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Synthesis of clay-sized iron oxides under marine hydrothermal conditions

¹ The Open University, P.O. Box 39328 Tel Aviv, Israel, and ² The Seagram Center for Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot, Israel

^* E-mail: nuritg{at}oumail.openu.ac.il

(Received 3 January 2001; revised 16 September 2001)

Goethite, lepidocrocite, magnetite and akaganeite were synthesized in 0.8 M, 2 M and 5 M NaCl solutions at various temperatures (25, 40, 60°C) under slightly acidic to slightly alkaline pH with or without Si additions. Elevated temperatures prevent complete oxidation of initial Fe²⁺ solutions and magnetite and siderite precipitate, accompanied by goethite and lepidocrocite. At higher salinity, O₂ solubility is reduced and its distribution is limited, leading to coprecipitation of lepidocrocite, akaganeite and goethite.

Lepidocrocite morphology changes from plates at pH 5.5 through rods at pH 7 to multi-domainic crystals at pH 8.2, due to enhanced crystal growth along the c axis. Salinity and temperature have opposite effects on lepidocrocite crystallinity.

Goethite crystals are multi-domainic and twinning appears only at elevated temperatures. Increases in temperature and salinity improve goethite crystallinity as observed by IR spectra. Addition of Si up to Si/Fe = 0.1 retards crystal growth and Si-OH-stretching bands appear. At Si/Fe = 1 most of the precipitate is short range ordered.

Platy and rod-shaped lepidocrocite from the Thetis and Atlantis II Deeps, were probably formed under the slightly acidic conditions of the hydrothermal brines. The Si concentration was greater in Atlantis II Deep than in Thetis Deep, leading to larger lepidocrocite and goethite crystals in the latter.

Multi-domainic goethite could have precipitated throughout. Pure phase goethite might have precipitated in the less concentrated brine, whereas mixtures of goethite and lepidocrocite might have precipitated in the more concentrated brine, depending mainly on oxidation rate and oxygen mobility within the brine.

KEYWORDS: akaganeite, goethite, hydrothermal brine, IR, lepidocrocite, magnetite, synthesis, TEM

This article has been cited by other articles:

				N. Taitel-Goldman, N. Taitel-Goldman, C. Bender Koch, and A. Singer Si-ASSOCIATED GOETHITE IN HYDROTHERMAL SEDIMENTS OF THE ATLANTIS II AND THETIS DEEPS, RED SEA Clays and Clay Minerals, February 1, 2004; 52(1): 115 - 129. [Abstract] [Full Text] [PDF]