Kinetics and Mechanisms of Acid‐pH Weathering of Pyroxenes

Abstract

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

Emerging investigator series: kinetics of diopside reactivity for carbon mineralization in mafic–ultramafic rocks

“…Within this class, diopside (CaMgSi 2 O 6 ) is of great interest due to its natural reactivity, 27,28 abundance, and complex multi-ion composition, which provides a multitude of possible carbonate products once exposed to injected CO 2 . Extensive work 29–49 has been focused on pyroxene dissolution kinetics and mechanisms and also on pyroxene carbonation. 26,50–56 However, there is a dearth of reported results 54,55,57,58 for the carbonation of diopside or its reactivity in wet scCO 2 fluids, which are vital to understand the fate and transport of anthropogenic subsurface CO 2 in mafic and ultramafic rocks.…”

Reactivity of Basaltic Minerals for CO2 Sequestration via In Situ Mineralization: A Review

“…The reactivity of basaltic minerals is determined based on their dissolution rate and grain size that will further help in assigning reactivity rating to different basalt types. Various research group have utilized various methodologies to report dissolution rates [144], but one of the most reliable ways is to report the dissolution rates in terms of weathering of metal ions [145][146][147]. Plagioclase exhibits high dissolution rates but has comparatively larger grain sizes.…”

“…Comparative reactivity of basaltic minerals as a function of two intrinsic properties, i.e., grain size and dissolution rate for carbonation[145][146][147][148].…”

Resolving Nanoscale Processes during Carbon Mineralization Using Identical Location Transmission Electron Microscopy

“…Forsterite (Mg 2 SiO 4 ) is a model mineral of interest in the carbon mineralization community due to its prevalence, simple composition, and rapid carbonation kinetics. , Its dissolution − and carbonation ,,,,,− behavior, including its reactivity in wet scCO 2 fluids, ,,,,,− is well-documented. In addition to olivine, pyroxene single-chain silicates are also important constituents of mafic-ultramafic rocks whose dissolution − and carbonation ,,,− behavior has been closely scrutinized. Diopside (CaMgSi 2 O 6 ), a member of the pyroxene group, though relatively less studied with regard to carbon mineralization, ,,− is of interest due to its reactivity, ubiquity, and more ...…”