Saturday, 15 July 2006
128-7

Tropical Weathering Profile in a Biotite Schist.

Maria da G. de V. X. Ferreira, Universidade Católica de Pernambuco, Rua do Princípe, 526 - Boa Vista, Recife, PE, 50.050-900, Brazil, Mateus RosasOSAS Ribero, Universodade Federal Rural De Pernambuca, Rua Dom Manoel De Medeiros S/Nº. Dois Ormaos, Recife, Pe, 52171-900, Brazil, and Edmilson S. de Lima, Universidade Federal de Pernambuco, Av. Prof. Moraes Rego, 1235 - Cidade Universitária, Recife, PE, 50.670-901, Brazil.

Parent material is one of the factors controlling weathering intensity and soil development. In a rock, such as granite, texture (mainly grain size), mineralogical composition and fracture density are the most important factors. In addition to these parameters, schistosity orientation in relation to the terrain surface is also an important feature in soil developed from metamorphic rocks, like schists. The studied area is located in the Coastal Humid Zone of Northeastern Brazil, which has an altitude of 360 m. The relief is strongly rolling, with round tops and V valleys reflecting the regional foliation structure. The climate is warm and humid, with annual precipitation between 1200 and 1600 mm. The rainy season goes from April to August, and precipitation is around 200 mm/month. The original vegetation was a sub-deciduous tropical forest, replaced by a banana plantation. The parent material is a garnet biotite-gneiss with the schistosity parallel to the surface. The studied soil profile is located in the mid slope position and was described, sampled and analyzed according to standard procedures, i.e., horizon by horizon. Physical and chemical analysis (including ICP-AES), mineralogy of the soil fractions, pedography of undisturbed samples were performed and weathering indexes were calculated. The pedogenic processes, mainly physical-chemical weathering, generate a regolith of approximately 14 m thick. The soil was classified according to the World Reference Base for Soil Resources as a Haplic Lixisol (Typic Eutrophic Red Argisol in the Brazilian System). Total chemical analyses indicated progressive upward loss of Na, Ca, K and Mg. However, K and Mg do show a greater retention than Na and Ca. These bulk chemistry changes in the soil/saprolite composition are consistent with the mineralogical and geological characteristics of the studied area. Weathering and leaching in feldspars cause loss of Ca, Na and K, leaving behind, as a residue, kaolinite group minerals, whereas 50% of Mg and K remain in the phyllosilicates. The leaching of these elements is higher at two zones: 13 and 16 m depth. There was a positive correlation among mineralogical and geochemical features (mica and clay percentages and weathering index). pH in water ranged from 4.9 in the Ap horizon to 6.3 in the C horizon. In the sand fraction, pedographic studies showed quick argilization of feldspars (even sodic members) and the expected quartz preservation. Biotite gets smaller going from 5 mm in the rock to 0.1 mm in the Bt1 horizon. However, it shows an unexpected behavior, resisting to a hot, humid and oxidizing environment. In the silt and clay fractions, biotite degraded to irregular interstratified phases. Kaolinite was very well crystallized in the clay fraction. The percentage of sand (breaking down of biotite) grew from the C horizon down, but it was not followed by an increase in permeability, because voids were not created as they are during sedimentation. On the other hand, the tendency of kaolinite to flocculate improved soil permeability on horizon with higher clay content. The breaking down of lepidoblastic texture (mainly platy minerals) of the parent materials developed a chitonic texture (grains surrounded by plasma) in the Bt2 horizon (0.95-1.45 m) and a porfiritic one in the Bt1 (0.75-0.95 m). Iron oxide was present under two forms that could be erroneously taken as pedofeatures if the parent material had not been studied. Ghosts of quartz and garnet, which was transformed into iron oxide by metamorphism, created the first pseudopedofeature, whereas iron oxide bands, parallel to foliation, generated by the fibrolitization of biotite, responded for the second one. Therefore, it is very important to study the parent material when soil genesis is the concern.

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