  Mineralization of carbon dioxide is often seen as an attractive alternative
to classical Carbon Capture and Storage (CCS) technologies, allowing the
sequestration of \ce{CO2} as a solid mineral with no risk of aquifer
contamination or leakage back to the atmosphere. While olivine and pyroxenes
are known to easily and quickly react with dissolved \ce{CO2}, fresh
peridotites are quite rare and ultramafic rocks usually contain significant
amounts of serpentine, which presents a lower reactivity. The purpose of this
study was then to analyze the reactivity of two natural rocks: a partially
serpentinized lherzolite and a fully altered serpentinite. Results confirm that
serpentine is much slower to react and gets altered only if the activity of
\ce{CO2} is high enough and if all olivine and pyroxenes have already been
consumed. Resulting carbonates are mostly Mg-rich calcite or Mg-depleted
dolomite with the occurrence of eitelite (\ce{Na2Mg(CO3)2} in the case of high
Na activities. The carbonation of these serpentinite was however associated in
some cases with a heavy precipitation of hazardous asbestiform chrysotile,
which could be a potential threat for engineered carbonation processes.
