Generating porosity during olivine carbonation via dissolution channels and expansion cracks
<p>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 surfa...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/9/879/2018/se-9-879-2018.pdf |
| Summary: | <p>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 NaHCO<sub>3</sub> 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
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. In the olivine cup with a grain size of
∼  80 µm (coarse-grained cup), dissolution planes developed
within 30 h, 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 the reaction proceeded and eventually disintegrated the aggregate
after 140 h. 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. 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
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.</p> |
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| ISSN: | 1869-9510 1869-9529 |