Iron oxides and smectites in sediments from the Atlantis II Deep, Red Sea
Schwertmann, Udo; Friedl, Josef; Stanjek, Helge; Murad, Enver; Bender Koch, Christian
European Journal of Mineralogy Volume 10 Number 5 (1998), p. 953 - 968
published: Oct 5, 1998
manuscript accepted: Apr 3, 1998
ArtNo. ESP147051005010, Price: 29.00 €
Abstract Solid-phase analysis was carried out on a 11 m sediment core of the Atlantis II Deep using chemical analysis, X-ray diffraction (XRD), electron microscopy (TEM, EDX), Mossbauer (MS) and infrared (FTIR) spectroscopy. Iron oxides and smectites are the dominant minerals. Among the Fe oxides an extremely poorly crystalline nanometer-sized (nano-) hematite with strong anisotropic XRD peak broadening comprised most of the upper part of the column whereas well crystalline hematite prevailed in its lower part. Akaganéite dominated in an intermediate layer. The nano-hematite consisted of irregular particles ca. 5 nm in diameter and of 2.4-2.8 nm thick curled fibers as revealed by TEM. Mainly due to its small size the nano-hematite exhibits complex MS spectra. Both, the nano- and the well crystalline hematite, could be subdivided on the basis of their magnetic hyperfine field and quadrupole shift. Further characterization and semiquantification was based on MS spectra recorded at various temperatures. Diand trioctahedral Fe-rich smectites comprised the clay silicate fraction of the samples. The trioctahedral ones dissolved in acid oxalate and were magnetically ordered at 4.2 K. It is speculated that the variety in Fe-oxide mineralogy reflects the time dependent variations of the formation environment. Although we were not successful in reproducing nano-hematite in the laboratory by varying parameters such as temperature (50-125°C) and chemical composition of the aqueous system we believe that variation of soluble Si concentration is responsible for the large range in crystal perfection of hematite. Akaganéite is most likely the result of FeIII hydrolysis at higher chloride concentration.