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Metastable Si-Fe phases in hydrothermal sediments of Atlantis II Deep, Red Sea

¹ The Open University of Israel, P.O. Box 39328 Tel Aviv, 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)

Amorphous silicate sediments comprise most of the upper part of the sedimentary column of the Atlantis II Deep, whereas more crystalline phases are found in the lower part. The objective of this study was to focus on the newly-formed, metastable, hydrothermal phases, that eventually transform into clays or Fe oxides.

Short-range ordered Si-Fe phases (Si/Fe = 0.2–0.6) rounded to elliptical in shape, and amorphous Si-Fe platy phases (Si/Fe = 0.09–pure Si) comprise some of the amorphous sediments in Atlantis II Deep. The rounded particles have distinct electron-dense (thicker, relatively ordered) margins and a less crystalline inner core. They have rhombohedral symmetry with unit-cell parameters of a = 0.504 nm and c = 1.08 nm. They presumably transform into crystalline clay minerals in statu nascendi (at the formative stage) in the sediments. Synthesis in NaCl brines designed to simulate crystallization of the rounded particles, was performed successfully. Similar rounded phases (Si/Fe = 0.26) were synthesized under saline, neutral, hydrothermal conditions (40°C, pH 7, 2 M NaCl) with initial Si/Fe = 1.5. We suggest that the metastable rounded particles precipitate from the hot brine created by the encounter between Red Sea Deep Water and the hydrothermal brine underneath. Within a few thousands years they disintegrate and transform into clay minerals, probably nontronite.

KEYWORDS: amorphous, Atlantis II Deep, hydrothermal, short range ordered, Si-Fe phase, rounded particles

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