Reaction Progress of Olivine Alteration as a Function of Time and CO2 Supply at 150°C – An Experimental Study

Abstract:

Increased anthropogenic CO₂ emissions have caused an imbalance in the Earth’s carbon cycle causing climate changes. Interaction of CO₂ with water and rock plays important role in geochemical cycle of carbon and among potential ways of reducing atmospheric CO₂ by sequestration into carbonates. Olivine, among the main constituents of mafic and ultramafic rocks, is rich in Mg and Fe that can react with CO₂ to form Mg-Fe carbonates like siderite and magnesite.

Laboratory experiments were performed in batch-type titanium reactors to study the interaction between CO₂-rich water and olivine under hydrothermal conditions. The experimental were conducted at 150°C, for 1 to 4 weeks, the CO₂ concentration was 5 to 30 mmol/kg and the initial pH was around neutral. The olivine used was forsterite 93% from Åheim Olivine Pit-Norway. The alteration products and fluid chemistry were monitored as a function of time. Moreover, the results were supported with reaction path modeling to further constrain the olivine alteration and carbonate mineral formation mechanism.

Initially forsterite dissolved releasing elements into solution. During this stage limited alteration products were formed and the solution pH was buffered by H⁺ uptake by olivine dissolution and CO₂ concentration. Upon further reactions secondary phases started to form including chrysotile, brusite and mainly solid solution of iron-containing magnesite. This resulted in decrease of CO₂ concentrations and eventually increased solution pH. Experiments with higher initial CO₂ concentrations required longer period until CO₂was consumed. Furthermore experiments with longer duration had quantitatively larger amount of precipitated minerals.

Findings of this research represent platform for kinetic numerical simulations of olivine alteration and therefore provide insight into carbon storage in mafic rocks under in-situ conditions.