Linking nm-scale measurements of the anisotropy of silicate surface reactivity to macroscopic dissolution rate laws: New insights based on diopside

The interfacial zone between a bulk fluid and a mineral surface is where all exchange of matter and energy occurs during chemical weathering. However, our knowledge is still limited with respect to understanding where and how the rate-determining dissolution reactions take place. A complicating factor is the commonplace formation of amorphous Si-rich surface layers (ASSLs), which may hinder contact between the fluid and the mineral surface. To address the role of ASSL, we investigated the dissolution of a common silicate (diopside), and related the bulk dissolution rate with the nanoscale dissolution rate and surface chemistry of its individual prevalent faces. While ASSL were evidenced on all of the investigated faces, only those formed on (1 1 0) and (11¯0) were passivating, thereby controlling the reactivity of the underlying faces. The (1 1 0) and (11¯0) faces intersect the highest density of Mg–O–Si and Fe–O–Si bonds, and this specificity may explain the passivating behavior of the corresponding ASSL. Moreover, we evidenced an inverse relation between aqueous silica concentration and the bulk dissolution rate of crushed diopside grains, which suggest that the (1 1 0) and (11¯0) faces are predominant in a powder. By considering ASSL as a separate phase that can control silicate dissolution rates, extrapolated laboratory-based rates at conditions relevant to the field can be lowered by up to several orders of magnitude, thereby decreasing the large gap between laboratory and natural rates. This has important implications for more accurately modeling chemical weathering reactions, so important today for the C cycle and CO2 sequestration.