Crystal Chemistry of Hydrous Iron Silicate Scale Deposits at the Salton Sea Geothermal Field

-The crystal chemistry of Fe-Si scales deposited from geothermal brines at Salton Sea, California, was studied by powder X-ray diffraction and spectroscopic techniques including infrared, 5VFe M6ssbauer, 27A1 and 29Si nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure (EXAFS). Scales precipitated at near 250~ from dissolved ferrous iron and silicic acid are composed of hisingerite. This phase is shown to possess the same local structure as nontronite and is a poorly-crystallized precursor of the ferric smectite. A clear distinction can be made at the local scale between hisingerite and 2-line ferrihydrite because, even in their most disordered states, the former possesses a two-dimensional and the latter a three-dimensional anionic framework. At temperature near 100*C Fe-Si scales are a mix of Al-containing opal and hydrous ferrous silicate, whose local structure resembles minnesotaite and greenalite. This hydrous ferrous silicate is very well ordered at the local scale with an average Fe coordination about Fe atoms of 6 +_ 1. The difference in crystallinity between the ferrous and ferric silicate scales was related to variations of growth rates of clay particles precipitated from ferrous and ferric salt solutions. The low crystallinity of the ferric smectite suggests that the oxidation of ferrous iron occurs before polymerization with silica. Key Words--Crystal chemistry, EXAFS, Ferrihydrite, Hisingerite, Hydrous iron silicate, M6ssbauer, NMR. I N T R O D U C T I O N Over the past decade, considerable effort has been made to generate electricity from steam produced at the Salton Sea geothermal field located in the imperial Valley, southeastern California, USA (Hoyer et al 1991). The flashing o f steam from hyper-saline brines produces iron-rich siliceous scale deposits that rapidly foul piping and other brine-handling equipment. These scales are brown-black, hard, and vitreous material. Unti l methods (acidification or crystallization-reaction clarification) were developed to inhibit the formation o f these unusual scales, continuous operation o f production facilities was drastically hindered. Characterization of the poorly-crystalline, iron-rich silica scale deposits was required to understand the deposition mechanism and to develop techniques to avoid their deposition (Gallup 1989, Gallup and Reiff 1991). These initial studies, using spectroscopic techniques suggested that iron in the scales deposited at high temperature was primarily present in the ferric oxidation state and that the iron silicate phase resembled hisingerite. The present study seeks to determine the chemical environment o f iron and silicon in these deposits, with emphasis on determining the possible polyhedral linkages between Fe(O,OH)6 octahedra and SiO4 tetrahedra. We use several complementary methods, includCopyright 9 1995, The Clay Minerals Society ing powder X-ray diffraction, M6ssbauer and infrared spectroscopy, 27A1and 29Si-nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). E X P E R I M E N T A L