Multiple diagenetic processes as revealed by iron-sulfide mineralogy in sediments from the Erhjen-chi section southwestern Taiwan

Abstract

Electron microscopy techniques were used to determine compositional and structural properties and formation processes of iron-sulfide minerals in the Erhjen-chi section (Plio-Pleistocene Chiting Formation and Gutinkeng Mudstone) SW Taiwan Greigite occurred as a main constituent of millimeter- to centimeter-sized plant-fossil ichnofossil and semi-spherical iron-sulfide nodules but was not observed in the sediment matrix of all studied samples These nodules contained early-formed aggregates of granular or -elongated nanocrystals of greigite (~Fe3S4) about 100 nm in size with the aggregates surrounded and nodule cavities partially filled by coarser grained greigite crystals up to 1 μm in size locally intergrown with platy crystals with a composition of smythite (~Fe9S11) Iron-sulfide nodules were partially pyritized and marcasitized in the vicinity of the Gutingkeng Fault HRTEM analyses showed that the greigite nanocrystals invariably exhibited a high density of {111} planar defects and intense diffraction streaks parallel to the greigite directions implying close packed layers irregularly interrupted by stacking mistakes and/or changes of iron occupancy The coarser grained crystals of greigite were characterized by a defect-free domain epitaxially or topotaxially rimmed by areas showing layers with a 1 71-nm periodicity that might be hypothesized to include interstratified greigite-like 0 57 nm and smythite-like 1 14 nm units in the close packed layer sequence Electron diffraction patterns of the platy crystals exhibited a 1 14-nm periodicity along the c* direction consistent with the rhombohedral lattice of smythite with c = 3 42 nm Local interstratification of smythite with single or multiple greigite-like 0 57 nm layers was observed as well These results demonstrated two generations of greigite with distinct microstructures and late formation of smythite with complex transitional structures The greigite-dominated nodules were largely modified by recrystallization of pre-existing early-diagenetic greigite and smythite neoformation

Date of Award	2019
Original language	English
Supervisor	Wei-Teh Jiang (Supervisor)