臺灣西南海域永安海脊甲烷冷泉沉積物中之自生硫複鐵礦

Previous studies indicated the presence of a sulfidization front record in sediments at 10-15 meters below seafloor at the active methane seepage site MD052911 in the Yung-An ridge area off southwestern Taiwan. The sulfidization front is marked by precipitation of high concentrations of acid volatile sulfide (AVS) arising from reactions between downward diffusion of dissolved sulfide produced by sulfate reduction due to intense anaerobic oxidation of methane in the sulfate-methane transition zone and upward diffusive dissolved iron from depth. This study shows X-ray diffraction data indicating that the iron-sulfide mineral constituent of the black AVS at 1,483-1,487 cm below seafloor (peak AVS) is dominated by greigite (Fe₃S₄ thiospinel) with minor mackinawite (tetragonal FeS). Electron microscopic analyses suggest that the greigite forms aggregates of short and elongated prismatic crystals sub-micrometers in size, showing well-developed cubo-octahedral faces. The prismatic elongation is parallel to one of the〈100〉directions. Small aggregates of greigite crystals are often observed to occur within the fissures of phyllosilicate cleavage and locally show parallel growth along〈100〉, forming long crystal chains with a serrate outline. The greigite was formed as a result of incomplete sulfidization of mackinawite in the sulfidization front, probably involving a dissolution and re-precipitation mechanism. Greigite is an important transitional metastable phase in non-steady state early diagenesis of the iron-sulfide system which is presently still not well understood. This study adds to the limited body of direct observation and characterization of authigenic greigite from marine sediments.