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Solid Earth An interactive open-access journal of the European Geosciences Union

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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
13 Apr 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).
Generating porosity during olivine carbonation via dissolution channels and expansion cracks
Tiange Xing1, Wenlu Zhu1, Florian Fusseis2, and Harrison Lisabeth1,3 1Department of Geology, University of Maryland, College Park, 20742, USA
2School of Geosciences, University of Edinburgh, Edinburgh, EH9 3FE, UK
3Department of Geophysics, Stanford University, Stanford, 94305, USA
Abstract. 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 cup (made of porous olivine aggregates with a mean grain size of either ~ 5 or ~ 80 μm) filled with loose olivine sands with grain sizes of 100–500 μm. The whole sample assembly was reacted with a NaHCO3 aqueous solution at 200 °C, under a constant confining pressure of 13 MPa and a pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the 3-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. In the olivine cup with a grain size of ~ 80 μm (coarse-grained cup), dissolution fractures developed within 30 hours, before any precipitation was observed. In the experiment with the olivine cup of ~ 5 μm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as reaction proceeds and eventually disintegrate the aggregate after 140 hours. Detailed analysis show that these are expansion cracks caused by the volume mismatch between the expanding interior and the nearly constant surface. Nanotomography images of the reacted olivine cup reveal pervasive etch-pits and worm-holes in the olivine grains. We interpret this perforation of the solids to provide continuous 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 dissolution and reaction-induced fracturing should account for the observed self sustainability of the reaction.
Citation: Xing, T., Zhu, W., Fusseis, F., and Lisabeth, H.: Generating porosity during olivine carbonation via dissolution channels and expansion cracks, Solid Earth Discuss.,, in review, 2018.
Tiange Xing et al.
Tiange Xing et al.
Tiange Xing et al.


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Publications Copernicus
Short summary
The carbonation reaction of olivine is associated with a positive volume change that could clog the pore space and prevent further reaction. This contradicts with the observation of fully carbonated outcrops. The mechanism behind this observed sustainability is poorly understood. Our study reveals that the reaction-induced fracturing and the dissolution channels are the main mechanisms that contribute to the sustainability of carbonation reactions. This provides new insights to the CO2 minerals.
The carbonation reaction of olivine is associated with a positive volume change that could clog...