Kaolinite to Halloysite-7 Å Transformation in the Kaolin Deposit of SÃo Vicente de Pereira, Portugal

Iuliu Bobos1, Joelle Duplay2, João Rocha3 and Celso Gomes1
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@geo.ua.pt

Abstract: 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 02,11 and 13,13 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

Clays and Clay Minerals; December 2001 v. 49; no. 6; p. 596-607; DOI: 10.1346/CCMN.2001.0490609
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)