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Mineralization of carbon in ultramafic mine tailings can occur on a scale that is significant relative to the greenhouse gas emissions of a mine. The secondary carbonate minerals that form in mine tailings are safe and durable traps for carbon and their presence can represent substantial disposal of atmospheric CO,. Carbonate minerals precipitate in situ by dissolution of silicate mine residues at the Diavik Diamond Mine, Northwest Territories, Canada, and the Mount Keith Nickel Mine, Western Australia, Australia. Differences in climate and tailings management practices have resulted in widespread mineralization and preservation of hydromagnesite [Mg(5)(CO(3))(4)(OH)(2)center dot 4H(2)O] in the desert at Mount Keith and limited mineralization of nesquehonite [MgCO(3)center dot 3H(2)O], calcite, and sodium carbonate minerals in the tundra at Diavik. We have used crystallographic methods to quantify capture and storage of carbon at these ultramafic-hosted mines. Our results demonstrate that on the order of 10(2) to 10(4) tonnes/year of CO(2) can be fixed in a mine tailings facility as the result of naturally-occurring weathering processes.