Crystal structure analysis of wodginite from Bernic Lake, Manitoba

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Elphick, Patricia Margaret
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A crystal structure analysis using densitometer-measured intensities for 190 reflections recorded on an integrated precession instrument, has been carried out for a wodginite from Bernic Lake, Manitoba. The mineral is monoclinic, and the specimen analysed has true cell dimensions a=9.516 Ao, b=11.452 Ao and c=5.112 Ao, B=91o08' and space group C2/c, but it has a strong sub-cell with a'=4.758 Ao, b'=5.726 Ao, c'=5.112 Ao, B=91o08' and space group P2/c, with cell content Z=1x[(Mn+2 0.77 Fe+3 O.25 Sn+4 0.61 Ti+4 0.35 Nb+5 0.04 Ta+5 1.98)O-2 8]. The structure analysis described here is based on the small sub-cell. The starting structure was based on one third of the unit cell of the columbite-tantalite structure (Sturdivant, 1930). The structure was analysed by least squares and Fourier analyses. A final structure, which was similar to that of tantalite apart from some variation in the oxygen positions, was obtained which gave an R factor of 10.5%. In the sub-cell used for the structure analysis all the Ta atoms are ordered into one 2-fold metal site, M1, and the other metal atoms are in the other 2-fold metal site, M2. The eight oxygen atoms are in two 4-fold general positions. Both metal sites are pseudo-octahedrally coordinated by oxygens with M1(Ta)-0 distances from 1.908 to 2.129 Ao, and M2-0 from 1.948 to 2.203 Ao, 0-0 distances around M1(Ta) vary from 2.532 to 2.924 Ao, and around M2 from 2.792 to 2.966 Ao.