The fossil wood of East Cairo, Egypt: A mineralogical view

As part of the characterization of the petrified wood of East Cairo at the New Cairo Petrified Forest, representative samples collected from the area were studied by X-ray powder diffraction (XRD) and optical microscopy. The samples, as indicated by XRD analysis, are composed of quartz ± some goethite ± minor moganite ± minor gypsum ± trace calcite. The absence of moganite in some samples has been attributed to the transformation of moganite to quartz or to leaching processes. Under the optical microscope, some of the 10 petrified-wood samples are permineralized with mainly microcrystalline quartz (chalcedony), some with a combination of chalcedony and microgranular quartz, and some with microgranular quartz. The sequence of silicification is likely to have been either opaline precursor to chalcedony and quartz, or directly to chalcedony and quartz. The crystallinity indices (C.I.) of the quartz in the samples studied, measured using a standard X-ray powder diffraction procedure, are consistent with the petrographic findings. The chalcedonic samples have the least C.I. values, and the microgranular quartz samples the highest values.