Carbonation of calcium-magnesium pyroxenes: Physical-chemical controls and effects of reaction-driven fracturing

Abstract

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.

Rapid Marcasite to Pyrite Transformation in Acidic Low-Temperature Hydrothermal Fluids and Saturation Index Control on FeS2 Precipitation Dynamics and Phase Selection

“…This is because at low temperatures, CDR is much faster than solid-state reactions, as has been demonstrated in numerous hydrothermal mineral phase trans- formations in sulfides, 60,74−82 tellurides, 83,84 phosphates, 85,86 carbonates, 87−91 oxides, 92,93 and silicates. 94,95 Fast hydrothermal transformation of marcasite to pyrite is supported by the observations of some natural deposits. For example, in the giant Jinding (Mesozoic−Cenozoic Lanping Basin, southern China) hydrothermal Zn−Pb deposit (80−190 °C; near-neutral pH), pyrite abundance increased gradually at the expense of marcasite through three stages of mineralization; only pyrite was observed in stage 3, when the highest temperatures were reached.…”

“…While the catalytic role of hydrothermal fluids in the marcasite to pyrite transformation requires further detailed studies, it is likely that the rapid transformation is due to CDR mineral replacement reactions rather than solid-state transformation. This is because at low temperatures, CDR is much faster than solid-state reactions, as has been demonstrated in numerous hydrothermal mineral phase transformations in sulfides, ,− tellurides, , phosphates, , carbonates, − oxides, , and silicates. , …”

Carbonation and serpentinization of diopsidite in the Altun Mountains, NW China

“…The magmatic diopsidite can be exposed over a large area constituting several square kilometres 54 , although the global amount of diopsidite cannot be determined. A few experimental studies have probed the carbonation process of diopside 13 , 14 . The abundant calcite forms during carbonation coupled with serpentinization, as presented here in the study of serpentinized diopsidite (Fig.…”

“…In nature, large volumes of mafic–ultramafic rocks have been studied to record carbonation processes, especially basaltic rocks, which are rich in calcium, magnesium, iron oxides and highly porous 9 – 12 showing very promising potential for carbon storage. Neither carbonation nor serpentinization of diopside-rich diopsidite have ever been described in detail for natural samples, although some experimental studies have explored carbonation processes of diopside 13 , 14 .…”

Formation of Strontianite and Witherite Cohesive Layers on Calcite Surfaces for Building Stone Conservation

“…The replacement of the primary crystal takes place along fractures since they provide a path for the infiltration of the fluid phase. It has been interpreted that cracks form due to the stress generated by the increase in the molar volume. , Furthermore, it has been stated that the nucleation of crystals of the secondary phase on point and line defects of the primary phase surfaces generates stress-concentrating environments from where fracturing propagates, similarly as that observed during metal corrosion. − Surprisingly, despite the significant increase in the molar volume associated to the transformation of calcite into strontianite (+5.6%) and the very large one associated to its transformation into witherite (+24.1%), no cracks are observed in crosscut sections of calcite crystals partially replaced by either strontianite or witherite, even after 2 years interaction with the fluid. Gillet et al (1987) observed that during the pseudomorphic replacement of abiogenic aragonite by calcite, which involves a molar volume change of +8.4%, fracturing only occurred when calcite crystals in the overgrowth reached sizes well over 50 micrometres.…”