Abstract

Samples of silicified wood from the New Cairo Petrified Forest—evidence of a Paleogene forest also abundant at other sites in Egypt—were analyzed for mineralogy, organic carbon (OC), and loss on ignition (LOI) using X-ray diffraction, optical microscopy, and inductively coupled plasma methods. The OC content ranged from 0.04 to 0.16 wt.% and the LOI from 0.4 to 4.3 wt.%. The samples comprising quartz (78.3–100%) ± goethite (0.3–17.3%) ± mogánite (2–4.5%) ± gypsum (2%) ± calcite (0.5–0.6%) are classified into: (1) Silicified Wood A and (2) Silicified Wood B. The first is petrified with only microgranular quartz and the second with chalcedony ± cryptocrystalline quartz. The quartz precipitated in tracheids and on cell walls in the samples, while the chalcedony formed in the vessels, veins, and holes. Both silica polymorphs appear to have formed directly from primary solutions, rather than from diagenetic transformation of a silica phase. They most likely have originated from weathering in the area, as there are no other petrological sources of silica in the area.

The abundance of SiO2, SO42–, and MnO decreases from the Silicified Wood A to most of the Silicified Wood B samples, while Fe2O3, LOI, MgO, and Na2O increase. The P, Ba, and Sr are associated with the manganese in the samples, while V, Cr, Co, Ni, As, Y, Zr, and to a lesser extent, Nb, Hf, La, and Ce are associated with the iron component (mainly goethite). The Silicified Wood A samples have lower Mg/Camolar ratios than the Silicified Wood B samples and, hence, formed under relatively wetter conditions. The Silicified Wood A and the Silicified Wood B samples are depleted in Y, Nb, Hf, La, Ce, and Th with respect to the local sediments and soils. These depletions suggest a slightly acidic environment such as a swampy or stagnant pond. The thorium to uranium ratios in the silicified wood samples analyzed are significantly low (0.013–0.3), and thus are consistent with secondary U inputs. The U contents range from trace amounts to 17.2 ppm. Uranium concentrations are resident in gypsum in these samples.

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