Bobfergusonite from the Nancy pegmatite, San Luis range, Argentina: Crystal-structure refinement and chemical composition.

TAIT, K. T., HAWTHORNE, F. C., CERNÝ, P., Y GALLISKI, M. A.,

CANADIAN MINERALOGIST

Mineralogical Association of Canada

Lugar: Ottawa; Año: 2004 vol. 42 p. 705 - 716

0008-4476

A second occurrence of bobfergusonite, ideally Na2 Mn2+5 Fe3+ Al (PO4)6, has been discovered at the Nancy pegmatite, San Luis Range, Argentina. The Nancy pegmatite is a small, poorly evolved pegmatite of the beryl?columbite?phosphate subtype of rare-element pegmatites. Bobfergusonite from the Nancy pegmatite is monoclinic, a 12.796(3), b 12.465(2), c 11.001(2) Å, 97.39(3), V 1740.1(5) Å3, P21/n, Z = 4, has been refined to an R-index of 2.6% for 2959 observed (|Fo| ? 4F) reflections measured with MoK X-radiation on a Bruker P4 diffractometer equipped with a CCD detector. The crystal used for the collection of the X-ray intensity data was subsequently analyzed with an electron microprobe. The unit formula derived from the refined site-scattering values and electron-microprobe results is (Na1.10 0.90) (Na0.90 0.10) Mn2+(Mn2+0.89 Ca0.11) (Fe2+0.91Fe3+0.49 Mn2+0.32 Mg0.28) (Fe3+0.42 Fe2+0.28 Mg0.30) (Al0.94 Fe3+0.06) (PO4)6. In bobfergusonite, there are six M sites, each coordinatedby six O-atoms in an octahedral arrangement with <M?O> distances ranging from 1.918 Å at M(6) to 2.237 Å at M(2).There is strong order between the divalent and trivalent cations over the six M sites, and there is also strong order between Al and Fe3+. There are five X sites, X(1)?X(5), each having a wide dispersion of X?O distances. The X(1) site is mainly occupied by Mn2+ and has octahedral coordination. The X(2) and X(3) sites are each [8]-coordinated, the X(4) and X(5) sites are [7]-coordinated, and X(2)?X(5) are all approximately half-occupied by Na. In all of the alluaudite-related structures, the M sites form linear edgesharing trimers, and the alluaudite, wyllieite and bobfergusonite structures show subtly different patterns of cation order?disorder.Bobfergusonite crystals from both known localities, the Nancy pegmatite (Argentina) and the Cross Lake pegmatite (Manitoba), are considerably disordered.Keywords: bobfergusonite, crystal structure, phosphate, Argentina.