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ON THE MECHANISMS OF APOPHYLLITE ALTERATION IN AQUEOUS SOLUTIONS. A COMBINED AFM, XPS AND MAS NMR STUDY

¹ Institut för Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
² Institut för Mineralogie und Geochemie, Universität zu Köln, Zölpicher Str. 49b, 50674 Köln, Germany
³ Institut för Physik mit Ionenstrahlung (Exp. Physik III), Ruhr-Universität Bochum, 44780 Bochum, Germany

^* E-mail address of corresponding author: guntram.jordan{at}ruhr-uni-bochum.de

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 Ca²⁺ 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 multistep 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

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