Reaction-induced fracturing during serpentinite carbonation promoted by selective dissolution of brucite

Although ultramafic rocks have a high potential for mineral carbonation, their low porosity and thereby slow reaction kinetics remain challenges for artificial mineral carbonation of ultramafic rocks. Field observations suggest that carbonation of ultramafic rocks proceeds with reaction-induced fracturing, caused by the solid volume increase during carbonation^{[e.g., 1,2]}. However, such reaction-induced fracturing has not been clearly reproduced in laboratory settings for carbonation. Here we show a clear experimental example of macroscopic reaction-induced fracturing caused by carbonation of brucite-bearing serpentinite.

Cylindrical cores (6 mm in diameter and 5 mm in height) of fine-grained brucite-bearing serpentinite were reacted with 1M NaHCO₃ solution or CO₂-saturated water at 90–200°C during batch experiments for one week. Clear macroscopic fractures were observed for samples reacted with NaHCO₃ solution at 150 and 200°C. These samples were fractured by two types of tensile fractures: (a) diagonal fractures that cut the inside of the cylindrical samples, and (b) regularly spaced, vertical short fractures on the sample surface. Diagonal fractures are partly filled with magnesite and are cut by surficial vertical fractures. Reaction front is characterized by formation of porous serpentine that surround the original serpentine-brucite mixture. Magnesite-serpentine mixture further surrounds the porous serpentine, forming mesh texture-like magnesite-serpentine networks.

Above observations suggest that selective dissolution of brucite at the reaction front increase the Mg concentration and pH in the local solution, leaving porous serpentine. Magnesite preferentially precipitates at pre-existing micro cracks, causing local volume increase. These reaction and volume increase exert tensile stress inside the sample, causing macroscopic diagonal fractures. The diagonal fractures promote fluid transport and reaction within the inner part of the sample. The volume increase inside the sample induces tensional stress on the surface of the sample, causing surficial vertical fractures, which further enhance carbonation reactions.

We propose selective dissolution of brucite and preferential magnesite precipitation at pre-existing micro cracks induce macroscopic fracturing, create fluid flow paths and new reactive surface area, and accelerate carbonation of massive serpentinite. Such heterogeneous distribution of minerals with contrasting reactivity would be important for self-enhanced mineral carbonation.

 

1 Kelemen et al., 2011 Annual Review of Earth and Planetary Sciences, 39, 545–576.

2 Uno et al., 2022 Proceedings of the National Academy of Sciences, 119, e2110776118.