Magnetite solid solutions

Fig. 12. The phase relationships in the system Er Oj-FejOj-Fe at 1200°C. The numbers mean values of —log P(Oj) in fields equilibrated with three solid phases. The letters R, P, G and M represent stoichiometric compositions of Er Oj, ErFeOj, Er3Fe Oi2 and Fej04, respectively. Mj means the end-member of the magnetite solid solution with a chemical composition of FCj 95,04. The other letters as in table 5. (Kitayama and Katsura 1976.)...

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

Capobianco, C.H., Hervig, R.L., Drake, M. 1994. Experiments on crystal/liquid partitioning of Ru, Rh and Pd for magnetite and hematite solid solutions crystallised from silicate melt. Chemical Geology, 113, 23-43. 

There are structural analogues of a number of iron oxides in the Fe-H-O system. Under certain conditions, continuous solid solutions exist between the two members of a pair. The magnetite-ulvospinel and the hematite-ilmenite pairs are well-known examples. The principle in going from the Fe oxide to the Ti-containing phase is to replace two Fe by one Fe" and one Ti , thereby increasing the unit cell size. 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

It can be readily understood that the structure of the oxide, from which the reduced catalyst is prepared, plays an important role for the properties of the catalyst. This dependence has been proved experimentally by the influence which the rate of cooling of the oxides of a given catalyst composition shows upon the catalytic properties of the reduced catalyst. This effect can be interpreted by considering that in the reduced catalyst the promoters are distributed all over the surface and that it is, of course, highly important how they are distributed. This distribution cannot be independent of the way in which the promoters are present in the oxidic state, whether in solid solution in the magnetite, as separate crystals or as amorphous glassy layers. 

Wyckoff and Crittenden (7), using the Debye-Scherrer technique, have investigated FesCh, and Fe304 promoted with FeOAUOa and with K2OAI2O3. Their results indicate that the promoters do not produce separate phases, but form solid solution with magnetite. 

A THE OXIDATION BOUNDARY FOR MAGNETITE-ZINC FERRITE SOLID SOLUTIONS... 

Subsequent calculation of the high-temperature equilibrium of magnetite with wiistite, taking into account the values of the activity of the components of the solid solutions and the nonstoichiometry of the phases (Kurepin, 1975), made it possible to obtain the following equilibrium ... 

Mechanism of the Promoter Effect. The action of the so-called structural promoters (stabilizers), such as A1203, is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron - aluminum spinels [33], [289]-[291]. The solid solutions of Fe304 and the spinel FeAl204 have a miscibility gap below 850 °C... 

The process(es) by which the precursor(s) of magnetite in these iron formations precipitated is not well understood. A possible explanation involves the oxidation of Fe by reaction with seawater to produce Fe and H2 followed by the precipitation of green rust —a solid solution of Fe(OH)2 and Fe(OH)3—and finally by the dehydration of green rust to magnetite (but see below). Figure 4 shows that the boundary between the stabihty field of Fe(OH)2 and amorphous Fe(OH)3 is at a rather low value of pe. The field of green rust probably straddles this boundary. Saturation of solutions with sideiite along this... 

Oxidation Boundary for Magnetite-Zinc Ferrite Solid Solutions... 

The solid solution (ss) on the left has been represented as a mixture consisting of 1 — X moles of magnetite and x moles of zinc ferrite per formula unit of ss. It is assumed that the zinc ferrite moiety remains passive while any changes relating to the oxygen/cation composition occur exclusively in the magnetite component. 

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