Modification of surface charge properties during kaolinite to halloysite-7angstrom transformation

The surface charge properties of well and poorly ordered kaolinite and hall oysite-7 Angstrom, representing three different stages of kaolinite to hall oysite-7 Angstrom transformation identified in the kaolin deposit of Sao Vi cente de Pereira (Portugal), were studied. Mineralogical (X-ray diffraction , Fourier transformation-infrared) and chemical data (analytic electron mic roscopy) showed that the gradual transformation from kaolinite to halloysit e-7 Angstrom minerals was accompanied by an increase in hydration and a dec rease in Si/Al ratio. In particular, the replacement of Si(IV) by Al(III) i n the tetrahedral layer caused an electrical charge unbalance and a modific ation of surface charge properties during the kaolinite to halloysite trans formation. Accordingly, the cation exchange capacity (CEC) gradually increa ses when passing from well-ordered kaolinite to halloysite-7 Angstrom, atte sting the direct correspondence existing among the structural order of the samples, number of tetrahedral substitutions, and CEC. Electrophoretic expe riments provided further evidences of the origin of surface charge properti es and the variation encountered during kaolinite to halloysite-7 Angstrom transformation. The curves of zeta-potential versus pH show the same pH dep endency for the three minerals, but a gradual increase of zeta-potential wh en passing from well ordered kaolinite to halloysite-7 Angstrom through the poorly ordered kaolinite. The results can unambiguously be attributed to t he increase of permanent charge due to the higher degree of isomorphic subs titution, more than to the increase of structural and ionizable water. (C) 1999 Academic Press.