Iridium abundance

In Gubbio, Italy, a 1 cm layer of clay between extensive limestone formations marks the boundary between the Cretaceous and Tertiary Periods. This clay layer was known to have been deposited about 65 million years ago when many life forms became extinct, but the length of time associated with the deposition was not known. In an attempt to measure this time with normally deposited meteoritic material as a clock, extensive measurements of iridium abundances (and those of many other elements) were made on the Gubbio rocks. Neutron activation analysis was the principal tool used in these studies. About 50 elements were searched for in materials like the earth s crust, about 40 were detected and about 30 were measured with useful precision [26-28]2. 

Efforts to find trace-metal evidence of extraterrestrial impact at the Ordovician-Silurian boundary have been unsuccessful (e.g., Orth et al, 1986 Wang et al, 1992). Peaks in iridium abundance at the boundary have been linked to reductions in sedimentation rate the persistent cosmic source of iridium is otherwise diluted by high terrigenous or biogenic sedimentation. 

Wang K., Chatterton B. D. E., Attrep M., Jr., and Orth C. J. (1992) Iridium abundance maxima at the latest Ordovician mass extinction horizon, Yangtze Basin, China, terrestrial or extraterrestrial Geology 20, 39-42. 

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

In contrast to the rhodium process the most abundant iridium species, the catalyst resting state, in the BP process is not the lr(l) iodide, but the product of the oxidative addition of Mel to this complex. 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

Osmium is the 80th most abundant element on Earth. As a metal, it is not found free in nature and is considered a companion metal with iridium. It is also found mixed with platinum- and nickel-bearing ores. It is recovered by treating the concentrated residue of these ores with aqua regia (a mixture of 75% HCl and 25% HNO). The high cost of refining osmium is made economically feasible by also recovering marketable amounts of platinum and nickel. 

Iridium is the 83rd most abundant element and is found mixed with platinum, osmium, and nickel ores. The minerals containing iridium are found in Russia, South Africa, Canada, and Alaska. 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

Osmium occurs in nature, always associated with other platinum group metals. It usually is found in lesser abundance than other noble metals. Its most important mineral is osmiridium (or iridosmine), a naturaUy occurring mineral alloyed with iridium. 

Ruthenium occurs in nature natively, found in minor quantities associated with other platinum metals. Its abundance in the earth s crust is estimated to be 0.001 mg/kg, comparable to that of rhodium and iridium. 

Metallic liquids can also experience fractional crystallization. The abundances of trace elements such as gold, gallium, germanium, and iridium and the major element nickel in various classes of iron meteorites vary because of the separation of crystalline metal phases (kamacite or taenite). 

Cobalt, rhodium and iridium - Most stable complexes of Co111 are low-spin and thus diamagnetic. Together with the 100% natural abundance of the 59Co nucleus, this... 

Calculate the atomic mass of iridium. Iridium has two isotopes. Iridium-191 has a mass of 191.0 amu and a percent abundance of 37.58%. Iridium-191 has a mass of 193.0 amu and a percent abundance of 62.42%. Show all your work. 

As in years past, the published thiophene literature dwarfs the selenophene and tellurophene literature. However, there was a significant increase in the number of interesting selenophene and tellurophene references last year. The preparation of T)5-selenophene complexes of chromium, magnesium, ruthenium and iridium were studied <950M332>. These studies took advantage of the NMR-active selenium isotope 77Se (7.58% natural abundance) as a tool to directly study binding of selenophene derivatives to a catalyst surface. 

Aubrites are depleted in siderophile elements relative to chondritic abundances. Excluding Shallowater, bulk rocks have Cl-normalized Fe/Si ratios of 0.006-0.055. Shallowater, with a higher modal metal content (3.7% versus <0.7% Watters and Prinz, 1979), has a Cl-normalized Fe/Si of 0.33 (Easton, 1985). Trace siderophile elements are also depleted for example, excluding dark samples, iridium varies from —10 X Cl to 10 X Cl. Dark clasts from Khor... 

The refractory component comprises the elements with the highest condensation temperatures. There are two groups of refractory elements the refractory lithophile elements (RLEs)—aluminum, calcium, titanium, beryllium, scandium, vanadium, strontium, yttrium, zirconium, niobium, barium, REE, hafnium, tantalum, thorium, uranium, plutonium—and the refractory siderophile elements (RSEs)—molybdenum, ruthenium, rhodium, tungsten, rhenium, iridium, platinum, osmium. The refractory component accounts for —5% of the total condensible matter. Variations in refractory element abundances of bulk meteorites reflect the incorporation of variable fractions of a refractory aluminum, calcium-rich component. Ratios among refractory lithophile elements are constant in all types of chondritic meteorites, at least to within —5%. 

---