Iron-Deficient Magnetite

Since the spinel phase must be prepared at low temperatures (by hydrothermal synthesis or by careful oxidation of magnetite at a temperature T 300 C, for example) it has been widely suspected that some incorporation of hydrogen is needed to stabilize it. However, Schrader and Buttner have shown that pure y-Fe203 does exist, and Coey et al. have been able to prepare Fe3 j04 in the compositional range 0 x 0.08 by quenching non-stoichiometric magnetite prepared at 1450 °C. There is no evidence that hydrogen is needed to stabilize the system. 

The introduction of Fca-atom vacancies modifies the energy diagram of Fig. 3. The preservation of local charge neutrality requires that each Fea-atom vacancy have five of its six nearest neighbors as Fee ions, which means that the ai(, ) states of five Fea ions neighboring a vacancy, , are raised an energy Ea above the Fermi energy. They become acceptor states inaccessible to the mobile electrons at low temperatures. It is possible to express this situation with the structural formula 

As pointed out by Coey et al. , such a description can account well for two observations  

The significant point is that the bulk Fea matrix seen by the mobile ai(J,) electrons retains a valence ratio Fea /Fca = 1. As the volume of this matrix decreases, so does Ty, but the basic electrostatic Coulomb repulsions within the matrix remain the same, so the electron-phonon coupling may retain its essential character. The initial analysis of the room- 

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In one scenario,the dissolution of magnetite entails formation of an unstable iron-deficient oxide phase produced by the more rapid migration of iron ions out of the oxide matrix the reaction rate is governed by decomposition of this peroxide-like species. Gorichev et al. ° determined a value for j of 2.2 0.2 in HaS04, but it was apparent that the fit of the data was poor. The Erofeev reaction does not appear to be suitable for the treatment of data prior to the induction period. 

If there is no constant influx of fluid of a certain composition, decomposition of magnetite ceases. The limiting case is a dry system closed to CO2. By analogy with systems closed to water, in such a system with constant pressure P — Pf = const) the fluid phase disappears entirely, and the Mgt + Sid + Hem association (system Fe-C-O) becomes bivariant and can exist stably below the P-T curve (see Fig. 77) in the stability field of the Sid -1- Hem (+ fluid) association. From these considerations the Mgt -I- Sid + Hem association cannot be used to judge the low-temperature limit of mineral formation the upper limit is fixed quite definitely inasmuch as removal of CO2 begins at P P and the reaction proceeds irreversibly to the right. The extensive occurrence of magnetite in oxide-carbonate iron-formations of low-rank metamorphism apparently indicates the absence of equilibrium or even a deficiency of COj and special dry conditions. 

Wustite is a nonstoichiometric microheterogeneous solid. Its idealized structure is rather similar to the magnetite structure. Both lattices are built from a cubic, close packed, oxygen lattice with the iron ions located only in octahedral voids for wustite, and in both octahedral and tetrahedral voids for magnetite. Wustite is thermodynamically metastable below 833 K but can be easily supercooled to room temperature. The composition of the material is variable and always deficient in iron Fei j,0 with x varying from 4.5% to 11%. Electroneutrality is preserved by the presence of Fe ions. These ions and the holes in the iron sublattice are not statistically distributed throughout the material but form a variety of clusters centered around the ferric ions placed in tetrahedral interstitial sites. ... 

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