THE IRON MINERALOGY OF EOCENE FOSSIL WOOD-A MOSSBAUER STUDY OF SAMPLES FROM THE PETRIFIED FOREST, NEW CAIRO, EGYPT

摘要

As part of an investigation of iron species in fossil wood at the New Cairo Petrified Forest, representative samples, collected from the area, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), and Mossbauer spectroscopy at room temperature. The samples are composed of silica, with similar to 1.2 to similar to 17 wt.% Fe2O3, 0.21-0.92 wt.% Al2O3, and 0.001-0.71 wt.% MnO. The other oxides-including TiO2, MgO, CaO, Na2O, K2O, and P2O5-constituted similar to 0.24 to similar to 1.4 wt.% of the fossil wood. Mossbauer spectra of the samples, fitted using the hyperfine distribution method, showed a major sextet of Fe3+ in goethite (64.75-100%), another sextet of Fe3+ in hematite (27.62-31.94%), and a doublet (2.01-5.72%) that is attributable to octahedral Fe3+ in superparamagnetic phases. The presence of the iron in the trivalent state in all samples is consistent with high oxygen fugacity environments post-fossilization. Equivalent weights of goethite and hematite in these samples calculated using the relative spectral areas determined by Mossbauer spectroscopy and the iron contents by ICP-MS were 0.84-18.52 and 0.13-0.45 wt.%, respectively. Changes in goethite or hematite abundances in the samples had no effects on their hyperfine parameters.