Layered materials such as clay minerals or oxides contribute significantly to soil reactivity because of their large specific surface area that promotes adsorption/desorption reactions. Among them, Foug`erite is a Fe(II)/Fe(III) layered double hydroxide that was identified twenty years ago in hydromorphic soils. Foug`erite has a layer structure resembling that of the so-called Green Rust (GR) whose color is due to the mixed Fe valence in their structure. GR layers are positively charged because of the presence isomorphic substitutions (e.g. Fe3+ in place of Fe2+) and possibly vacancies. Layer charge compensation at the mineral scale is ensured by anions located in the interlayer space. These interlayer anions can be exchanged with anions present in surrounding pore water. Consequently, GR participate to the biogeochemical cycling of anions in the environment, including nutrients or pollutants. In this study, we carried out a detailed analysis of green rust structure in order to better quantify these biogeochemical cyclings. In particular, we performed laboratory and synchrotron experiments that allowed deciphering the mechanisms of chloride for sulphate exchange. The effect of particle size was investigated using two types of samples, one being nanometric and the other one micrometric, that we synthesized and characterized. It could be demonstrated that sulphate replaces chloride in the interlayer space. These experiments shed light on the relationship between structure and reactivity of these phases.