ON THE MECHANISMS OF APOPHYLLITE ALTERATION IN AQUEOUS SOLUTIONS. A COMBINED AFM, XPS AND MAS NMR STUDY K IRILL A LDUSHIN 1,2 ,G UNTRAM J ORDAN 1, *, M ICHAEL F ECHTELKORD 1 ,WOLFGANG W. S CHMAHL 1 , H ANS -WERNER B ECKER 3 AND WERNER R AMMENSEE 2 1 Institut fu ¨r Geologie, Mineralogie und Geophysik, Ruhr-Universita ¨t Bochum, 44780 Bochum, Germany 2 Institut fu ¨r Mineralogie und Geochemie, Universita ¨t zu Ko ¨ln, Zu ¨lpicher Str. 49b, 50674 Ko ¨ln, Germany 3 Institut fu ¨r Physik mit Ionenstrahlung (Exp. Physik III), Ruhr-Universita ¨t Bochum, 44780 Bochum, Germany Abstract—Apophyllite, a hydrous K-Ca-phyllosilicate, reacts with acidic aqueous solutions at room temperature. Various analytical methods have been applied to study the mechanism of the reaction and its characteristics, i.e. the changes in chemical composition, modifications in crystal structure and alterations in surface morphology. In contact with acidic solution, protonation of the terminal, non-bridging oxygen at the silicate tetrahedra takes place and the interlayer cations K + and Ca 2+ are removed. The protonation and ion removal causes the interlayer spacing to increase. Atomic force microscopy shows that the increase takes place discontinuously and, therefore, reflects a discontinuous reaction that comprises a two- or three- step protonation. Additionally, three structurally different protonation sites have been detected by nuclear magnetic resonance spectroscopy which also differ in the amount of close-by hydrogen, although in pristine apophyllite all terminal oxygen positions at silicate tetrahedra are structurally equivalent. In many clay minerals such structurally different protonation sites have not been detected so far. Thus, the multi- step protonation process in apophyllite clearly demonstrates the vast sensitivity of the protonation reaction on small structural variations in phyllosilicates. Key Words—Acidic Leaching, Apophyllite, Atomic Force Microscopy, Cation Exchange, Crystalline Silicic Acid, Dissolution, Nuclear Magnetic Resonance Spectroscopy, Phyllosilicates, Surface Alteration, Swelling. INTRODUCTION Detailed studies of reactions of phyllosilicates with acidic solutions provide insight into fundamental natural processes, such as weathering, formation of minerals, or geochemical cycling (e.g. Nagy et al., 1991; Wieland and Stumm, 1992; Walther, 1996). These investigations are also important for industrial applications, e.g. production of sorbents, catalyst carriers, decontaminants (Corma and Perez-Pariente, 1987; Ravichandran and Sivasankar, 1997; Saito et al., 1997; Temuujin et al., 2001). The reactions with acidic solutions cause a selective leaching of cations located in the octahedral sheet of phyllosilicates and, in some cases, lead to the formation of an amorphous phase or at least to a significant loss of crystallinity of the product (Frondel, 1979; Kaviratna and Pinnavaia, 1994; Aznar et al., 1996). In other cases, cation depletion does not cause decomposition of the structure and the product retains the structural features of the parental mineral (Lagaly et al., 1975; Frondel, 1979; Pabst, 1958; Rodriguez et al., 1994; Kosuge et al., 1995). Generally, phyllosilicate reactions in acidic solution have been studied by X-ray diffraction (XRD), magic angle spinning nuclear mag- netic spectroscopy (MAS NMR), infrared (IR) spectro- scopy and BET surface area analysis. Morphological changes have typically been studied ex situ by scanning electron microscopy (SEM) and in situ by atomic force microscopy (AFM). Atomic force microscopy is known as a powerful method for the in situ investigation of surface-liquid interfaces and has successfully been applied to the investigation of various alteration processes on mineral surfaces as well as to crystal growth and dissolution (e.g. Shiraki et al., 2000; Teng et al., 2001; Astilleros et al., 2002; Deuster et al., 2003; Duckworth and Martin, 2003; Peskleway et al., 2003), even under hydrothermal conditions (Jordan et al., 1999; Higgins et al., 2002; Aldushin et al., 2004). Studies by AFM on the behavior of phyllosilicates in acidic solution have been performed by e.g. Bosbach et al. (2000), Bickmore et al. (2001), Brandt et al. (2003), Liu et al. (2003) and Aldushin et al. (2004). Apophyllite [KCa 4 Si 8 O 20 (F,OH)·8H 2 O] is a hydrous sheet silicate. Its structure is tetragonal (P4/mnc, a = 8.96 A ˚ , c = 15.8 A ˚ ; Colville et al., 1971). Perpendicular to the c axis, layers composed of silicate tetrahedra alternate with layers of Ca, K, F and OH (Figure 1). The silicate layers of apophyllite are composed of intercon- nected four- and eight-membered rings, with the terminal, non-bridging tetrahedra apices of the four-membered rings alternately pointing up and down along the c direction. The terminal apices of adjoining layers oppose each other and form a :Si–O–Ca–O-Si: type bonding via Ca 2+ , thus forming a kind of a framework with alternating voids. Apophyllite, therefore, has many Clays and Clay Minerals, Vol. 52, No. 4, 432–442, 2004. Copyright # 2004, The Clay Minerals Society 432 * E-mail address of corresponding author: guntram.jordan@ruhr-uni-bochum.de DOI: 10.1346/CCMN.2004.0520404