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

Abstract

Rosemaryite, ideally NaMnFe 3+ Al(PO 4 ) 3 , has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R 1 = 4.01 %, in the P2 1 /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, NaFe 2+ Al(PO 4 ) 3 , and qingheiite, Na 2 MnMgAl(PO 4 ) 3 , but aluminium predominantly occurs in the M(2a) site, not in the M(2b) site as observed in ferrowyllieite, Na 2 Fe 2+ 2 Al(PO 4 ) 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 Fe 3+ and two Fe 2+ doublets. The Fe 2+ component corresponding to 2/3 of the Fe 2+ spectral area and having a smaller quadrupole splitting of 2.63 mm/s at 15 K, is assigned to the Fe 2+ on the M(2a) site, and the Fe 2+ component with the larger quadrupole splitting of 3.17 mm/s at 15 K, is assigned to the Fe 2+ on the M(1) site. Fe 3+ is located only at the M(2a) and M(2b) sites, and the Fe 3+ component corresponding to 3/4 of the Fe 3+ and exhibiting the larger quadrupole spitting of 0.77 mm/s at 15 K, is most likely associated with Fe 3+ 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 H 2 O 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.