Osmium core composition

Pegram W. J. and Turekian K. K. (1999) The osmium isotopic composition change of Cenozoic sea water as inferred from a deep-sea core corrected for meteoritic contributions. Geochim. Cosmochim. Acta 63(23-24), 4053-4058. 

Figure 5 Marine Os/ Os record for the past 80 My in all oceans from H202-leached metalliferous and hydrogenetic sediments. Note that the pronounced excursion to low ratios at the K/T boundary (65 My) is explained by a meteorite impact. (Reprinted from Geochi mica et Cosmochimica Acta, 63, Pegram WJ, Turekian KK. The osmium isotopic composition change of Cenozoic sea water as inferred from a deep-sea core corrected for meteoritic contributions, 4053-4088, Copyright (1999), with permission from Elsevier Science.)...

In this section mantle evolution curves are presented for neodymium (Nd), hafnium (Hf), lead (Pb) and osmium (Os) isotopes. A summary of these isotopic systems is given in Text Box 3.2. Earlier studies based upon the study of Sr isotopes in the mantle (e.g. Bell et al., 1982) are now known to be unreliable because of its high geochemical mobility (Goldstein, 1988). The significance of the mantle evolution curves described here is that they demonstrate that the mantle does not operate as an isolated system but that it has evolved in its composition over time, in response to core formation, crust extraction, and the recycling of crustal material. 

The size of the osmium-copper clusters of interest in the catalyst considered here is such that the number of metal atoms which could be present in a full surface layer is significantly higher than the number that would be located in the interior core. For a stoichiometry of one copper atom per osmium atom, there are, then, too few copper atoms to form a complete surface layer around the osmium. It should be realized that parameters derived from the EXAFS data on the osmium-copper clusters are average values, since there is very likely a distribution of cluster sizes (9) and compositions in a silica-supported osmium-copper catalyst. 

Phospholipids of the composition present in cells spontaneously form sheetlike phospholipid hilayers, which are two molecules thick. The hydrocarbon chains of the phospholipids in each layer, or leaflet, form a hydrophobic core that is 3-4 nm thick in most biomembranes. Electron microscopy of thin membrane sections stained with osmium tetroxide, which binds strongly to the polar head groups of phospholipids, reveals the bilayer structure (Figure 5-2). A cross section of all single membranes stained with osmium tetroxide looks like a railroad track two thin dark lines (the stain-head group complexes) with a uniform light space of about 2 nm (the hydrophobic tails) between them. 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

This difference in copolymer composition distribution was confirmed by transmission electron microscopy of microtomed sections stained with hydrazine and osmium tetroxide to show much larger butyl acrylate-rich particle cores for the semi-continuous latexes (39). 

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