Iron oxidation and order-disorder in the (Fe2+, Mn)(Ta, Nb)2O6 → (Fe2+, Mn)Fe3+ (Ta, Nb)2O8 transition

Heat treatments in air and in vacuum have been performed on crystal and powder of the natural tantalite (Mn0.882+Fe0.092+)(Ta0.865+Nb0.145+)(2)O-6(2-), as well as on powder of synthetic Fe2+(Nb0.65+Ta0.45+)(2)O-6. Crystal parameters and hyperfine interactions were obtained by use of x-ray diffraction and Mossbauer spectroscopy. It is shown that the partially ordered natural sample is completely ordered after heat treatment in vacuum. Conversely, heat treatment in air induced the tantalite [(Mn0.882+Fe0.092+)(T0.865+Nb0.145+)(2)O-6(2-)] --> wodginite [(Mn2+, Fe2+)Fe3+(Ta, Nb)(2)O-8] transformation on the powdered sample. When applied to the crystal sample, the heat treatment in air produced a mixture of two phases: the one in large amount is the ordered (Mn0.882+F0.092+)(Ta0.862+Nb0.145+)(2)O-6(2-), the other, in minor amount, is (Mn-0.88(2+), Fe-0.09(2+))Fe3+(Ta, Nb)(2)O-8. The former arrives from cation ordering in the bulk portion of the sample, while the latter results from the near-surface oxidation. The Mn content as well as the oxidant atmosphere appears to play an important role in the transition mechanism. The same heat treatment applied to the synthetic columbite induces a different reaction: ferrocolumbite is transformed into the ixiolite Fe3+(Nb0.6Ta0.4)O-4 with a minor amount of (Nb, Ta)(2)O-5.