Can the presence of structural phosphorus help to discriminate between abiogenic and biogenic magnetites?

摘要

The influence of phosphate on the competitive formation of magnetite and lepidocrocite and the properties of magnetite prepared from mixtures of Fe(II) and Fe(III) salts were studied. Products were prepared at 90 ℃ and pH 12.5 (series 1), 50 ℃ and pH 7 (series 2) and 20 ℃ and pH 8 (series 3). The P/Fe atomic ratio in the initial solution ranged from 0 to 3% and the pH was kept at the desired value with NaOH or KOH. Air was used as oxidant in series 2 and 3. All products, which were characterized by X-ray diffraction, transmission electron microscopy, chemical analysis and IR spectroscopy, contained a phase intermediate between magnetite and maghemite (referred to as magnetite in this paper). The products of series 1 consisted only of magnetite at all P/Fe ratios, whereas both magnetite and lepidocrocite formed in series 2 and 3 above a certain P/Fe ratio. On increasing the P/Fe ratio in the initial solution, the magnetite crystals became smaller and more oxidized (i.e. closer to maghemite) and the lepidocrocite/magnetite ratio increased. The P associated with magnetite was partly in the form of occluded P, i.e. non-surface-adsorbed phosphate. IR spectra suggested this P to be structural and occurring as low-symmetry PO4 units. Because abiogenic magnetites produced in various environments incorporate structural P but some well-characterized biogenic magnetites seem to contain no P or be formed in P-poor environments, we hypothesize that natural magnetites containing occluded P are unlikely to be biogenic. However, more studies are needed to discard the presence of P in biogenic magnetites.