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KAOLINITE TO HALLOYSITE-7 Å TRANSFORMATION IN THE KAOLIN DEPOSIT OF SÃO VICENTE DE PEREIRA, PORTUGAL

¹ Department of Geosciences, University of Aveiro, 3810-Aveiro, Portugal
² Centre de Geochémie de la Surface, Universite "Louis Pasteur", 67084-Strasbourg Cedex, France
³ Department of Chemistry, University of Aveiro, 3810-Aveiro, Portugal

E-mail of corresponding author: juliu{at}geo.ua.pt

The transformation of kaolinite to halloysite-7 Å was identified in the kaolin deposit of São Vicente de Pereira (SVP), using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). Both the 02, 1 and 1, 13 reflections show changes in the XRD patterns along the kaolinite to halloysite-7 Å transition, and the FTIR spectra show changes corresponding to both OH^– and Si–O–stretching bands and Al–O–Si–bending vibrations. The interlayer water content in the kaolinite structure increases during transition. The two-layer periodicity of well-ordered kaolinite and rolling up of kaolinite plates are observed using high-resolution transmission electron microscopy (HRTEM). Long and short tubes exhibit halloysite-7 Å. No structural Fe was found in the kaolinite samples. Analytical electron microscopy (AEM) indicates no substitution of Al³⁺ for Si⁴⁺. The Si/Al ratio shows values of ~1 for the kaolinite and rolled kaolinite plates. The ²⁷Al magic angle spinning neutron magnetic resonance (MAS-NMR) spectra display a resonance centered at ~1 ppm, assigned to six-coordinated aluminum. The transformation of kaolinite to halloysite-7 Å is controlled by surface reaction.

Key Words: Kaolinite to Halloysite-7 Å Transformation • Hydration • Rolling and Folding • Surface Reaction Control • Portugal

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