Authigenic rhabdophane from brown iron ore of the oxidation zone of the Babaryk massive sulfide occurrence (South Urals): scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) study

Abstract

Rhabdophane (Ce0.34−0.43Nd0.13−0.14Ca0.06−0.29La0.08−0.11Y0.05−0.12Pr0.03−0.05Sm0.02−0.05Gd0.02−0.05Fe0−0.04Dy0.00-0.01)0.97-1.01((P0.69−0.96S0.04−0.31)1.00O4)⚫H2O is found in a Fe3+-oxyhydroxide nodule (brown iron ore) collected from the upper part of the oxidation profile of the Babaryk massive sulfide occurrence (South Urals, Russia) at a 1.6 m depth. The structural and microtextural features of rhabdophane are revealed by electron backscattered diffraction (EBSD); the chemical composition and distribution of the main components are determined on a scanning electron microscope (SEM) equipped with an energy-dispersive analyzer (EDA); the bulk contents of rare earth elements (REEs) and other elements in rock samples are analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Rhabdophane forms spherulitic aggregates up to 35 µm in size with a fine-grained core and radial radiant rims composed of prismatic crystals. The chaotically oriented aggregates of its particles of various sizes including prismatic crystals and spherulitic intergrowths also fill fractures up to 200 µm long and 20–30 µm thick in goethite. The zonal radial radiant structure of the rhabdophane aggregates and their occurrence in fractures of goethite unambiguously indicate the authigenic origin of rhabdophane. The chemically heterogeneous rhabdophane grains always contain Y, Ca and S and rarely Fe and Sr and are Th- or U-free. Contrasting zonation of Ca, S and Y contents is characteristic of spherulites. The band contrast of the EBSD patterns shows a good crystallinity of prismatic crystals regardless of the chemical composition even for Ca–S-rich zones. On the other hand, the Ca- and S-rich fine-grained centers of the spherulites do not yield any distinguishable diffraction patterns. There is a strong negative correlation in pairs (Ca+Sr)–P and (REEs+Y)–S and a positive correlation in pairs (Ca+Sr)–S and (REEs+Y)–P, which indicates the isomorphism according to the scheme (REEs+Y)3+ + (PO4)3− ↔ (Ca+Sr)2+ + (SO4)2−. Thus, the chemical composition of rhabdophane does not completely correspond to the rhabdophane–tristramite/brockite series because of the absence of tetravalent U or Th. In contrast to similar samples from the deeper part of the oxidation zone, the brown iron ore with rhabdophane is enriched in light rare earth elements (LREEs) and P. The REEs were probably sourced from ore-bearing volcanomictic rocks, while P could also have been derived from the soil. The enrichment in REEs and P and the formation of rhabdophane are related to the alternation of dry and wet periods, the P input, and sorption–desorption of REEs from Fe3+ oxyhydroxides and/or clay minerals due to pH changes and variable composition of pore water.