Olivine reactivity with CO2 and H2O on a microscale: Implications for carbon sequestration

The silicate mineral olivine, (Mg,Fe)2SiO4, reacts exothermally with CO2 and forms secondary minerals, including carbonates. Therefore olivine reaction is a promising process for carbon sequestration, to convert carbon dioxide from the atmosphere to mineral form. The purpose of this study was (1) to explore the composition, structure and reactivity of olivine surfaces during exposure to air and to water at ambient conditions, (2) to investigate the effect of elevated CO2 pressure and temperature, and (3) to identify the secondary minerals.Olivine surfaces have been examined with atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), before and after reaction with CO2.

Experiments were carried out in pure water equilibrated with CO2 at total pressures up to 80bars, at temperatures 25°C and 120°C and both in the absence and presence of oxygen. New formation products appeared on the olivine surface as a homogeneous layer of bumps, less than 100nm in diameter, within hours of exposure to air.

Olivine crystals, exposed to water, dissolved and secondary minerals formed within days. Colonies of bacteria populated olivine surfaces on samples stored in water for more than 4days at room temperature. Loosely attached material formed on olivine surfaces and could easily be scraped away with the AFM tip.

A red precipitate formed when crystals where reacted at increased temperatures and CO2 partial pressures for less than 4days. The new phases were identified as goethite, hematite, silica and carbonate minerals. Olivine surfaces oxidize and iron oxides form even when oxygen is absent, suggesting hydrolysis, where water is converted to hydrogen and oxygen.