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

A. Manceau1, Ph. Ildefonse2, J.-L. Hazemann1, A-M. Flank3 and D. Gallup4
1 Environmental Geochemistry Group, LGIT-IRIGM, University of Grenoble and CNRS, BP53, 38041 Grenoble Cedex 9, France
2 Laboratoire de Minéralogie et Cristallographie, Tour 16, 4 place Jussieu, 75252 Paris Cedex 05, France
3 LURE, Bat. 209D, Université Paris-Sud, 91405 Orsay Cedex, France
4 Unocal Corporation, P.O. Box 76, Brea, California 92621

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, 57Fe Mössbauer, 27Al and 29Si nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure (EXAFS). Scales precipitated at near 250°C 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 • Mössbauer • NMR

Clays and Clay Minerals; June 1995 v. 43; no. 3; p. 304-317; DOI: 10.1346/CCMN.1995.0430305
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)