On the mechanisms of apophyllite alteration in aqueous solutions. A combined AFM, XPS and MAS NMR study

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 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 multi-step protonation process in apophyllite clearly demonstrates the vast sensitivity of the protonation reaction on small structural variations in phyllosilicates.

Basal plane reactivity of phyllosilicates studied in situ by hydrothermal atomic force microscopy (HAFM)

“…Based on the data shown in Fig. 7, the activation energy for the transformation of pristine apophyllite into the altered state HH at pH 5.6 is 42 ± 5 kJ/mol which is close to the activation energy of 57 ± 2 kJ/mol for the P fi T fi H transformation at a b c pH 3 (Aldushin et al, 2004b). The formation of high hillocks could be measured up to about 110°C.…”

“…Thus high rough hillocks on the apophyllite surface can be perceived as a porous material containing water molecules within voids formed by the cross-linked silicate layers. This suggestion is supported by NMR investigations of apophyllite treated for 11 h at pH 1.6 (Aldushin et al, 2004b) which show, aside from various protonated states, the formation of Si(OSi) 4 -type silica with variable bonding parameters.…”

“…Two first alteration stages (T and H) are proton promoted, and represent protonation of the non-bridging tetrahedral oxygens, cation release, and separation of silica sheets (Aldushin et al, 2004b). Prolonged treatment may hydrolyze the bridging Si-O-Si bonds within the layers.…”

“…In such a way, the reaction can proceed layer by layer into the bulk crystal (Aldushin et al, 2004a). Investigations of the reaction mechanism (Aldushin et al, 2004b) showed that the protonation and ion release within an interlayer takes place in two different sequences of individual transformations. One sequence comprises two transformation steps, the other three steps.…”

Study on water-dispersible colloids in saline-alkali soils by atomic force microscopy and spectrometric methods

“…Figure 3a shows the XPS spectra of the water-dispersible colloids extracted from the agriculture soils and the saline-alkali soils before and after HPMA treatment. The colloidal surface of the salinealkali soils sample contains C, O, Si, Al, and Ca (Aldushin et al, 2004;Arnarson and Keil, 2001). This result is in accordance with that from the XRD measurement.…”