Original paper

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

Hatert, Frédéric; Hermann, Raphaël P.; Fransolet, André-Mathieu; Long, Gary J.; Grandjean, Fernande

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+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, Na2FeM2+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.

Keywords

rosemaryitephosphate mineralstructure refinementmossbauer spectroscopyinfrared spectroscopy