Scanning electron microscopy and sulfur isotopic characterization of pyrite in silicified wood fragment in Japan

Five samples of fine-grained, authigenic pyrite seams in silicified woods from Tertiary marine to brackish sediments in Japan were examined on surface morphology by scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectrometry (EDX) and on sulfur isotopic composition (δ³⁴S). The surface morphology as revealed by SEM varied from long fibrous (pseudomorph after original plant fiber), spongy, blocky/cubic to framboidal (spherical aggregates of minute crystals). Except two samples from Chikuho coal mine area in northern Kyushu (Ebitsu: spongy fabric, δ³⁴S= +3.5 %₀, +12.2%₀), all pyrite samples were characterised by fairly low δ³⁴S values ranging from -40.0 to -20.1 %₀, suggesting to be the products from steady state sulfate reduction, under an open system, of seawater-sulfate by sulfate-reducing bacteria. Pyrite in coal deposits from the Ebitsu area is also bacteriogenic, but probably formed under a closed system where original plant debris were deposit. Any clear trend between surface morphology and δ³⁴S of the pyrite was not found.