Oxide-sulfide cores

Scheme 5.8 Synthesis of clusters containing iron-oxide-sulfide cores.

The growth mechanism for the IF-MS2 (M = Mo,W) materials by the sulfidi-zation of the respective oxide nanoparticles has been studied in detail (12, 31). The growth mechanism is schematically illustrated in Fig. 4 (31a). Here oxide nanoparticles are sulfidized on the surface at temperatures between 800 and 950 °C in an almost instantaneous reaction. Once the first sulfide layer enfolds the oxide nanoparticle its surface is completely passivated, and hence sintering of the nanoparticles is avoided. In the next step, which may last a few minutes, reduction of the oxide nanoparticle core by hydrogen takes place. In the third step, which is rather slow and may take a few hours, depending on the size of the nanoparticles... 

Oxygen enrichment increases the bed pressure, see Figure 11. According to fluid dynamic evaluations, the bed pressure increase seems to be caused by the increase of the density and grain size of the bed. Many calcine particles were found to have a dense oxidized surface layer on the sulfide core. Figure 12, in the microstructural studies. 

Antimicrobial agents are used where there is a need to inhibit bacterial and fungal growth. The additives can consist of copper, germanium, zinc and zinc compounds, metal oxides or sulfides, metal zeofltes, as well as silver and copper oxide-coated inorganic core particles (154—159) (see Industrial ANTIMICROBIAL AGENTS). 

The planets nearest the Sun have a high-temperature surface while those further away have a low temperature. The temperature depends on the closeness to the Sun, but it also depends on the chemical composition and zone structures of the individual planets and their sizes. In this respect Earth is a somewhat peculiar planet, we do not know whether it is unique or not in that its core has remained very hot, mainly due to gravitic compression and radioactive decay of some unstable isotopes, and loss of core heat has been restricted by a poorly conducting mainly oxide mantle. This heat still contributes very considerably to the overall temperature of the Earth s surface. The hot core, some of it solid, is composed of metals, mainly iron, while the mantle is largely of molten oxidic rocks until the thin surface of solid rocks of many different compositions, such as silicates, sulfides and carbonates, occurs. This is usually called the crust, below the oceans, and forms the continents of today. Water and the atmosphere are reached in further outward succession. We shall describe the relevant chemistry in more detail later here, we are concerned first with the temperature gradient from the interior to the surface (Figure 1.2). The Earth s surface, i.e. the crust, the sea and the atmosphere, is of... 

Note It is sometimes convenient to describe the element distribution between sulfides and oxides as chalcophiles (occurring in the Earth s crust as sulfides) and lithophiles (predominating as oxides and halides in the Earth s crust) (see Fig. 1.5). This geochemical classification includes also the siderophiles (remaining as metals or alloys, especially in the Earth s core) and the atmophiles (which occurs largely in volatile form in the atmosphere and dissolved in the oceans). 

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

ISP contain four basic core structures which have been characterized crystallographically both in model compounds and in ISP (Rao and Holm, 2004). These are (Figure 13.15), respectively, (A) rubedoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom bound to four Cys residues—the iron atom can be in the +2 or +3 valence (B) rhombic two-iron-two-sulfide [Fe2-S2] clusters—typical stable cluster oxidation states are +1 and + 2 (the charges of the coordinating cysteinate residues are not considered) ... 

Hydrothermal alteration is laterally extensive and manifest as Fe-carbonate, Fe-oxide (supergene ), sericite, calcite and minor chlorite. Clay alteration is so pervasive that much of the core disintegrates via clay hydration processes after very short exposure to the atmospheric conditions. Although limited XRD work has not shown the presence of swelling clays sericite which is abundant does have the capacity to swell (Eberl et al. 1987). Additional intense clay alteration is controlled by faulting and is spatially related to sulfide emplacement. 

Preliminary work has shown that mineralization at Williams Brook includes the sulfide phases pyrite and honey-brown sphalerite. Oxide phases including magnetite and hematite are prevalent throughout the core and heavily concentrated in the first 10 m of the drill... 

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