Lanthanides relative abundances

Levinson (1966) has discussed the problems of nomenclature. The mineralogists propensity for sticking names on complex natural chemical phases (minerals) that give no clue whatsoever as to the nature of those phases runs amuck with lanthanide minerals. In those, a simple shift in lanthanide relative abundances changes the phase from that of mainly one chemical element to that of mainly another, with irresistible possibilities for a new and unrelated mineral name. 

The earth s crust is again a good source of lanthanides. Although the name rare earths is still used to denote the lanthanide elements, and scandium and yttrium, in the strictest sense of the word rare they are more plentiful than many of our common elements. It comes as a surprise to many people when a comparison of the relative abundance of the lanthanides and other elements in the earth s crust is made. Table 4... 

Promethium is a typical element of the lanthanide series. The relative abundances of the lanthanides are plotted in Fig. 14.1 as a function of the atomic number. This figure illustrates Harkin s rule the abundance of elements with even atomic numbers is appreciably higher than that of elements with odd atomic numbers. For element 61 the natural abundance is zero. 

Figure 14.1. Relative abundances of the lanthanides (after V. M. Goldschmidt).

Lanthanides have historically been called "rare earths" because it was originally believed that these elements were sporadically distributed in nature. Actually, they are relatively abundant in the Earth s crust, even... 

Masuda A.,-1962, Regularities in variation of relative abundances of lanthanide elements and an attempt to analyse separation-index patterns of some minerals. J. Earth Set. Nagoya f/nm, 10, 173-187. 

Despite the. similarities in the chemical reactivity of elements in the lanthanide series, their abundances in Earth s crust vary by two orders of magnitude. This graph shows the relative abundance as a function of atomic number. How do you explain the sawtooth variation across the series ... 

Fig. 2. (a) Raw lanthanide abundance data for Australian shales and Cl chondritic meteorites, showing the inherently higher concentrations of even-numbered elements (the Oddo—Harkins effect, due to the greater stability of even-numbered nuclides), (b) The lanthanide pattern resulting from normalising the Australian shale abundance data to the Cl chondritic values. This normalisation illustrates both the relative abundance and fractionation of the lanthanides compared to values typical of the primordial solar nebula. (Data are from table 4.) ... 

The second source of information about the early solar nebula comes from the primitive meteorites, which provide ages of 4.55 Ae. Although many classes of meteorites show elemental fractionations, the Type 1 carbonaceous chondrites (or Cl, where I = Ivuna, the type example of this class of meteorites) have a composition (excluding the volatile elements by which are meant in this context H, C, N, O and the rare gases) which is close to that of the relative abundances, normalised to Si, derived from the solar spectra. Lanthanide data are given in table 6. A comparison of the solar and meteoritic data is plotted in fig. 3 which shows the close correspondence between the two sets. This similarity is one of the pieces of evidence that we are dealing with the overall composition of the original solar... 

In this section, we discuss the question of the bulk planetary abundances of the rare earth elements. Central to the problem of planetary abundance determinations is the assumption that the composition of the original solar nebula, for the non-gaseous elements, is given by the composition of the Cl meteorites. It is accordingly of interest to see what evidence is available from the planets, and how it relates to the primordial nebula values. In the previous section, we have seen that although the moon is enriched in the lanthanides relative to those in the primordial solar nebula by about 2.5 times, the pattern is probably parallel to that of Cl. The evidence for an apparent depletion in the heavy lanthanides is readily explicable as a consequence of early lunar magma ocean crystallisation of phases such as olivine and orthopyroxene, which selectively accept Gd-Lu. 

Lanthanide abundances in natural waters are extremely low (table 19, fig. 33). This observation is well illustrated by Haskin et al. (1966b), who calculated that the entire mass of lanthanides in the oceans is equivalent to that in about a 0.2 mm thickness of sediment of the same areal extent. The lanthanide patterns of normal ocean waters are significantly enriched in the heavy lanthanides relative to the light lanthanides, when compared to terrigenous sedimentary rocks. Ocean waters are relatively depleted in Ce a reflection of preferential incorporation of this element in... 

Fig. 44. Lanthanide abundance patterns in Australian shales ranging in geological age from mid-Proterozoic to Triassic. There is no change in the relative abundance patterns over a period of about 1.5 billion years. (See table 26 for sample details.)...

The rare earths are not really rare in nature. Cerium is reported to be more abundant in the earth s crust than lead and tin, and even the rarer elements, europium and lutetium are much more abundant than the platinum group elements. Except for scandium, these rare earths have never been found in nature as individual rare earths, but wherever they are found, they occur as mixtures of these elements in some combined form. The relative abundance of the individual rare earths can, however, vary considerably in these mixtures, depending on where they are found. In general, the even atomic numbered elements are from three to ten times as abundant as the odd numbered adjacent elements in the lanthanide series, and in the earth s crust, the light (lower atomic number) lanthanides are more abundant than the heavies. 

Rare-earth elements, in contrast to their historical name, are relatively abundant in the Earth s crust, and they occur in many economically viable ore deposits throughout the world with estimated worldwide reserves of 110 million tonnes. For instance, cerium (Ce), which is the most abundant rare earth, has a relative abundance of 66.5 mg/kg, similar to that of zinc, while thuhum (Tm), which is the least abundant, has a relative abundance of 0.52 mg/kg, greater than that of cadmium and silver. The abundance of lanthanides in nature shows an even-odd alteration with atomic number. As a general rule, owing to their extremely similar chemical properties, especially valences and ionic radii, geochemical processes often concentrate these elements in the same minerals, where elements are intimately mixed, and therefore they always occur in the same ore deposits. Nevertheless, owing to its smaller atomic and ionic size, scandium only occurs in rare-earth ores in minor amounts. 

Relative abundances of the isotopes of the polyisotopic lanthanides (and barium, hafnium). 

Lanthanides, especially cerium, fulfil the basic requirements for alternative corrosion inhibitors the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors they have a low toxicity and are relatively abundant in nature. Cerium has a high afimity for oxygen and the bond between cerium and oxygen is unlikely to be broken under the potentials applied. For some aluminium alloys, cerium precipitation from aqueous solutions of cerium salts was observed on cathodic intermetalhc compounds and in some instances, the oxide covered the entire specimen surface [14-19]. 

Of all the materials sampled in the laboratory, the class of meteorites called chondrites is believed to come the closest to retaining the nonvolatile elements of the solar system in their primitive relative abundances. If the processes that formed those meteorites did not appreciably fractionate the nonvolatile elements, then surely they did not separate yttrium and the members of the lanthanide series from each other. Thus, from analyses of chondritic meteorites, the relative elemental abundances of Y and the lanthanides in the solar system are known to a high degree of confidence. 

Special (and somewhat rare) classes of stars have much higher abundances of the lanthanides in their atmospheres than does the sun, evidence of unusual stellar processes. On the whole, most stellar matter appears to have relative abundances of heavy elements similar to those of the sun (e.g.. Unsold, 1969). This suggests that statistical aspects have overcome the contributions of individual stars to the overall evolution of interstellar gas composition. Alternatively, current ideas about the origin of matter may be incorrect the universe... 

A main feature of the lanthanide distribution in these materials is a substantial depletion in Ce. Excess Ce is found in authigenic ferromanganese nodules (e.g., Goldberg et al., 1963 Ehrlich, 1968 Glasby, 1972-73). Presumably, the selective uptake of Ce by these common oceanic materials accounts for the relative deficiency of that element in ocean water. Concentrations of lanthanides in most biogenic and authigenic oceanic materials are relatively low, and the proportions of those materials in common ocean sediments are low, so their relative abundance distributions do not appreciably affect the overall abundances for the sediments that contain them. 

The close similarity of the lanthanide distributions in ocean floor volcanics to that of the chondrites is further evidence that Earth has the same overall average relative lanthanide abundances as the chondrites. Otherwise, the uniformity of these melt products of the mantle found in all the oceans of the world would seem to be fortuitous. The distribution is not unmodified from that of the chondrites, and the lavas are not primitive or first-generation melting products of a primitive terrestrial mantle. Variations in lanthanide concentrations and relative abundances among ocean floor basalts are mainly the result of minor inhomogeneities in the mantle source regions, small differences in conditions of partial melting, and crystal fractionation of the lavas prior to eruption. 

The higher lanthanide concentrations and the light-lanthanide enrichment of average continental material relative to chondrites has been attributed to extraction of these elements from the mantle. A residue is left behind, whose average lanthanide distribution must complement that of the continental crust, assuming that the overall relative lanthanide abundances for the Earth are the same as those in the chondrites. The ocean floor igneous suite appears to be derived from that residue. Are there rocks that can plausibly be considered samples of that residue, and which have distributions deficient in light lanthanides relative to the chondrites Are there rocks that represent primitive terrestrial mantle from which the lanthanides and other incompatible elements have not yet been extracted ... 

The differences in lanthanide distributions between such ultramafic accumulates and their parent liquids emphasize the difficulties in inferring genetic relationships between some types of related materials just from lanthanide distributions alone. Frey et al. (1971) noted that monomineralic ultramafic rocks (e.g., nearly pure olivine or pyroxene) seemed to have more fractionated relative abundances than rocks with several minerals, for which mineral selectivities for certain lanthanides in many cases tend to balance each other. Garmann et al. (1975) reported lanthanide concentrations for three dunites (nearly pure olivine rocks) all three had distributions somewhat enriched in lighter lanthanides, similar to that from the Muskox intrusion (Frey et al., 1971). 

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