Atomic mechanisms of diopside (CaMgSi2O6) surface hydroxylation

Understanding how silicate surfaces interact with water is essential for predicting weathering, reactivity, metamorphic reactions as well as interface design. The initial adsorption of water at very low surface coverage governs subsequent hydroxylation, dissolution, and element release. Here, we use density functional theory to compare low-index diopside (CaMgSi 2 O 6 ) surfaces and their terminations, and then focus on the lowest energy and experimentally relevant (110) facet as a reference for hydration analysis. Upon cleavage and relaxation, the surface develops a pronounced surface–subsurface stratification, with Si and Mg exposed in the outer layer and Ca in the subsurface. Relaxation leads to a local increase in the coordination of undercoordinated Si through short-range interactions with subsurface oxygen atoms, resulting in the formation of protruding SiO 4 motifs that act as reactive centres without inducing large-scale structural rearrangement. Early hydration is characterized by configurations in which water-derived hydroxyl and proton species are stabilized at the surface, rather than by intact molecular adsorption. The first water derived hydroxylation event shows the hydroxyl group preferentially binds to protruding Si-centered reactive motif, and the released proton preferred localizes at the neighbouring bridging oxygen. This event restores near-bulk tetrahedral coordination at Si and passivates the protruding site. After this initial dissociation event, the local rectivity is altered: further hydroxylation becomes more favorable at an undercoordinated Mg site, whereas + and Mg-H 2 O configuration become closer in energy, indicating increased local proton flexibility. These results provide an atomistic picture of the early low-coverage hydration on diopside (110), which is interpreted as a site-resolved mechanistic description of incipient hydration, rather than a full equilibrium thermodynamic description of the aqueous interface.