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European Journal of Mineralogy; January, February 2003; v. 15; no. 1; p. 127-136; DOI: 10.1127/0935-1221/2003/0001-0127
© 2003 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Cation and valence distribution in Fe-phosphates of the lazulite type

Regina SELKE*, Peter SCHMID-BEURMANN and Ladislav CEMIC

Institut für Geowissenschaften, Universität Kiel, Ludewig-Meyn-Str. 10, D-24098 Kiel, Germany

* e-mail: rs{at}min.uni-kiel.de

Samples of the lazulite-scorzalite, MgAl2(PO4)2(OH)2 — FeAl2(PO4)2(OH)2 solid-solution were synthesized hydro-thermally at 500°C and 0.3 GPa in compositional steps of 0.125 inxFe = Fe/(Fe + Mg) and an additional sample withxFe = 0.0625. The oxygen fugacity was controlled using the QFI-buffer. Under these conditions the solid-solution shows total miscibility verified by X-ray powder diffraction, infrared spectroscopy in the OH stretching region, and 57Fe-Mössbauer spectroscopy. The linear dependence of the molar volume from composition indicates an ideal behaviour of the volumetric properties. The compositional dependence of the lattice parameters a0, b0, and β indicates two different mechanisms for the Fe-incorporation below and above xFe = 0.125. The Fe3+ content determined by 57Fe-Mössbauer spectroscopy increases significantly with decreasing xFe of the solid solutions, from about 5 % of the total iron content to approximately 13.8 %, and was interpreted quantitatively on the basis of an oxidative dehydrogenation reaction. The results indicate a change in the substitution mechanism from an isomorphic substitution which is dominated by simple Mg {Leftrightarrow} Fe2+ exchange above xFe = 0.125 to a point defect reaction, in which Fe3+ gains a strong influence on the development of the lattice parameters.

Key-words: lazulite, scorzalite, substitution mechanism, infrared spectroscopy, Mössbauer spectroscopy.






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