Olivine, (Mg,Fe)2SiO4, is one of the most prevalent silicate minerals in the solar system. It composes a major portion of the upper mantle of the Earth and is found on the Moon, Mars, asteroids (e.g. Vesta), and a variety of other extraterrestrial bodies such as meteorites. As an easily weathered mineral phase in the terrestrial regime, olivine has long been investigated in terms of its ongoing chemical weathering-dissolution process with water and CO2, alteration of oceanic crust and reactions occurring during subduction. This presentation summarizes the results from three complementary studies designed to interrogate the structure and dynamics of water at the olivine surface. Density functional theory (DFT) was used to assess the effect of alkaline earth (AE: Ca, Sr, Ba) and transition metal (TM: Cr, Mn, Fe, Co, Ni) dopants on molecular water adsorption on the forsterite(010) surface. Dopants were modeled by replacing a single Mg cation on the forsterite(010) surface, which is sufficient to characterize the local water molecule interactions with the dopant. DFT predicts that water adsorbs strongest on the undoped forsterite surface, and there is no simple trend relating the cation dopant charge or ionic radii to the adsorption energy of water. Quasielastic neutron scattering (QENS) was coupled with classical MD to assess the structure and mobility of water on a fully hydroxylated (010) surface of synthetic nano-forsterite. The water structure on the hydroxylated surface is composed of a well-ordered immobile first layer that interacts with both the OH and H groups, a second continuous strong layer that interacts with the first layer and OH groups, and a more mobile top layer. Using the same nano-forsterite we probed the vibrational properties of interfacial water (three different loadings) using inelastic neutron scattering (INS) again coupled with MD simulations. At lower water loadings ("dry" and 0.5 monolayers), the INS spectra exhibited red shift (lower frequency) of the water libration band and strengthening (blue shift, higher frequency) of the O-H stretching modes, implying weakening of the H-bonding acting on the water molecules. Higher loadings involved multi-layer adsorption producing a significant shift of the vibrational bands, implying increased H- bonding strength and disorder between water molecules. It is important to note that the experimental and molecular simulation results agree quite closely for both studies.

Probing the olivine-water interface: Experimental and molecular simulation perspectives

ACS Spring Meeting 2021

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