Iron oxidation and order-disorder in the (Fe2+, Mn)(Ta, Nb)(2)O-6 -> (Fe2+, Mn)Fe3+ (Ta, Nb)(2)O-8 transition

Heat treatments in air and in vacuum have been performed on crystal and pow der 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 par ameters and hyperfine interactions were obtained by use of x-ray diffractio n and Mossbauer spectroscopy. It is shown that the partially ordered natura l 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] transformat ion on the powdered sample. When applied to the crystal sample, the heat tr eatment 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 m inor amount, is (Mn-0.88(2+), Fe-0.09(2+))Fe3+(Ta, Nb)(2)O-8. The former ar rives from cation ordering in the bulk portion of the sample, while the lat ter results from the near-surface oxidation. The Mn content as well as the oxidant atmosphere appears to play an important role in the transition mech anism. The same heat treatment applied to the synthetic columbite induces a different reaction: ferrocolumbite is transformed into the ixiolite Fe3+(N b0.6Ta0.4)O-4 with a minor amount of (Nb, Ta)(2)O-5.