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

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, β = 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.