Sulphide spinels

These general remarks will now be illustrated by reference to individual oxide spinel phases. Sulphide spinels are discussed on p. 285. 

Few experimental data exist for the free-energy composition relationships in mixed-oxide or -sulphide solutions, for example (Fe,Cr)304 or (Fe,Cr)i cS i . so it is necessary to use various solution models to estimate these data. Furthermore, few data exist for the free energies of formation of compounds of fixed stoichiometry, such as the sulphide spinel FeCr2S4, and these must be estimated. 

A very great number of (mainly oxides or sulphides) have a spinel-type... 

Wall V. I and England R. N. (1979). Zn-Ee spinel-silicate-sulphide reactions as sensors of metamorphic intensive variables and process. Geol Soc. Amer. Abstr. Progr, 11 534. 

Kovalenko V. 1., Solovova I. P., Ryabchikov 1. D., Ionov D. A., and Bogatikov O. A. (1987) Fluidized C02-sulphide-silicate media as agents of mantle metasomatism and megacrysts formation evidence from a large druse in a spinel-lherzolite xenolith. Phys. Earth Planet. Int. 45, 280-293. 

Some sulphides with structures closely related to the spinel structure are noted on p. 618.)... 

The sulphides of iron include Fe3S4 (spinel structure), Fe7Sg (pyrrhotite), FeS (troilite), and FeS2 (pyrites and marcasite). Ferrous sulphide rarely has the Fe S ratio precisely equal to unity, though stoichiometric FeS can be prepared. A microcrystalline form prepared by precipitation has been examined by X-ray powder photography and also by electron diffraction. This form (mackinawite) has... 

The spinel structure is found among a very large number of oxides AB2X and also in a limited number of sulphides, selenides, fluorides and cyanides of the same composition, some of which are recorded in table 8.09. The cubic unit cell of this structure is shown in fig. 8.21 and contains 32 X ions. Each A ion is tetrahedrally co-ordinated by four and each B ion is octahedrally co-ordinated by six X neighbours, and each X ion is bound to one A and to three B ions. The co-ordination may therefore be summarized thus ... 

For all these reasons an increase in chromium concentration may be the best solution to increase the sulphidation resistance of iron-aluminium alloys. The present work has shown that chromium has a very beneficial effect on the sulphidation resistance in oxygen deficient gases. Such improvement of sulphidation resistance in H2S environments has also been found in nickel-aluminium alloys, where the corrosion rate decreased significantly, when 8 wt% chromium were added [22], Whether chromium increases the rate of alumina formation or contributes to the formation of protective spinell type oxides Fe(Cr,Al)203 cannot be decided from the available results. It is also possible that chromium catalyses the transition from a less to a more protective type of A1203. It was for example found that chromium enhances the transition from 0-Al2O3 to a-Al203 in Ni-Al at temperatures above 1000°C [30],... 

In summary, the elements Cu and Zn may be quite mobile during metamorphism and alteration and concentrations may diverge from expected smooth patterns. Anomalies in Ni and Cr concentrations may reflect the role of olivine (Ni) and clinopyroxene or spinel (Cr). Ni and Cu can also be concentrated into sulphide melts. Ti anomalies indicate the role of Fe-Ti oxides. 

The most important spinels from the magnetic point of view are the oxides 2,3. Some of the sulphides also have interesting magnetic properties... 

It has been found that a number of compounds exist which do not rigidly obey the law of fixed proportions. In the spinel group of compounds, of which MgAlgO is the type, certain constituents of the lattice may be lost but the lattice still retains its structure. Spinel itself may undergo the continuous transition MgAl204M 2A1q/304 (y-alumina). The excess of one component is due either to the existence of gaps in the lattice where that component should be, or to an interstitial excess-of another component. The behaviour is shown by certain oxides, sulphides and halides. 

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