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Mechanism of wollastonite carbonation deduced from micro- to nanometer length scale observations

¹ Institut de Physique du Globe de Paris, Centre de Recherches sur le Stockage Géologique du CO₂, 4 Place Jussieu, 75005 Paris, France
² Laboratoire de Géologie, UMR 8538 du CNRS, École Normale Supérieure, 24 Rue Lhomond, 75005 Paris, France
³ Institut de Minéralogie et de Physique des Milieux Condensés, CNRS, Université Paris 6 et 7, 140 rue de Lourmel 75252, Paris, France
⁴ Géochimie de l’Environnement, Laboratoire de Géophysique Interne et Tectonophysique, CNRS, UMR C5559, OSUG, Université Joseph Fourier, 38041, Grenoble Cedex 9, France
⁵ Centre Pluridisciplinaire de Microscopie Electronique et de Microanalyse, Université d’Aix Marseille III, Faculté des sciences de Saint Jérôme, Avenue Normandie Niemen, 13397, Marseille Cedex 20, France

Correspondence: ^* E-mail: daval{at}ipgp.jussieu.fr

The microstructural evolution of CaSiO₃ wollastonite subjected to carbonation reactions at T = 90 °C and pCO₂ = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO₂. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites.

Key Words: CO₂ sequestration • wollastonite • carbonation • dissolution-reprecipitation • diffusion modeling • leached layers • FIB-TEM • Rietveld refinement