Abstract

The Fe end-members scorzalite [Fe2+Al23+(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 (ΔEQ = −3.99 mm/s) in scorzalite is in quantitative agreement with the calculated value (ΔEQ = −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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