Abstract:
Wodginite (Ta, NB, Mn, Fe, Sn, Ti) 2 0 4, mangano-tantalite (Mn, Fe) (Ta, Nb, Ti, Sn) 2 0 6 and pseudo-ixiolite (Ta, Nb, Mn, Fe, Sn, Ti) 2 0 4 are the three major Ta-oxides occurring in the Tanco pegmatite at Bernic Lake, Manitoba, wodginite being by far the most abundant. Microlite Ca2 Ta2, O6 (O, OH,F) and tapiolite (Fe, Mn) (Ta, Nb) 2 0 6 occur in minor amounts. A detailed crystallographic study was carried out on the three major Ta-oxides which are closely related crystallographically. Orthorhombic pseudo-ixiolite has the simplest cell (a=4.76, b=5.75, c=5.16 *A). Upon heating to 1000*C for several hours over a range of oxygen fugacities, the pseudo-ixiolites are converted to orthorhombic tantalites with the a-period three times that of pseudo-ixiolite and with a small increase in comparative a-dimension, with b remaining constant and c decreasing slightly. This change from pseudo-ixiolite to tantalite is considered to result from disordering-ordering of the cations by x-ray photographs show that the two phases are not completely disordered or ordered... The wodginites generally have lower Mn and higher Fe, Sn and Ti contents, and higher Ta/NB ratios than either tantalite or pseudo-ixiolite... The Ta-minerals occur in distinct zones within the pegmatite. The pseudo-ixiolite occurs in the quartz-plagioclase-K-feldspar-spodumene-muscovite zone and wodginite occurs in the K-feldspar-muscovite-plagioclase-quartz-beryl zone just inside the pseudo-ixiolite bearing zone. The albitic aplite contains both tantalite and wodginite. The zoning of these Ta-minerals within the pegmatite is discussed in relation to the paragenesis of the body.
