A STRUCTURAL, INFRARED, AND MöSSBAUER SPECTRAL STUDY OF ROSEMARYITE, NAMNFE3+AL(PO4)3

Abstract

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) Å 3, Z = 4. The crystal structure and cation distributions are similar to those of ferrorosemaryite, NaFe2+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. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.