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Abstract. Members of the solid solution seriesZnl_xF%A1204 (x = 0.2, 0.4, 0.6 and 1.0) with spinel structure were synthesized by direct solid-state reaction of the simple metal oxides and metallic iron in evacuated silica ampoules at 1175 ~ C. Two aliquots of the single-phase spinels obtained for each composition were annealed under vacuum at 1075 ~ C and 725 ~ C for 48 hours and then quenched in liquid nitrogen.The cation distributions of all the quenched samples were determined by X-ray powder diffraction, using the Rietveld method of structural refinement. The degree of inversion increases with iron content and for spinels with the same chemical composition with quenching temperature. The relative areas estimated for the contributions to the M6ssbauer spectra of tetrahedrally-and octahedrally-coordinated Fe 2+ suggest that most of Zn 2+ cations remain at the tetrahedral site, as expected from the relative cation site preferences.Failure to quench the equilibrium cation distributions, suggested by deviations between the observed composition dependence of the cation distribution and that expected from the thermodynamic model of O'Neill and Navrotsky (1983, 1984), may be explained by an enhancement of cation diffusion rates in the Znl_xF%A1204 (0 < x < 1) spinels caused by the presence of cation vacancies. Fe3+/vacancy defects are easily formed in these spinels due to partial oxidation of Fe z+ at high temperature.

ABSTRACT:The REE contents in the clay-sized fractions of nineteen Cretaceous and Pliocene continental sediments, Portugal, were found to be correlated with kaolinite. No correlation is apparent between clay mineralogy and La/Lu or Eu/Sm ratios. The Eu anomaly is generally smaller (or even absent) in the clay-sized fraction than in the corresponding whole sediment, probably due to a greater ability of the clay minerals to accommodate Eu (as Eu 2+) rather than the other REE released during weathering.It is well known that the rare earth element (REE) distribution in a sediment depends on the relative importance of several factors such as: (i) the REE contents in the parent rocks and their distribution in the mineral phases; (ii) the nature and the REE contents of the fluids passing through the parent rocks; (iii) the partitioning behaviour of the REE between the fluids and the mineral phases of those rocks; (iv) the ability of the secondary minerals formed during the reactions to accommodate the REE; (v) the changes which take place during the transport process; and (vi) the chemical reactions during and after deposition

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