examined by electronic microscopy and selected specimens were prepared for measuring the anisotropy of magnetic susceptibility. The knowledge of the ore facies domains is of great importance for optimization of the mining works.

Pancas: The Kingdom of Bornhardts

Underlain by Neoproterzoic granitic rocks, the region around the town of Pancas, located in the Espirito Santo State, eastern Brazil, comprises a spectacular landscape dominated by bornhardts of various shapes. Sugarloafs, turtlebacks, domes and pinnacles separated by long and rectilinear valleys occupy an area of ca. 17,400 ha, which was declared natural monument by the Brazilian government in 2008. The development of the bornhardt province seems to involve the following stages: (i) nucleation of a system of vertical and widely spaced joints in the granitic bedrock at sometime in the Late Neoproterozoic, (ii) intensive weathering along the joints probably during the Eocene and (iii) uplift and subsequent erosion between the Late Miocene and the Pliocene, thereby leading to the exhumation of the bornhardts.

DENUDAÇÃO QUÍMICA E REBAIXAMENTO DO RELEVO EM BORDAS INTERPLANÁLTICAS COM SUBSTRATO GRANÍTICO: DOIS EXEMPLOS NO SE DE MINAS GERAIS

Os interfluvios entre as grandes bacias hidrograficas do Brasil oriental correspondem, muitas vezes, a degraus morfologicos que dividem planaltos escalonados. Esses degraus sao feicoes escarpadas resultantes da diferenca de intensidade dos processos erosivos nas cabeceiras dessas grandes bacias. Esse artigo apresenta um estudo da denudacao quimica em dois degraus dessas bordas interplanalticas localizadas no sudeste de Minas Gerais: (i) o degrau de Cristiano Otoni (250m) divisor das bacias dos rios Sao Francisco (terras altas ou planalto superior) e Doce (terras baixas ou planalto inferior) e, (ii) o degrau de Sao Geraldo (450m) que divide as bacias dos rios Doce (terras altas ou planalto superior) e Paraiba do Sul (terras baixas ou planalto inferior). Para entender a dinâmica da denudacao quimica nessas bordas escarpadas, foram monitorados o total de solidos dissolvidos (TDS), o Eh, o pH, a vazao de 21 bacias hidrograficas distribuidas ao longo de ambas vertentes dos degraus (terras altas e frente da escarpa) no final dos periodos chuvoso (abril) e de estiagem (agosto) em um mesmo ano hidrologico (2009). As bacias amostradas situam-se sobre o mesmo substrato litologico (granitoides). Utilizando os dados de TDS e vazao foram calculadas as taxas de denudacao quimica (ton.ano-1.km-²) e as taxas medias de rebaixamento do relevo (m.Ma-1) para as bacias amostradas. Os resultados revelaram que as aguas fluviais dos degraus estudados apresentam assinaturas geoquimicas distintas: (i) nas terras altas, os valores de pH sao sempre acidos (≈ 6,35) e nas escarpas, levemente basicos (≈ 7,10); (ii) os valores de TDS (carga dissolvida) nas terras altas (≈ 10,00 mg.L-1) sao menores que os encontrados nos frente das escarpas (≈ 35,00 mg.L-1 para Sao Geraldo e, ≈ 18,00 mg.L-1 para Cristiano Otoni). As taxas de rebaixamento do relevo ocasionadas pela denudacao quimica sao semelhantes em ambas as terras altas (≈ 2,40 m.Ma-1). Em contrapartida, a frente da escarpa da Serra de Sao Geraldo apresenta taxa de denudacao quimica mais elevada (7,06 m.Ma-1) do que o Degrau de Cristiano Otoni. (4,25 m.Ma-1). Essa diferenca foi interpretada como diretamente relacionada a altura das escarpas, 450 e 250 m, respectivamente.

Domínios geomorfológicos na área de ocorrência dos depósitos de espongilito da região de João Pinheiro, Minas Gerais, Brasil

Important erosive processes that took place after the Cretaceous Era were responsible for the evolution of the landscape in the Joao Pinheiro region where spongillite deposits occur. The progression of these erosive processes on the carbonate rocks led to the development of negative karstic features where the ponds present formed the spongillite deposits. Based on field studies and analysis of multispectral images, four geomorphological domains were identified: i) domain 1, represented by a plateau related to sandstones from the Areado Group that present the highest altitudes in the area; ii) domain 2, that constitutes a desiccated area related to pelites from the Areado Group; iii) domain 3, comprising a karstic planation surface associated with carbonate rocks from the Bambui Group and Pre-Bambui unit covered by Cenozoic sediments where the ponds occur; iv) domain 4, composed of gutter shaped valleys enclosing meandering of underfit streams (of the Prata and Paracatu rivers). These valleys cut through the karstic planation surface (domain 3) where Pleistocene sediments occur and characterize the more recent morphological domain.

Environmental factors related to the production of a complex set of spicules in a tropical freshwater sponge

Adverse natural conditions will, generally, induce gemmulation in freshwater sponges. Because of this environmental dependence, gemmoscleres are given exceptional value in taxonomic, ecological and paleoenvironmental studies. Other spicules categories such as microscleres and beta megascleres have received little attention with regard to their occurrence and function during the sponge biological cycle. Metania spinata, a South American species common to bog waters in the Cerrado biome, produces alpha and beta megascleres, microscleres and gemmoscleres. To detect the environmental factors triggering the production of all these kinds of spicules, the species annual seasonal cycle was studied. Artificial substrates were devised, supplied with gemmules and placed in Lagoa Verde pond which contained a natural population of M. spinata. Field monitoring was conducted for eight months in order to observe the growth of sponges and spicules formation. Samples of water were taken monthly for physical and chemical parameters determination. The appearance of the alpha megascleres was sequentially followed by that of microscleres, gemmoscleres and beta megascleres. The first ones built the new sponge skeleton, the last three were involved in keeping inner moisture in the sponge body or its gemmules. The water level, temperature and the silicon (Si) concentration in the pond were the most important factors related to this sequential production of spicules, confirming environmental reconstructions based on the presence or absence of alpha megascleres and gemmoscleres in past sediments.

Bulk density as method of evaluating reducibility of hard hematite ore in direct reduction process

Abstract Massive hematite ore (MHO) is a special high grade iron ore used as lump ore in the process of obtaining direct reduced iron (DRI). The influence of porosity on the reducibility has been investigated using optical and scanning electron microscopes. Hematite is the main component of the samples and occurs as granular crystals (10 μm), microplates (1 μm) and euhedral martite (10-30 μm). Quartz, maghemite, kenomagnetite and goethite are minor components. Primary micropores (Å to 1 μm) are associated with microplaty crystals that fill cavities between granular hematite. Secondary micropores (Å to 5 μm) related to euhedral martite crystals are the most important. The total porosity of weathered samples, measured using nitrogen adsorption and mercury injection, attains values up to 18%, whereas unweathered samples have a porosity less than 2·5%. Varajão et al. showed that reducibility is strongly enhanced by porosity and this work shows that reducibility can be indirectly evaluated by bulk density.