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.
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
