The Iron Mineralogy of Eocene Fossil Wood—a Mössbauer Study of Samples From the Petrified Forest, New Cairo, Egypt

Abstract

Abstract 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 ~1.2 to ~17 wt.% Fe 2 O 3 , 0.21−0.92 wt.% Al 2 O 3 , and 0.001−0.71 wt.% MnO. The other oxides—including TiO 2 , MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 —constituted ~0.24 to ~1.4 wt.% of the fossil wood. Mossbauer spectra of the samples, fitted using the hyperfine distribution method, showed a major sextet of Fe 3+ in goethite (64.75–100%), another sextet of Fe 3+ in hematite (27.62–31.94%), and a doublet (2.01–5.72%) that is attributable to octahedral Fe 3+ 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. The observed changes in Mossbauer spectra of goethite in the samples studied have been attributed to poor crystallinity, rather than element substitutions. This resulted in a reduction of the average magnetic hyperfine field (to ~27 T) and an increase in the hyperfine field distribution widths. The effects of poor crystallinity were much less pronounced in the hematite spectrum, with the samples having average room-temperature magnetic hyperfine fields of ~50.2 T, as expected for bulk hematite. Silica substitutions for iron in the fossil wood samples cannot be ruled out, but the effects of such processes on hyperfine field distributions of the iron are unknown.