Modern authigenic amorphous and crystalline iron oxyhydroxides in subsurface Ordovician dolostones (Jinan, North China Block): Biomineralization and crystal morphology

Abstract

Iron oxide/oxyhydroxide precipitation in nature can involve various mechanisms that are associated with environmental conditions. Amorphous and crystalline Fe oxyhydroxides associated with extracellular polymeric substances (EPS) are common in some of the pores and fractures of subsurface Ordovician dolostone (Jinan, North China Block). These precipitates, dominated by ferrihydrite nanogranules (20–30 nm), acicular goethite nanorods and nanofibers (>200 nm), and various twinned goethite polymorphs (~1–3 mm), record a series of crystallization stages that reflect the mechanisms of iron mineralization. For Fe oxyhydroxide formation this involved: (1) biogenic ferrihydrite and goethite nanoparticle formation, (2) ferrihydrite–goethite transformation and development of nanogranule-attached goethite, (3) goethite twin development, and (4) multi-generation overgrowths on twined goethite. This process shows that the Fe was fixed biogenically with crystallization taking place via a nonclassical nanogranule-aggregation pathway. As such, it highlights the complex mechanisms of crystal growth and morphological development involved with Fe oxyhydroxide mineralization in nature. The distribution of the precipitates, concentrated in larger voids ~11 m below ground surface, demonstrates their sensitivity to ambient conditions such as redox and Fe supply.