Chromium abundance

Wide variations of 5°Cr/54Cr are expected within presolar grains, and perhaps also in the ratio 5°Cr/52Cr but the small chromium abundance within the grains has rendered the measurement impractical so far. Also, the 5°Cr/52Cr ratio is often assumed to be normal in order that corrections for mass-dependent fractionation can be made. Such normalization suppresses manifestvariability in 5°Cr but of course itcan be recovered from the data if one selects some other assumption about the mass fractionation. 

Liang Y. and Elthon D. (1990) Evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts. Nature 343, 551-553. 

D(Ti) varies between 0.5 and 2.8, rather wildly. The chromium abundance is 8 times (0.9 unit) higher in the Sun than that of titanium. D(Cr) varies between... 

Carbon steel is an alloy of iron with small amounts of Mn, S, P, and Si. Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, etc. Iron is a cheap, abundant, useful, and important metal. 

The most abundant natural steroid is cholesterol. It can be obtained in large quantides from wool fat (15%) or from brain or spinal chord tissues of fat stock (2-4%) by extraction with chlorinated hydrocarbons. Its saturated side-chain can be removed by chromium trioxide oxidation, but the yield of such reactions could never be raised above 8% (see page 118f.). 

Zirconium occurs naturally as a siUcate in zircon [1490-68-2] the oxide baddeleyite [12036-23-6] and in other oxide compounds. Zircon is an almost ubiquitous mineral, occurring ia granular limestone, gneiss, syenite, granite, sandstone, and many other minerals, albeit in small proportion, so that zircon is widely distributed in the earth s cmst. The average concentration of zirconium ia the earth s cmst is estimated at 220 ppm, about the same abundance as barium (250 ppm) and chromium (200 ppm) (2). 

Chromium [7440-47-3] Cr, also loosely called chrome, is the twenty-first element in relative abundance with respect to the earth s cmst, ranking with V,... 

Zn, Ni, Cu, and W, yet is the seventh most abundant element overall because Cr is concentrated in the earth s core and mantle (1,2). It has atomic number 24 and belongs to Group 6 (VIB) of the Periodic Table and is positioned between vanadium and manganese. Other Group 6 members are molybdenum and tungsten. On a toimage basis, chromium ranks fourth among the metals and thirteenth of aU mineral commodities in commercial production. 

Titanium-Based Casting and Wrought Alloys. Titanium-based alloys offer an attractive alternative to gold alloys and to the base-metal alloys that contain nickel or chromium. On a volume basis the cost of titanium is roughly comparable to that of the chromium-containing alloys, but the price of titanium tends to be more stable because its ores are abundant and widely distributed (see Titaniumand titanium alloys). 

Chromium, 122 ppm of the earth s crustal rocks, is comparable in abundance with vanadium (136 ppm) and chlorine (126 ppm), but molybdenum and tungsten (both 1.2 ppm) are much rarer (cf. Ho 1.4 ppm, Tb 1.2 ppm), and the concentration in their ores is low. The only ore of chromium of any commercial importance is chromite, FeCr204, which is produced principally in southern Africa (where 96% of the known reserves are located), the former Soviet Union and the Philippines. Other less plentiful sources are crocoite, PbCr04, and chrome ochre, Cr203, while the gemstones emerald and ruby owe their colours to traces of chromium (pp. 107, 242). 

Chromium (atomic mass = 51.9961 amu) has four isotopes. Their masses are 49.94605 amu, 51.94051 amu, 52.94065 amu, and 53.93888 amu. The first two isotopes have a total abundance of87.87%, and the last isotope has an abundance of2365%. What is the abundance of die third isotope Estimate the abundances of die first two isotopes. 

C02-0084. The pie chart in Figure 2-17 shows the isotopic abundances for chromium. Sketch the mass spectrum of this eiement. 

Uranium is not a very rare element. It is widely disseminated in nature with estimates of its average abundance in the Earth s crust varying from 2 to 4 ppm, close to that of molybdenum, tungsten, arsenic, and beryllium, but richer than such metals as bismuth, cadmium, mercury, and silver its crustal abundance is 2.7 ppm. The economically usable tenor of uranium ore deposits is about 0.2%, and hence the concentration factor needed to form economic ore deposits is about 750. In contrast, the enrichment factors needed to form usable ore deposits of common metals such as lead and chromium are as high as 3125 and 1750, respectively. 

In this method, chromium is extracted and preconcentrated from seawater with trifluoroacetylacetone [H(tfa)] which complexes with trivalent but not hexavalent chromium. Chromium reacts with trifluoroacetylacetone in a 1 3 ratio to form an octahedral complex, Cr(tfa)3. The isotopic abundance of its most abundant mass fragment, Cr(tfa)2 was monitored by a quadrupole mass spectrometer. 

Representatives of this phylum have been used extensively as indicators of stressed environments. Population structure and species diversity of free-living nematodes inhabiting sediments in the New York Bight were moderately influenced by the heavy-metal content of sands. In medium-grained sands, species diversity was inversely correlated with increased concentrations of chromium and other metals. Sands containing 3.0 to 21.5 mg Cr/kg were also marked by high relative abundances of one or two nematode species the tolerance of these species to chromium stress probably exceeded that of the normal nematode inhabitants of such sediments (Tietjen 1980). 

Occurrence. The ore of Cr of higher commercial importance is chromite (FeCr204). Other minerals are crocoite PbCr04 and chrome ochre Cr203. About 2% Cr in emerald Be3Al2Si6018 is the source of its green colour. Chromium is comparable in abundance in the earth s crustal rocks with V and Cl. 

Papanastassiou DA (1986) Chromium isotopic anomalies in the Allende meteorite. Astrophys J 308 L27-L30 Papanastassiou DA, Wasserburg GJ (1969) Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects. Earth Planet Sci Lett 5 361-376 Papanastassiou DA, Wasserburg GJ (1978) Strontium isotopic anomalies in the Allende meteorite. Geophys Res Lett 5 595-598... 

Sheilds WR, Murphy TJ, Catanzaro EJ, Garner J (1966) Absolute isotopic abundance ratios and the atomic weight of a reference sample of chromium. J Res Natl Bur Standards 70A(2) 193-197 Shukolyukov A, Lugmair GW (1998) Isotopic evidence for the Cretaceous-Tertiary impactor and its type. Science 282(5390) 927-929... 

Chromium is the 21st most common element found in the Earths crust, and chromium oxide (Cr Oj) is the 10th most abundant of the oxide compounds found on Earth. It is not found in a free metallic state. 

Chromium has 4 stable isotopes with the following abundances (Rosman and Taylor 1998)... 

SNII events alone explain the observed solar abundance distribution between oxygen and chromium. This can be taken as a major theoretical achievement. Complementary sources of hydrogen, helium, lithium, beryllium, boron, carbon and nitrogen are required, and these have been identified. They are the Big Bang, cosmic rays and intermediate-mass stars. Around iron and a little beyond, we must invoke a contribution from type la supernovas (Pig. 8.5). These must be included to reproduce the evolution of iron abundances, a fact which suggests... 

The dominant nuclear species resulting from processes at temperatures between 4 and 6 billion k include titanium-44, chromium-48, rron-52 and 53, nickel-56 and 57 and zinc-58, 60, 61 and 62. Isotopic abundances resulting from radioactive decay of these nuclei are compatible with terrestrial and meteoritic measurements relating to calcium-44, titanium-48, chromium-52 and 53, iron-56 and 57, and nickel-58, 60, 61 and 62. 

Chromium occurs in the minerals chromite, Fe0 Cr203 and crocoite, PbCr04. The element is never found free in nature. Its abundance in earth s crust is estimated in the range 0.01% and its concentration in sea water is 0.3 qg/L. The element was discovered by Vaquelin in 1797. 

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