Caulinita natural proveniente da região amazônica (PP-0559) foi caracterizada morfologica e estruturalmente com o objetivo de contribuir para o entendimento do papel da caulinita em reações redox no meio ambiente. Através de microscopia eletrônica de transmissão e difração de elétrons, confirmou-se a estrutura triclínica em cristais orientados, com diâmetros entre 0,2 e 2 mm e cerca de 50 nm de espessura. Os cristais anisotrópicos após orientação foram estudados por ressonância paramagnética eletrônica em função da direção do campo magnético aplicado. Foram estudadas as principais linhas da impureza de Fe +3 substitucional e centros paramagnéticos induzidos por radiação, possíveis centros oxidantes da caulinita.We report a structural and morphological characterization of the natural kaolinite PP-0559, from the Brazilian Amazon region, with the aim to contribute to the understanding of this clay mineral in environmental redox reactions. The triclinic structure was confirmed by transmission electron microscopy and electron diffraction studies. It exhibits oriented crystals, with diameters between 0.2 and 2 mm and about 50 nm of thickness. The anisotropic crystals after orientation were studied by electron paramagnetic resonance as a function of the applied magnetic field. The main EPR absorption lines of the substitutional Fe +3 impurity and radiation induced paramagnetic centers were studied as possible oxidizing centers of kaolinite.
Keywords: kaolinite, clay minerals, EPR spectroscopy
IntroductionClay minerals are the main colloidal soil fraction. Their physico-chemical properties are fundamentally influenced by their composition and surface reactivity. They present acidic catalytic and redox properties that promote the polymerization of organic residual substrates which strongly contribute to their humification. 1 The mechanisms promoting organo-mineral complex formation are very important also in the maintenance of a micronutrient reservoir for crops. These clay mineral systems are therefore important for agricultural activities in tropical countries. 2 Stable defects in the kaolinite structure, which are detectable through electron paramagnetic resonance spectroscopy (EPR), have been associated with their habilities to polymerize amines and amino acids. 3 Thus, the comprehension of the structural and surface properties of clay minerals is fundamentally important to various scientific enterprises, such as soil and environmental sciences, as well as special advanced materials.Kaolinite, Al 2 Si 2 O 5 (OH) 4 is a dioctahedral layered hydrated aluminosilicate of the 1:1 type with two distinct interlayer surfaces. One side of the layer is gibbsite-like with aluminum atoms coordinated octahedrically to corner oxygen atoms and hydroxyl groups. The other side of the layer is constituted by a silica-like structure, where the silicon atoms are coordinated tetrahedrically to oxygen atoms (Figure 1). The adjacent layers are linked by hydrogen bonding involving aluminol (Al-OH) and siloxane (Si-O) groups. These bonding f...