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The weathering of primary silicates, and their carbonation in particular, is key for the geochemical cycling of elements, strongly affecting the C cycle and the long-term regulation of the Earth's climate. The knowledge on the controlling factors and mechanisms of aqueous carbonation of primary silicates is however still far from complete. This precludes a better understanding of their chemical weathering in nature and is a strong handicap to implement effective Carbon Capture and Storage (CCS) strategies. Here, dissolution-carbonation reactions of two abundant Ca-Mg pyroxenes, augite and diopside, have been investigated in experiments conducted at hydrothermal conditions, in the presence/absence of different carbonate sources (NaHCO 3 and Na 2 CO 3 ). We show that the main reaction products are low-magnesium calcite and amorphous silica. A higher conversion of augite (~38 wt%) than diopside (~15 wt%) was achieved. The presence of abundant Fe and Al (and minor Na) in the former pyroxene strongly enhances the release of cations to the solution, and contributes to the formation of abundant secondary crystalline silicates as well as 2 carbonates (Na-phillipsite and magnesium silicate hydrate, MSH). In particular, Na-phillipsite nucleates in etch pits exerting a crystallization pressure ~100 MPa exceeding the tensile strength of augite and causing extensive fracturing. This takes place via an interface-coupled dissolutionprecipitation mechanism, despite the bulk system being undersaturated with respect to this phase.Limited reaction-induced fracturing was also observed following MSH precipitation within diopside crystals. Reaction-induced fracturing increases exposed reactive surface area and creates channels for solution flow, thereby contributing to the progress of the reaction via a positive feedback loop.Ultimately, our results help to understand differences in the kinetics and mechanisms of chemical weathering of these two abundant rock forming inosilicates relevant for CCS strategies, showing that secondary phase formation (other than carbonates) are fostered by moderately alkaline pH and the presence of alkali metals, resulting in reaction-driven fracturing that enables the progress of silicate carbonation for an effective, safe, and permanent CO 2 mineral storage. We also show that under our experimental conditions the precipitation of amorphous silica and calcite cannot generate sufficient pressure as to create fracturing, an effect that limits carbonation of Mg-Ca-Fe pyroxenes.

Coal-fired power plants are main contributors to atmospheric CO 2 emissions. They also produce huge amounts of coal fly ash (CFA) waste, which is typically landfilled, posing significant environmental risks due to its high content of potentially toxic elements (PTE). However, CFA is an alkaline aluminosilicate-rich waste, which offers the possibility of CO 2 mineral capture and the production of economically-relevant mineral by-products such as zeolites. Yet, the combined carbonation and zeolite production from CFA resulting in PTE trapping has never been explored. Here we show that under mild hydrothermal conditions (150 °C) and depending of process parameters such as pH and background alkali metal ion in alkaline (bi)carbonate solutions, a carbonation efficiency of up to 79%, with a net CO 2 mineral capture of 0.045 g/kg CFA can be achieved, even when using a low Ca and Mg (3.72 wt% CaO, 1.74 wt% MgO) Class F fly ash. Moreover, amorphous zeolitic precursors and different crystalline zeolites (yield 1 up to 60 wt%) are simultaneously obtained, and PTE in CFA are effectively trapped into the newly formed calcite, zeolitic precursors, and zeolite phases. These results have important implications for carbon capture and storage, as well as for the safe reutilization and disposal of CFA waste.

Pyroxenes are among the main constituent of basic and ultrabasic igneous rocks. Because they form at high P and T conditions They are not thermodynamically in equilibrium at the Earth surface and will transform into more stable phases by an irreversible thermodynamic process (Frings & Buss, 2019). Weathering is a first-order process that enables life on Earth, as the solutes released during chemical weathering serve as nutrients, and these reactions represent the first step of the biogeochemical cycle of most elements (Frings & Buss, 2019). In this sense, chain silicates are important contributors of Mg, Ca and Fe to natural waters (Marini, 2006). Also, the kinetics of primary silicate chemical weathering controls erosion rates and soil formation (Frings & Buss, 2019). Weathering of Ca, Mg-containing silicates is as well of relevance due to its role in controlling the composition of the atmosphere acting as long-term sink for CO 2 , and therefore in the global climate (e.g., Chen & Brantley, 1998).Chemical weathering under acidic conditions occurs in a wide range of scenarios on Earth, including volcanic environments, soils as well as monument stones subjected to acid rain (Simão et al., 2006), or acid mine drainage sites (e.g. Hellmann et al., 2012). Additionally, from a technological point of view, these pH conditions are relevant for in situ geological CO 2 storage in rocks (pH around 3, Shao et al., 2013). This process requires the release of the divalent metal cations contained in the structure of the silicate mineral, which then react with dissolved CO 2 and precipitate as carbonates (mineral carbonation, Daval et al., 2009). The rate limiting step for this process is considered to be the release of divalent cations from the silicate structure, and in the specific case of pyroxenes, their low reactivity is claimed to be responsible for the overall low conversion into carbonates, compared for example with olivine and serpentine under similar conditions (Meyer, 2014). Thus, knowledge of the kinetics and mechanisms of pyroxene weathering under different experimental conditions may help to optimize the conditions to obtain maximum carbonate yields.Despite the importance of the weathering of Ca, Mg-bearing pyroxenes, there are relatively few studies that have addressed the dissolution kinetics of these minerals, and in particular of augite (Schott & Berner, 1985;Siegel & Pfannkuch, 1984), as compared to other minerals such as feldspars or olivine. Also, there

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