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American Mineralogist; April 2003; v. 88; no. 4; p. 481-488
© 2003 Mineralogical Society of America
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Electronic structure of Fe-bearing lazulites

M. Grodzicki1,*, G.J. Redhammer1, G. Amthauer1, V. Schünemann2, A.X. Trautwein2, B. Velickov3 and P. Schmid-Beurmann4

1 Institute of Mineralogy, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria
2 Institute of Physics, Medical University, Ratzeburger Allee 160, D-23536 Lübeck, Germany
3 Institute of Applied Geosciences, TU-Berlin, Ernst-Reuter-Platz 1, D-10587 Berlin, Germany
4 Institute of Mineralogy and Petrography, University of Kiel, Ludewig-Meyn-Str. 10, D-24098 Kiel, Germany

Correspondence: * E-mail: michael.grodzicki{at}sbg.ac.at

The Fe end-members scorzalite [Fe2+Al2 3+(PO4)2(OH)2] and barbosalite [Fe2+Fe3+ 2 (PO4)2(OH)2] of the lazulite series have been investigated by Mössbauer and diffuse reflectance spectroscopy, and by electronic structure calculations in the local spin density approximation. The measured quadrupole splitting ({Delta}EQ = –3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value ({Delta}EQ = –3.90 mm/s), as well as its temperature dependence. The optical spectrum of barbosalite can be resolved into three peaks at 8985 cm–1, 10980 cm–1, and 14110 cm–1. These positions correlate well with the two calculated spin-allowed d-d transitions at 8824 cm–1 and 11477 cm–1, and with an intervalence charge transfer transition at about 14200 cm–1. The calculated low-temperature magnetic structure of barbosalite is characterized by a strong antiferromagnetic coupling (J = –84.6 cm–1) within the octahedral Fe3+-chains, whereas a weak antiferromagnetic coupling within the trioctahedral subunit cannot be considered as conclusive. The analysis of the charge and spin densities reveals that more than 90% of the covalent part of the iron-ligand bonds arises from the Fe(4s,4p)-electrons. Clusters of at least 95 atoms are required to reproduce the available experimental data with quantitative accuracy.






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