Kinetics and Mechanisms of Acid-pH Weathering of Pyroxenes Monasterio Guillot, Luis Rodríguez Navarro, Carlos Manuel Ruiz Agudo, Encarnación his research has been funded by the Spanish Government (grant CGL2015-70642-R and CGL2015-73103-EXP), the European Commission (ERDF funds and ACT_ERA NET no. 691712, PCI2019-111931-2), the Junta de Andalucia (research group RNM179), and the University of Granada (Unidad Cientifica de Excelencia UCE-PP2016-05). LMG acknowledges funding by the Spanish Government FPI (Grant BES-2016-078468). The authors also thank the personnel of the Centro de Instrumentacion Cientifica (CIC; University of Granada) for assistance with ICP-OES, Micro-CT, and SEM analyses. All experimental results discussed here are presented in the main text and Supporting Information S1. The authors thank the anonymous reviewers for the extremely careful and fruitful revision performed, which has significantly improved the original version of the manuscript. Weathering of primary silicate minerals under acidic conditions occurs in contexts as varied as acid mine drainage, volcanic environments, soils, stone monuments subjected to acid rain or Geological Carbon Storage (GCS). Considering the abundance of pyroxenes on the Earth crust, knowledge of their weathering kinetics and mechanisms may help to optimize carbonate yield in GCS. Here we report experimental results from the reaction of the clinopyroxenes augite and diopside in acidic solutions. Dissolution at far-from-equilibrium conditions results in the formation of etch pits where crack initiate and propagate by stress corrosion and pressure exerted by swelling of an amorphous, gel-like Si-rich phase, which precipitates despite the undersaturation of the bulk solution and whose formation is highly controlled by the heterogeneity of the mineral surface and the local transport mechanism. These precipitates are commonly localized within deep etch pits and cracks, characterized by a low fluid renewal where high Si-concentrations can be reached locally, so that supersaturation with respect to amorphous silica can occur. Cracks and silica precipitates are most abundant in the case of augite weathered in flow-through experiments. This is related to its faster reaction rate compared to diopside, most likely due to its higher iron content. Finally, in the case of diopside an amorphous magnesium silicate hydrate (M-S-H) precursor forms, which represents an indirect evidence of the high pH conditions prevailing at the diopside-solution interface during dissolution. 2021-12-16T11:52:47Z 2021-12-16T11:52:47Z 2021-10-14 info:eu-repo/semantics/article Monasterio-Guillot, L., Rodriguez Navarro, C., & Ruiz-Agudo, E. (2021). Kinetics and mechanisms of acid-pH weathering of pyroxenes. Geochemistry, Geophysics, Geosystems, 22, e2021GC009711. [https://doi. org/10.1029/2021GC009711] http://hdl.handle.net/10481/72089 10.1029/2021GC009711 eng http://creativecommons.org/licenses/by-nc-nd/3.0/es/ info:eu-repo/semantics/openAccess Atribución-NoComercial-SinDerivadas 3.0 España Wiley