The magnesite deposits of Malentrata (Tuscany, Italy) were derived from serpentinite silicification-carbonation of the Ligurian ophiolite, and represent a natural analogue of in Situ CO2 mineral sequestration. Carbonation of magnesium silicate minerals (e.g. serpentine, olivine) at temperatures below 200 degrees C is an exothermal process, involving incorporation of carbon dioxide into stable carbonates (e.g. magnesite, dolomite). Serpentinites at Malentrata were transformed to a brownish friable rock characterized by the occurrence of opal, chromian montmorillonite, Fe-rich magnesite and minor iron sulfides and oxides. Widespread lenses of cohesive rocks occur within the altered serpentines, resulting from the complete silicification of the protolith. The pervasive alteration of serpentinite was accompanied by the formation of a network of magnesite and dolomite veinlets, and large magnesite-dolomite veins along major structures. Field observations, petrography and mineral chemistry define the followi