Crystal structure and topological affinities of magbasite, KBaFe3+Mg7Si8O22(OH)2F6: a trellis structure related to amphibole and carpholite

M. D. Welch, R. H. Mitchell, A. R. Kampf, A. R. Chakhmouradian, D. Smith, M. Carter
2014 Mineralogical magazine  
The crystal structure of magbasite from the Eldor carbonatite complex, Quebec, Canada, has been determined and indicates that the currently accepted formula should be revised to KBaFe3+Mg7Si8O22(OH)2F6. Magbasite is orthorhombic, space group Cmme (Cmma), with unit-cell parameters a 18.9506(3) Å, b 22.5045(3) Å, c 5.2780(1) Å, V 2250.93(6) Å3 (Z = 4). The structure has been solved and refined to final agreement indices R 1 = 0.026, wR 2 = 0.052, GooF = 1.116 for a total of 2379 unique
more » ... 9 unique reflections, and is a new kind of trellis motif related to amphibole and carpholite topologies. An amphibole-like I-beam ‖(100) of edge-sharing octahedrally-coordinated M(1,2,3) sites, which are filled by Mg, is sandwiched between double-chains of SiO4 tetrahedra ‖c. This I-beam is connected to side-ribbons ‖(010) of edge-sharing (Mg,Fe2+)O4(OH,F)2 and Fe3+O4(OH)2 octahedra to form a tunnelled box or trellis structure very like that of carpholite, for which the I-beams are pyroxene-like. K occupies a tunnel site analogous to the A site of amphibole. Ba occupies a cavity site at the corners where the I-beam and side-ribbon meet, and corresponds to the A site of carpholite. The structural relations between magbasite and carpholite are discussed.
doi:10.1180/minmag.2014.078.1.03 fatcat:scsbsczcezgk5dpqqky2s7lmla