Speaker:
Tiange Xing, a PhD student at U. Maryland, presents "Generation of Porosity during Olivine Carbonation via Expansion Cracks and Dissolution Channel" at the MIT Earth Resources Laboratory.
Live video will be available on the ERL Youtube Channel.
"The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces.
We conducted in situ dynamic X-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a sample setup that included a thick-walled fine-grained olivine cup filled with coarse-grained olivine sands. The whole sample assembly was reacted with a NaHCO3 aqueous solution at 200°C, under a constant confining pressure of 13 MPa and pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the three-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated.
The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. As the reaction proceeds, idiomorphic magnesite crystals on the surface of the olivine grains become measurable. The magnesite shows a near constant growth rate on the surface of olivine throughout the experiment, suggesting that the reaction is self-sustained. In our experiments, large fractures were generated as reaction proceeds, which eventually disintegrate the aggregate. Detailed analysis show that these are expansion cracks caused by the volume mismatch in the cup walls, between the expanding interior and the near-surface which keeps a nearly constant volume. Generation of reaction-induced fractures provides continuous fluid access and maintains the reaction. Nanotomography images of the reacted olivine cup reveal pervasive etch pits and wormholes in the olivine grains. We interpret this perforation of the solids to provide fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micron scale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled mechanism of reaction-induced fracturing and dissolution should account for the observed self-sustainability of the reaction."