Hafnium abundance

Setser, J. L., and W. D. Ehmann Zirconium and hafnium abundances in meteorites, tektites and terrestrial materials. Geochim. cosmochim. Acta 28, 769-782 (1964). 

Thus, it is possible that all of the three controls are acting on the hafnium contribution to seawater. Despite the low concentrations of neodymium in seawater, the high Nd/Hf ratios in Fe-Mn crusts indicate relatively lower hafnium abundances. Because zircon is not a crystallizing phase in basalts, hafnium is not sequestered in zircon in the oceanic cmst and it may be more available for dissolution due to hydrothermal processes compared to continental rocks. As a result the flux of hafnium from the oceanic crust into seawater, relative to the continental flux, may be higher than for neodymium, making it more visible. 

Titanium, which comprises 0.63% (i.e. 6320 ppm) of the earth s crustal rocks, is a very abundant element (ninth of all elements, second of the transition elements), and, of the transition elements, only Fe, Ti and Mn are more abundant than zirconium (0.016%, 162 ppm). Even hafnium (2.8 ppm) is as common as Cs and Br. 

Table 21.1 summarizes a number of properties of these elements. The difficulties in attaining high purity has led to frequent revision of the estimates of several of these properties. Each element has a number of naturally occurring isotopes and, in the case of zirconium and hafnium, the least abundant of these is radioactive, though with a very long half-life ( Zr, 2.76%, 3.6 x 10 y Hf, 0.162%, 2.0 X 10 5 y). 

Zirconium can be a shiny grayish crystal-Uke hard metal that is strong, ductile, and malleable, or it can be produced as an undifferentiated powder. It is reactive in its pure form. Therefore, it is only found in compounds combined with other elements—mosdy oxygen. Zirconium-40 has many of the same properties and characteristics as does hafhium-72, which is located just below zirconium in group 4 of the periodic table. In fact, they are more similar than any other pairs of elements in that their ions have the same charge (+4) and are of the same general size. Because zirconium is more abundant and its chemistry is better known than hafnium s, scientists extrapolate zirconium s properties for information about hafnium. This also means that one twin contaminates the other, and this makes them difficult to separate. 

Hafnium had lain hidden for untold centuries, not because of its rarity but because of its dose similarity to zirconium (16), and when Professor von Hevesy examined some historic museum specimens of zirconium compounds which had been prepared by Julius Thomsen, C. F. Rammelsberg, A. E. Nordenskjold, J.-C. G. de Marignac, and other experts on the chemistry of zirconium, he found that they contained from 1 to 5 per cent of the new element (26, 27). The latter is far more abundant than silver or gold. Since the earlier chemists were unable to prepare zirconium compounds free from hafnium, the discovery of the new element necessitated a revision of the atomic weight of zirconium (24, 28). Some of the minerals were of nepheline syenitic and some of granitic origin (20). Hafnium and zirconium are so closely related chemically and so closely associated in the mineral realm that their separation is even more difficult than that of niobium (columbium) and tantalum (29). The ratio of hafnium to zirconium is not the same in all minerals. 

The abundances of the elements of the titanium group were compared to those of the zinc group in Table 13-1. It will be recalled that unlike the zinc group metals, which are rare but easily isolated, the titanium group metals are abundant, but purified with difficulty. Note from the (very rough) figures given that titanium is 50 times as abundant as zinc, zirconium is 3000 times as abundant as cadmium, and hafnium 30 times as abundant as mercury. 

Zirconium comprises 0.016% (162 ppm) of the Earth s crast and, as a transition element, is only less abundant than Fe, Ti, and Mu. Hafnium is much less abundant at 2.8 ppm, but is stUl comparable in quantity to Cs and Br. The most important minerals of zirconium are zircon (ZrSi04), which is mostly mined in Australia, South Africa, the USA, and Sri Lanka, and baddeleyite (Z1O2), found mostly in Brazil. The estimated reserves exceed a billion tonnes. Australia and South Africa account for about 80% of zircon mining. All zirconium minerals are contaminated by small quantities of hafnium (0.5-2% of Zr content), but in a few (such as alvite, MSi04 XH2O, M = Hf, Zr, Th) the content of Hf is comparable with that of Zr. The above-mentioned similarities in the chemical behavior of these metals explain their close association in Nature and the similarity of their isolation procedures. 

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%. 

Chondritic relative abundances of strongly incompatible RLEs (lanthanum, niobium, tantalum, uranium, thorium) and their ratios to compatible RLEs in the Earth s mantle are more difficult to test. The smooth and complementary patterns of REEs in the continental crust and the residual depleted mantle are consistent with a bulk REE pattern that is flat, i.e., unfractionated when normalized to chondritic abundances. As mentioned earlier, the isotopic compositions of neodymium and hafnium are consistent with chondritic Sm/Nd and Lu/Hf ratios for bulk Earth. Most authors, however, assume that RLEs occur in chondritic relative abundances in the Earth s mantle. However, the uncertainties of RLE ratios in Cl-meteorites do exceed 10% in some cases (see Table 4) and the uncertainties of the corresponding ratios in the Earth are in same range (Jochum et ai, 1989 W eyer et ai, 2002). Minor differences (even in the percent range) in RLE ratios between the Earth and chondritic meteorites cannot be excluded, with the apparent exception of Sm/Nd and Lu/Hf ratios (Blicher-Toft and Albarede, 1997). 

Moreover, they are chemically similar. Both display very limited solubility in water, and differences in chemical behavior are mainly associated with atomic size. Lutetium and hafnium are also refractory elements during nebula condensation, and thus their relative abundance in the Earth are chondritic, however, at the time of writing the value of the bulk Earth Hf/ Hf ratio and the Lu decay constant are being debated (Table 1). Neodymium- and hafnium-isotope ratios in this paper will be expressed as parts per 10 deviations from the bulk Earth values, viz., as s d and sht- Eor example,... 

Hafnium is a moderately common element in Earth s crust. Its abundance is estimated to be about 5 parts per million. That makes it about as abundant as bromine, uranium, or tin. 

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