Gm. Dacosta et al., INFLUENCE OF NONSTOICHIOMETRY AND THE PRESENCE OF MAGHEMITE ON THE MOSSBAUER SPECTRUM OF MAGNETITE, Clays and clay minerals, 43(6), 1995, pp. 656-668
Several samples of large- and small-particle magnetite (Fe3O4), as wel
l as its thermal decomposition products formed at different temperatur
es and atmospheres, have been studied extensively by Mossbauer spectro
scopy (MS), both with and without an applied field of 6T. Synthetic mi
xtures of magnetite and poorly- or well-crystallized maghemite have al
so been studied. Large-particle magnetite (MCD > 200 nm), when heated
in air for 12 hours at T < 400 degrees C, transforms to a mixture of w
ell crystallized hematite and magnetite, the latter one remaining stoi
chiometric, according to the relative area-ratios obtained from MS. Th
ermal treatment at 1300 degrees C in a controlled O-2 partial pressure
, produced a mixture of stoichiometric and nonstoichiometric magnetite
, but the latter component seems to be composed of particles with diff
erent degrees of nonstoichiometry. The Mossbauer spectra of the decomp
osition products at T < 200 degrees C in air of small-particle magneti
te (MCD similar to 80 nm) could be successfully interpreted as a mixtu
re of magnetite and maghemite, rather than nonstoichiometric magnetite
. This suggestion is further supported by the experiments with the syn
thetic mixtures. It is clearly demonstrated that is not possible, even
by applying a strong external field, to separate the contribution of
the A-site of magnetite from that of maghemite.