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LEPIDOCROCITE IN HYDROTHERMAL SEDIMENTS OF THE ATLANTIS II AND THETIS DEEPS, RED SEA

¹ The Open University of Israel, P.O. Box 39328 Tel Aviv, Israel
² The Seagram Center for Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot, Israel
³ Chemistry Department, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, Frederiksberg, DK-1871 Frb.C., Denmark

^* E-mail address of corresponding author: nuritg{at}oumail.openu.ac.il

Lepidocrocite (-FeOOH) formation in the hydrothermal brines of the Thetis and Atlantis II Deeps in the Red Sea results in markedly different crystals (size and shape). The only foreign element associated with the crystals is Si and analyses of samples from the two deeps yielded average Si/Fe (molar) ratios of 0.03 and 0.11, respectively. The Si/Fe ratio does not affect formation of a perfect lattice along [010]. Direct observations of crystal morphology as well as X-ray diffraction patterns, Mössbauer and infrared spectra, all indicate that the Atlantis II Deep lepidocrocite is less crystalline than the Thetis Deep lepidocrocite. In one sample a poly-disperse size distribution was resolved indicating a fine-scale variation in precipitation conditions. Infrared spectroscopy suggests that the Si is adsorbed on the lepidocrocite surfaces, probably also forming polymers, as both Fe–O–Si and Si–O–Si bonds can be detected. The formation of the Atlantis II Deep lepidocrocite is due to fast oxidation of Fe²⁺. The blanket-like layer of lepidocrocite in Atlantis II and Thetis Deeps lepidocrocite was probably formed as a result of precipitation during an abrupt oxidation event of the brine, triggered by down-welling of a condensed oxidized brine, which originated in the northern part of the Red Sea. A difference in Si concentrations determined the different crystal properties of the lepidocrocite formed in the two deeps.

Key Words: Analytical High-resolution Transmission Electron Microscopy • Atlantis II Deep • Electron Diffraction • Hydrothermal Sediments • Infrared Spectroscopy • Mössbauer Spectroscopy • Si associated with Lepidocrocite • Thetis Deep

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