Aluminum is an active amphoteric metal element, but native aluminum (Al°) can be formed and preserved in special condi- tions so that its occurrence has unique geological significance. In this paper, the coarse fractions (> 63 μm) of 269 samples of surface sediments and of 165 sediment samples in four cores from the South China Sea (SCS) were picked out and analyzed. Al° particles were found in only five surface samples and three samples in two sedimentary cores. The particles are grayish or silver white in color with a strongly metallic luster and high plasticity, and they occur as irregular plats, elongated shapes and spherules with botryoidal structures. The identified Al° particles belong to cubical system, Fm3m space group with a varying from 4.028×10?1 nm to 4.059×10?1 nm. Their chemical compositions consist mainly of Al (95.07%–99.84%) and minor amounts of Si, Fe, Ti, S, Zn, Mg, Ca and trace amounts of K, Na, Cu, Co, P and Mn. Our results show that the identified Al° is authigenic, formed in situ in the prospective areas of gas hydrates, and that it is an unreported type of Al° occurrence. A possi- ble mechanism of Al° formation is proposed: under the strongly reducing micro environments at cold seeps, dissolved Al3+ in sediments is reduced to its metallic state by the strongly reducing agent of H2 through microbial processes and enzyme action. As a result, the Al° particles from the prospective gas hydrate areas in the SCS are the special products, representing the first report on Al° from the sediments at cold seeps. The proposed mechanism provides a theoretical basis for further studies on the special biogeochemical processes at cold seeps.
