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 an d spectroscopic techniques including infrared, Fe-57 Mossbauer, Al-27 and Si-29 nuclear magnetic resonance (NMR), and Fe and Si K-edge exten ded X-ray absorption fine structure (EXAFS). Scales precipitated at ne ar 250 degrees C from dissolved ferrous iron and silicic acid are comp osed of hisingerite. This phase is shown to possess the same local str ucture as nontronite and is a poorly-crystallized precursor of the fer ric smectite. A clear distinction can be made at the local scale betwe en hisingerite and 2-line ferrihydrite because, even in their most dis ordered states, the former possesses a two-dimensional and the latter a three-dimensional anionic framework. At temperature near 100 degrees C Fe-Si scales are a mix of Al-containing opal and hydrous ferrous si licate, whose local structure resembles minnesotaite and greenalite. T his hydrous ferrous silicate is very well ordered at the local scale w ith an average Fe coordination about Fe atoms of 6 +/- 1. The differen ce 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 f erric smectite suggests that the oxidation of ferrous iron occurs befo re polymerization with silica.