Microstructure and porosity evolution during experimental carbonation of a natural peridotite

Carbonation experiments (T: 100–200 °C; pCO2: 14.6–19.4 MPa) were performed under closed system conditions on a natural peridotite from Almklovdalen (W-Norway) to investigate the microstructure and porosity evolution as a function of reaction progress. The unreacted samples (cubes of 2.5–3 cm3) were composed mainly of forsteritic olivine (~ 80–90 vol.%), clinochlore, orthopyroxene, amphibole and chromite. Initial porosities were on the order of 6–8% due to the presence of abundant inter- and intragranular microfractures. Microstructural observations after 7–42 days of experimental treatment showed that olivine was by far the most reactive phase. After 7 days at 100 °C no carbonate products were formed, but longer reaction times produced small amounts of magnesite on the surface and within the fracture network of the peridotite cubes. Increasing temperatures led to faster reactions and pore space filling by magnesite. In addition, replacement rims (up to 50 μm thick) composed of magnesite and amorphous silica formed around individual olivine grains. The replacement began with the formation of an amorphous silica layer that retained the shapes of the initial olivine grains. This layer remained permeable during the reaction as it allowed further dissolution of the underlying olivine. Increasing extents of carbonation were accompanied by decreasing porosities and enhanced cementation of grain boundaries. About 50% of the initial porosity was closed when carbonation reached about 10%. The reacted samples showed no evidence of reaction-induced cracking indicating that the carbonation reaction was self-limiting in our experiments. These results may have important implications for the efficiency of in situ CO2 sequestration in peridotites.

► We studied peridotite carbonation in a closed system under hydrothermal conditions.
► Olivine dissolution was accompanied by magnesite and amorphous silica precipitation.
► Shapes of individual olivine grains were preserved and defined by layers of silica.
► Progressive carbonation led to closure of porosity and grain boundary cementation.
► Carbonation reactions took place in a negative, self-limiting feedback regime.