Formation of hydroxysulphate and hydroxycarbonate green rusts in the presence of zinc using time-resolved in situ small and wide angle X-ray scattering I. A. M. AHMED*, S. SHAW AND L. G. BENNING School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK ABSTRACT The formation and transformation of hydroxysulphate (GRSO 4 ) and hydroxycarbonate (GRCO 3 ) Green Rusts were studied in situ using synchrotron-based time-resolved small and wide angle X-ray scattering. The time-resolved data revealed, for the first time, the pH dependent transition from poorly- ordered schwertmannite (pH <6.5) into GRSO 4 (pH ~6.8) followed by GRCO 3 (at pH ~9.6). These data also showed that the addition of Zn to the starting sulphate Fe 2+ /Fe 3+ solution resulted in a change in size of the GR unit-cell due to substitution of Zn into the GR structure. Introduction IRON oxides and oxyhydroxides are common components within soils and aquifers where they can form via the weathering of Fe-bearing silicate (e.g. olivine) and sulphide (e.g. pyrite) minerals. Green Rusts (GR; e.g. Fe 4 2+ Fe 2 3+ (OH) 12 SO 4 .xH 2 O) are a class of mixed-valent Fe oxyhydroxides that tend to form during aqueous corrosion of metallic- iron, or in reductomorphic soils (Trolard et al., 1997) as a result of both abiotic and biotic processes (Genin, 2004). Their structure consists of layers of positively charge octahedrally coordinated Fe, intercalated with anionic species (e.g. SO 4 2À , CO 3 2À or Cl À ). The structural Fe 2+ (Fe 2+ /Fe 3+ may vary between 0.5 and 3, but typically = 2) within GR serves as a powerful electron donor for the reductive transformation of many redox sensitive elements (e.g. Cr 6+ ? Cr 3+ ). Also some divalent transition metals (e.g. Ni and Zn) can substitute directly for the Fe 2+ within the octahedral layers. The reduction and/or absorption of these elements leads to reduced mobility and bioavailability and therefore GR is potentially important for the immobilization of toxic (e.g. Se) and radioactive (e.g. U) species within contaminated land environments (O’Loughlin et al., 2003; Pepper et al., 2003; Refait et al., 2000; Williams and Scherer, 2001). However, the formation and stability of GR phases in the environment are still poorly understood. This is primarily due to fact that GR particles are highly reactive and oxidize rapidly in air making them difficult to study using conventional ex situ techniques. In this study we have characterised the formation of GR in situ using synchrotron-based time-resolved small and wide angle X-ray scattering (SAXS/WAXS). The aim was to explore the kinetics and mechanism of the abiotic GRSO 4 and GRCO 3 formation (nucleation and growth) and their transformation under controlled conditions, and also evaluate the effect of Zn on these processes. Materials and methods A programmable reaction system was used to precipitate GR at a fixed temperature (21ºC) and within an anoxic environment using a continuous flow of pure N 2 gas. This system allowed accurate addition of base and continuous logging/control of pH and redox potential (Eh). Pure GRSO 4 and GRCO 3 , as well as Zn-containing GRSO 4 and * E-mail: i.ahmed@see.leeds.ac.uk DOI: 10.1180/minmag.2008.072.1.159 Mineralogical Magazine, February 2008, Vol. 72(1), pp. 159–162 # 2008 The Mineralogical Society