Forsterite surface composition in aqueous solutions: a combined potentiometric, electrokinetic, and spectroscopic approach

Oleg Pokrovsky , Jacques Schott

Surfaces of natural and synthetic forsterite (Fo ₉₁ and Fo ₁₀₀) in aqueous solutions at 25°C were investigated using surface titrations in batch and limited residence time reactors, column filtration experiments, electrokinetic measurements (streaming potential and electrophoresis techniques), Diffuse Reflectance Infrared Spectroscopy (DRIFT), and X-ray Photoelectron Spectroscopy (XPS). At pH < 9, a Mg-depleted, Si-rich layer (<20 Å thick) is formed on the forsterite surface due to a Mg ²⁺ ↔ H ⁺ exchange reaction. Electrokinetic measurements yield a pH _IEP value of 4.5 corresponding to the dominance of SiO ₂ in the surface layer at pH < 9. In contrast, surface titrations of fresh powders give an apparent pH _PZC of about 10 with the development of a large positive charge (up to 10 ^-4 mol/m ² or 10 C/m ²) in the acid pH region. This may be explained by penetration of H ⁺ into the first unit cells of forsterite surface. The surface charge of acid-reacted forsterite is one or two orders of magnitude lower than that of unreacted forsterite with an apparent pH _PZC at around 6.5 and a pH _IEP value of 2.1 which is close to that for amorphous silica and reflects the formation of a silica-rich layer on the surface. XPS analyses indicate the penetration of hydrogen into the surface and the polymerization of silica tetrahedra in this leached layer. At pH > 10, a Si-deficient, Mg-rich surface layer is formed as shown by XPS analyses and the preferential Si release from the surface during column filtration experiments.

Domains: Sciences of the Universe [physics]

Oleg Pokrovsky, Jacques Schott. Forsterite surface composition in aqueous solutions: a combined potentiometric, electrokinetic, and spectroscopic approach. Geochimica et Cosmochimica Acta, 2000, 64, pp.3299-3312. ⟨10.1016/S0016-7037(00)00435-X⟩. ⟨insu-03642934⟩