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

Selke, R., Schmid-Beurmann, P. and Cemic, L. (2003) Cation and valence distribution in Fe-phosphates of the lazulite type. European Journal of Mineralogy, 15 (1). pp. 127-136. DOI 10.1127/0935-1221/2003/0015-0127.

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Samples of the lazulite-scorzalite, MgAl2(PO4)(2)(OH)(2) FeAl2(PO4)(2)(OH)(2), solid-solution were synthesized hydrothermally at 500degreesC and 0.3 GPa in compositional steps of 0.125 in x(Fe) = Fe/(Fe + Mg) and an additional sample with x(Fe) = 0.0625. The oxygen fugacity was controlled using the QFI-buffer. Under these conditions the solid-solution shows total miscibility verified by Xray powder diffraction, infrared spectroscopy in the OH stretching region, and Fe-57-Mossbauer 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 a(0), b(0), and beta indicates two different mechanisms for the Fe-incorporation below and above x(Fe) = 0.125. The Fe3+ content determined by Fe-57-Mossbauer spectroscopy increases significantly with decreasing x, 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 double left right arrow Fe2+ exchange above x(Fe) = 0.125 to a point defect reaction, in which Fe3+ gains a strong influence on the development of the lattice parameters.

Document Type: Article
Keywords: lazulite scorzalite substitution mechanism infrared spectroscopy Mossbauer spectroscopy solid-solution series ray-powder data mossbauer-spectroscopy stability scorzalite minerals tridymite system iron
Research affiliation: Kiel University
DOI etc.: 10.1127/0935-1221/2003/0015-0127
ISSN: 0935-1221
Date Deposited: 24 Jan 2012 06:14
Last Modified: 23 Sep 2019 23:56

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