Two-XRD-line ferrihydrite and Fe-Si-Mn oxyhydroxide mineralization from Franklin Seamount, western Woodlark Basin, Papua New Guinea

Large deposits of Fe-Si-Mn oxyhydroxide, intimately associated with active warm springs, cover the flanks and caldera of Franklin Seamount in the west ern Woodlark Basin, Papua New Guinea. The deposits are dominated by poorly crystalline Fe oxyhydroxide, so-called two-XRD-line ferrihydrite. Data on h ydrothermal samples of ferrihydrite from FranMin Seamount provide new insig hts into the atomic structure and chemical composition of this mineral. Ele ctron microscopy supported by X-ray and selected-area electron-diffraction (XRD, SAED) analysis shows that the Franklin Seamount ferrihydrite consists of soft friable agglomerates of randomly stacked colloidal platelets or cr ystallites, 20-90 Angstrom in diameter, which possess short-range atomic or der in two dimensions and highly variable specific surface-areas. Different ial thermal analysis indicates that the ferrihydrite is very stable, not tr ansforming to hematite until 570 degrees C. Semiquantitative energy-dispers ion spectroscopy (EDS) analysis with a spatial resolution of 35 A indicates that agglomerates of pure ferrihydrite contain important amounts of Si (on average, 7.5 wt.% Si) and a constant Si:Fe ratio (on average, 0.45, atomic proportions) plus minor but variable amounts of sorbed P, S and As. Indivi dual crystallites exhibit very little beam-induced damage during the EDS an alysis. We suggest that Si is capable of being incorporated within the stru cture of the ferrihydrite. This suggestion is supported by XRD patterns sho wing a significant shift in the 11 (hk) hump to higher values of d, compare d to Si-free ferrihydrite, and by the thermal stability of the mineral. EDS results and results of bulk analyses demonstrate the wide range of composi tions of two-XRD-line ferrihydrite; nevertheless, an average composition of (VI)(Fe)(2.9)(IV)(Si, Fe, Al)(1.3)(O, OH, H2O)(12) is proposed for the sam ples from Franklin Seamount.