AUTHIGENIC GREIGITE IN MUD FROM THE CONTINENTAL-SHELF OF THE YELLOW SEA, OFF THE SOUTHWEST KOREAN PENINSULA

This is the first reported occurrence of greigite in Korea, which was found in semi-consolidated near-surface mud on the continental shelf o f the southeastern Yellow Sea. The seafloor (70 m of water) in the sam pling site is composed of alternating massive sand and homogeneous mud , which are presumably part of the Holocene transgressive deposits. Gr eigite grains (0.1-2 mm in size) are disseminated or form a single lam ina in the mud. The mineral is strongly magnetic and black with metall ic luster; some grains have a reddish-brown tarnish caused by oxidatio n. In reflected light, greigite occurs as yellowish-white to pale crea my tint and isotropic globular aggregates. The mineral greigite has a chemical composition of Fe = 54.4, S = 42.4, As = 0.14, Si = 2.3 wt%, and the empirical formula on the basis of S = 4 is Fe2.95S4. Cell dime nsions (cubic) are measured as a = 9.8635 Angstrom, V = 959.6 Angstrom (3) and D = 4.094 g/cm(3). It probably formed in a reducing micro-envi ronment around decaying organic matter.