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DIVISION S-9—SOIL MINERALOGY

Chemical and Mineralogical Properties of Kaolinite-Rich Brazilian Soils

^a Rua dos Funcionários, 1540, Juvevê, Departamento de Solos, Universidade Federal do Paraná, Curitiba (PR), 80035-050, Brazil
^b Dep. of Agricultural Chemistry and Soil Science, The Univ. of Sydney, N.S.W. 2006, Australia
^c Departamento de Solos, Universidade Federal de Viçosa, Viçosa (MG), 36571-000, Brazil

* Corresponding author (b.singh{at}acss.usyd.edu.au)

Highly weathered kaolinitic soils in Brazil often have adequate levels of K and Mg to support plant growth. The source of K and Mg in these soils and their relationship with the dominant mineral, kaolinite, is addressed in this article. Crystallographic, chemical, and morphological properties of kaolinite, from selected highly weathered Brazilian soils, were investigated by x-ray diffraction (XRD), analytical electron microscopy (AEM), chemical, and thermal methods. Kaolinite properties showed significant variations such as d(001) spacing ranging between 0.713 to 0.728 nm, width at half height (WHH) between 0.30 to 0.97 °2, and dehydroxylation temperature between 489 to 518°C. Kaolinite in the clay fraction has relatively poor crystal order with a mean crystallinity index value(CI) of 12.7. The dominant forms of the clay-fraction kaolinite were elongated and rounded, with relatively lower proportions of hexagonal particles. The silt-fraction kaolinite showed a tendency to form subspherical large aggregates with high stability. The average Fe₂O₃ level in the kaolinite of the clay fraction (19.1 g kg^-1) was higher than that obtained for the silt fraction (6.6 g kg^-1). The smaller kaolinite particles of the clay fraction showed a lower degree of crystal order, higher K and Mg levels, and lower dehydroxylation temperatures. From the strong relationship between the asymmetry index (AI) of the (001) diffraction line and the level of K in kaolinite from the younger soils, we believe that both K and Mg in kaolinite are part of residual micaceous layers interleaved in kaolinite crystals.

Abbreviations: AEM, analytical electron microscopy • AI, asymmetry index • ALN, average layer number • CI, Hughes and Brown crystallinity index • EDS, energy dispersive spectrometry • DT, dehydroxylation temperature • ICP-AES, inductively coupled plasma atomic emission spectrometer • MCD, mean crystal dimension • SEM, scanning electron microscope • SSA, specific surface area • TEM, transmission electron microscope • WHH, width at half height • XRD, x-ray diffraction