MINERALOGY OF SIDERITE NODULES IN THE PENNSYLVANIAN COAL MEASURES OF WESTERN ARKANSAS

Nodules and concretions have been used with varying success to infer depositional environments, sedimentation rates, and pore-water geochemistry during early diagenesis of fluvial-deltaic shales associated with Carboniferous coal measures. In this study, X-ray diffractometry was used to determine the mineralogy of sideritic nodules from the McCurtain Shale member of the Pennsylvanian McAlester Formation in western Arkansas. The nodules are very fine-grained, dark brown to black with hematized margins, weakly magnetic, and significantly more resistant to erosion than the surrounding shale. Some of the nodules have visible pyrite cores. Fossils have not been observed in the nodules. Nodules collected from several bedding-parallel horizons were sectioned and powdered to less than 325 mesh. Mineral phases were identified by comparison with published XRD spectra from the International Center for Diffraction Data’s (ICDD) PDF 4+ 2018 database. The validity of phase identifications and modal proportions of phases in each sample were estimated using a relative intensity ratio model built into Jade 9 software. The nodules are composed of siderite (83-85 modal %), illite (4-7 %), and fluorapatite (3-6 %), with trace amounts of pyrite, hematite, and quartz. The mineralogy of nodules in the McAlester Formation implies slow sedimentary rates under anoxic conditions where reduction of iron exceeded sulfate reduction.