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Listwanite, a carbonate-altered serpentinite commonly associated with gold and mercury mineralization, also represents a natural analogue to CO2 sequestration via in situ carbonation of minerals. The reaction pathways and permeability structure controlling listwanite formation are preserved and exposed at Atlin, British Columbia, where listwanite extends tens of meters into surrounding wallrock. The overall mineralogical transformation is the same as that being considered for industrial sequestration of CO2. In nature, this reaction proceeds via subreactions that are fossilized as spatially distinct zones. Serpentine+ olivine+ brucite reacted with CO2 to form serpentine+ magnesite, then magnesite+ talc, and finally magnesite+ quartz. These mineralogical transformations are achieved isochemically, except for the volatile species H2O and CO2. Although the first stage of the reaction only accounts for about 5–15% of the carbonation potential of serpentinite, it is very widespread, and therefore may have sequestered a significant portion of the total bound CO2. Moreover, within intact bedrock, the progress of the magnesite+ talc reaction generates fracture permeability, which appears to have locally enhanced reaction. The first two reactions combined account for about half of the carbonation potential for serpentinite and have a small associated increase in the volume of solids, which limits porosity loss. They thus hold the greatest promise for in situ mineral carbonation. The carbonation reactions are controlled by the activity of CO2 in the fluid phase. Proponents of industrial mineral-carbonation processes therefore may seek to control the composition …