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

Abstract

Abstract--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.

Fractal Mechanisms in Coagulation/Flocculation Processes in Environmental Systems

“…These results indicate that Fe speciation in the aggregates is very similar to that observed in solution [28]. However, contrary to the dissolved species, and similar to other systems [84], Si was not found in the local environment of Fe in the precipitates. A significant Fe polymerization is observed by increasing the hydrolysis ratio from n = 1 to n = 1.5.…”

Effect of phosphate, silicate, and Ca on Fe(III)-precipitates formed in aerated Fe(II)- and As(III)-containing water studied by X-ray absorption spectroscopy

“…Suppression of corner-sharing Fe-Fe linkage at high Si/ Fe ratios has been reported for dissolved Fe(III)-clusters (Pokrovski et al, 2003) and Si-rich Fe(III)-hydrolysis products (Doelsch et al, 2000). Even though HFO-Si had a molar Si/Fe ratio of 0.56, second-shell Si was not included in the fit because its EXAFS contribution constructively interfered with the dominant contribution from edge-sharing Fe (Manceau et al, 1995). The contribution from edge-sharing Fe was thus slightly overestimated in the fit.…”

Transmission Electron Microscopy of Synthetic 2- and 6-Line Ferrihydrite

“…We follow this convention; however, only synthetic 6LFh samples are strictly consistent with the International Mineralogical Association definition (Fleischer et al, 1975;Farmer, 1992). Other commonly used names for ferrihydrite include "protoferrihydrite" (the name originally proposed for 2LFh), "amorphous ferric oxide," and "hydrous ferric oxide" (e.g., Chukhrov et al, 1973;Towe and Bradley, 1976;Farmer, 1992;Manceau et al, 1995).…”