期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 304, 期 -, 页码 258-280出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2021.02.016
关键词
Pyroxene; Carbonation; Fracturing; Fracture mechanics; Zeolite; Augite; Diopside
资金
- Spanish Government [CGL2015-70642-R]
- European Commission (ERDF)
- Junta de Andalucia (research group RNM-179)
- University of Granada (Unidad Cientifica de Excelencia) [UCE-PP2016-05]
- Spanish Government FPI [BES-2016-078468]
The weathering and carbonation of primary silicates are crucial for geochemical cycling of elements and climate regulation. The study investigated dissolution-carbonation reactions of Ca-Mg pyroxenes, revealing low-magnesium calcite and amorphous silica as main products. The presence of Fe and Al in the pyroxenes enhances cation release and contributes to secondary phase formation.
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 (NaHCO3 and Na2CO3). We show that the main reaction products are low-magnesium calcite and amorphous silica. A higher conversion of augite (similar to 38 wt%) than diopside (similar to 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 and amorphous silicates as well as carbonates (Na-phillipsite and magnesium silicate hydrate, MSH, as well as calcite). In particular, Na-phillipsite nucleates in etch pits exerting a crystallization pressure similar to 100 MPa exceeding the tensile strength of augite (i.e, failure under tension due to breaking of inter-atomic bonds as well as opening and propagation of subcritical fractures or flaws) 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 reactioninduced 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 CO2 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. (C) 2021 Elsevier Ltd. All rights reserved.
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