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A structural, infrared, and Mössbauer spectral study of rosemaryite, NaMnFe³⁺Al(PO₄)₃

¹ Laboratoire de Minéralogie, B18, Université de Liège, B-4000 Sart-Tilman, Belgium
² Département de Physique, B5, Université de Liège, B-4000 Sart-Tilman, Belgium
³ Department of Chemistry, University of Missouri-Rolla, MO 65409-0010, USA

^* E-mail: fhatert{at}ulg.ac.be

Rosemaryite, ideally NaMnFe³⁺Al(PO₄)₃, has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R₁ = 4.01 %, in the P2₁/n space group, with a = 12.001(2), b = 12.396(1), c = 6.329(1) Å, ß = 114.48(1)°, Vol. = 856.9(2) ų, Z = 4. The crystal structure and cation distributions are similar to those of ferrorosemaryite, NaFe²⁺Fe³⁺Al(PO₄)₃, and qingheiite, Na₂MnMgAl(PO₄)₃, but aluminium predominantly occurs in the M(2a) site, not in the M(2b) site as observed in ferrowyllieite, Na₂Fe²⁺₂Al(PO₄)₃. The topologies of the X(1a) and X(1b) crystallographic sites are identical to those found in ferrorosemaryite, and correspond to a distorted octahedron and to a distorted cube, respectively. The [7+1]-coordinated X(2) site is a very distorted gable disphenoid, similar to the A(2)’ site of the alluaudite structure. Mössbauer spectra have been obtained from 4.2 to 295 K, and fitted with a model including two Fe³⁺ and two Fe²⁺ doublets. The Fe²⁺ component corresponding to 2/3 of the Fe²⁺ spectral area and having a smaller quadrupole splitting of 2.63 mm/s at 15 K, is assigned to the Fe²⁺ on the M(2a) site, and the Fe²⁺ component with the larger quadrupole splitting of 3.17 mm/s at 15 K, is assigned to the Fe²⁺ on the M(1) site. Fe³⁺ is located only at the M(2a) and M(2b) sites, and the Fe³⁺ component corresponding to 3/4 of the Fe³⁺ and exhibiting the larger quadrupole spitting of 0.77 mm/s at 15 K, is most likely associated with Fe³⁺ on the M(2b) site. The infrared spectrum of rosemaryite shows absorption bands at 3450 and 1624 cm^–1, bands that arise from the vibrational modes of H₂O and confirm the presence of water in the channels of the wyllieite structure. A comparison of both the Mössbauer spectra and structural data of rosemaryite with those of other phosphates of the alluaudite and wyllieite groups, is also presented.

Key-words: Rosemaryite, phosphate mineral, structure refinement, Mössbauer spectroscopy, infrared spectroscopy.