The mechanisms and rates of olivine carbonation reactions have been the object of a number of studies, but the thermodynamic limitations and the kinetics of the elementary processes that control the overall reaction are still poorly understood and characterized.The main objective of this study is to probe the effect of Fe on the measured rates of olivine carbonation and its role in the formation of Si-rich surface layers, which can significantly inhibit olivine dissolution and limit the extent of the carbonation reaction. A series of batch and flow-through reactor experiments was conducted in pure water at 90 and 150 degrees C and under a CO2 partial pressure of 100 and 200 bar, using both a natural sample of Fe-bearing olivine (Fo(88)) and a synthetic sample of pure forsterite (Fo(100)). Experimental results show that Fe plays an ambivalent role in the carbonation rates of olivine. On one hand, the presence of Fe favors the formation of Fe-Si-rich protective layers at the interface between olivine and aqueous solution, slowing down the dissolution reaction and limiting the extent of carbonation, whereas pure silica coatings have little to no inhibiting effect on measured carbonation rates. On the other hand, Fe enhances olivine to carbonate conversion rates at low degrees of supersaturation, by promoting the formation of fast precipitating Mg-Fe carbonate solid solutions. The passivating properties of Fe-Si-rich layers originate from the strong Fe(III)-Si interaction and are linked to the permanence of oxidizing conditions in the aqueous fluid. As a consequence, under reducing conditions, olivine carbonation rates can be significantly increased by higher extents of dissolution and by the formation of ferroan magnesites (Mg,Fe)CO3, which nucleate faster than the pure Mg end-member.Forsterite and olivine carbonation reactions can be hindered by the formation of secondary Mg sheet-silicates but, at the conditions studied, the formation of such silicate phases was observed to be transitional and not affecting significantly the rates of carbonation at the end of one-month long experimental runs.This work presents new measurements of olivine carbonation rates and delivers relevant information that suggest new reference criteria for the assessment of the sequestration potential of CO2 repositories and the optimization of the mineral carbonation process in mafic and ultramafic rocks. Published by Elsevier Ltd.
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