MICROSTRUCTURE OF GOETHITE-KAOLINITE DYAD AND MECHANISM OF ITS FORMATION
Comparison of lab-prepared goethite-kaolinite association with goethite-kaolinite mixture as control was done for analysis of its microstructure and mechanism of its formation. It was found that in the association kaolinite was well wrapped by goethite, but in the mixture they coated neither. Compared to pure goethite or kaolinite, neither goethite or kaolinite in the association showed much change in the distance between major crystal planes; but the association showed a higher micropore volume and a lower average pore diameter. However, both pore parameters of the mixture were close to the average values of monomeric goethite and kaolinite. The surface fractal dimension D of the mixture and the association increased, and Dmixture was higher than Dassoeiation. Infrared spectra (IR) demonstrated that the vibration frequency of ≡Fe-OH increased and that of ≡ Al-OH, Al-O, Si-0 and Fe-O decreased in the dyads. The magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the cementation between goethite and kaolinite led to increased chemical shift of 29Si and 27Al towards positivity, which implied that cation exchange between ≡Fea OH2(m/2)+(n=1,2,3) and ≡Si-OH or ≡Al-OH20.5+, and anion exchange between ≡Fe-OH and ≡Si-O- or ≡Al-OH0.5-,surface coordination between O atoms on the surface of kaolinite and Fe atoms on the surface of goethite were the main mechanisms of cementation between goethite and kaolinite, where hydrogen bond and electrostatic attraction served as the main acting forces. Although the IR and NMR parameters of the association and mixture displayed similar changes, the variation of the former was more significant, indicating that the cementation strength between goethite and kaolinite in the association is greater than in the mixture.
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