IRON-OXIDES AND SMECTITES IN SEDIMENTS FROM THE ATLANTIS-II-DEEP, RED-SEA

Solid-phase analysis was carried out on a 11 m sediment core of the At lantis II Deep using chemical analysis, X-ray diffraction (XRD), elect ron microscopy (TEM, EDX), Mossbauer (MS) and infrared (FTIR) spectros copy. Iron oxides and smectites are the dominant minerals. Among the F e oxides an extremely poorly crystalline nanometer-sized (nano-) hemat ite with strong anisotropic XRD peak broadening comprised most of the upper part of the column whereas well crystalline hematite prevailed i n its lower part. Akaganeite dominated in an intermediate layer. The n ano-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 i ts small size the nano-hematite exhibits complex MS spectra. Both, the nano- and the well crystalline hematite, could be subdivided on the b asis of their magnetic hyperfine field and quadrupole shift. Further c haracterization and semiquantification was based on MS spectra recorde d at various temperatures. Di- and trioctahedral Fe-rich smectites com prised the clay silicate fraction of the samples. The trioctahedral on es dissolved in acid oxalate and were magnetically ordered at 4.2 K. I t is speculated that the variety in Fe-oxide mineralogy reflects the t ime dependent variations of the formation environment. Although we wer e not successful in reproducing nano-hematite in the laboratory by var ying parameters such as temperature (50-125 degrees C) and chemical co mposition of the aqueous system we believe that variation of soluble S i concentration is responsible for the large range in crystal perfecti on of hematite. Akaganeite is most likely the result of Fe-III hydroly sis at higher chloride concentration.