Lisa de Ruiter, Christine V. Putnis, Jörn Hövelmann, Håkon Austrheim

Although quartz is a stable mineral at Earth surface conditions, field samples have shown its rapid dissolution in combination with the precipitation of Mg-silicate phases. Atomic force microscopy (AFM) experiments were performed to investigate the dissolution of quartz and the precipitation of secondary phases in high-pH, Mg-rich solutions both in-situ and ex-situ. Experiments were conducted at room temperature with varying MgCl2 or MgSO4 concentrations (0.1-100 mM), pH (8.9-12) and ionic strength (<1-530 mM). The results suggest that quartz dissolves by the removal of nanoparticles on the time scale of minutes, and that a nm-scale gel-like layer of amorphous silica forms on the quartz surface and is thicker at higher pH. During the in-situ experiments, soft and poorly attached precipitates form on the surface when the Mg concentration is high (100 mM). After 20 hours in a high-pH, Mg-rich solution, solid Mg-rich precipitates can be observed at places on the surface where the gel-like silica layer is present, predominantly near surface edges where dissolution is enhanced. This suggests a coupling between the dissolution of quartz, that resulted in the gel-like layer, and the formation of secondary phases, indicating an interface-coupled dissolution-precipitation mechanism. The precipitates could not be precisely identified but evidence suggests they are likely to be amorphous Mg-silicate phases. Such a coupled reaction may provide a pathway for Mg-Si phase formation suitable as a new environmentally friendly cement.