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

Abstract

International audienceThe 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

Pseudomorphic replacement of diopside during interaction with (Ni,Mg)Cl2 aqueous solutions: Implications for the Ni-enrichment mechanism in talc- and serpentine-type phases

“…These observations are consistent with the mechanism proposed for the replacement of chain silicates at lower temperatures (e.g. Daval et al, 2009;Ruiz-Agudo et al, 2012).…”

Experimental study of the carbonation of partially serpentinized and weathered peridotites

“…It is not clear from the BSE images how these interfacial pores were connected to the external fluid reservoir because the calcite product itself mostly lacks visible porosity on the micrometer scale. However, it is possible that they were connected by nano-channels similar to those found in experimentally replaced feldspars (Niedermeier et al, 2009) or carbonated and silicified wollastonite crystals (Daval et al, 2009b). Our observation that magnesite occasionally started to form instead of calcite in deeper regions of the replacement rim (Fig.…”

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…For example, thermodynamic considerations suggest that both olivine and wollastonite (CaSiO 3 ) are favorable candidates for carbonation reactions Power et al 2013). However, the formation of a non-porous silica layer on the olivine surface armors it from further reaction, while a meso-porous silica layer forms on the surface of the wollastonite enabling the carbonation reaction to continue through the grain (Daval et al 2009. A similar situation is seen in the uptake of Cd by calcite (Prieto et al 2003(Prieto et al , 2013.…”

“…This textural evolution is driven both by internal (e.g., minimization of excess energy by coarsening) and external processes (e.g., continued that can be the only preserved remnants of such an alteration process (e.g., Putnis et al 2007a;Tooth et al 2011). Hence, textural evolution resulting from the CDR mechanism directly affects the long-term reactive behaviour of the products, a key feature for applications such as carbon sequestration (e.g., Daval et al 2009Daval et al , 2011 and nuclear waste storage (e.g., Geisler et al 2010(e.g., Geisler et al , 2015.…”