Tracking Olivine Dissolution Kinetics at the Grain Scale: Insights from 4D X-ray Microcomputed Tomography

Abstract

Olivine is a target mineral for carbon dioxide removal (CDR) via enhanced rock weathering, which requires an in-depth understanding of its low-temperature chemical weathering mechanism. Here, we examined the accelerated dissolution of a single olivine grain under acidified conditions (1 M HCl) at ambient temperature. To this end, we conducted a static dissolution experiment in which individual grains were submerged in solution without agitation and tracked over time using time-lapse X-ray micro-computed tomography (XCT) at a voxel size of 1.8 μm. XCT imaging was performed at five time points over a period of 12.5 days, capturing a total of five time steps. Image analysis allowed the quantification of the temporal evolution of the surface area and grain volume. This provided a mean dissolution rate of 2.88 ± 0.83 × 10-7 mol m-2 s-1, which remained largely constant over the time course of the experiment. Microstructural changes in the olivine grain were already noticed after 3.8 days. Most prominently, intragranular fracture networks expanded throughout the volume of the grain, likely initiated from the pre-existing microcracks on the grain surface. As a result of fracture expansion, the grain surface area increased to more than 3 times the initial value. The single-grain approach adopted here hence provides additional insight into the low-temperature weathering of minerals complementary to conventional dissolution experiments. If ambient weathering acts in a similar way as in the accelerated (high-acid) conditions examined here, then our results suggest that internal crack expansion could be an important driver of dissolution.