Lanthanide abundances minerals

The lanthanide abundance pattern for Shergotty (table 9, fig. 7) is unique among meteorites and indicates a complex prehistory in the Martian mantle from which they appear to have been derived, as basalts, by partial melting. The enriched pattern of the heavy lanthanides (Gd-Lu) resembles that of pyroxenes (the parent rocks appear to have been pyroxene cumulates). It provides no evidence that garnet was a residual phase in the source from which these basalts were derived, for, if so, the reciprocal pattern would be displayed. Leaching experiments show that most of the lanthanides are contained in accessory phases (whitlockite and apatite) rather than in the major mineral phases. 

Garnet has a unique lanthanide abundance pattern among the common rockforming minerals, concentrating the heavy lanthanides (Gd-Lu). Melts which have been in equilibrium with this mineral thus possess eomplementary patterns steeply enriched in the light lanthanides (La-Sm), and depleted in the heavy lanthanides (Gd-Lu). These patterns usually indicate that they have been derived from the mantle, where eclpgite is stable, rather than from the crust. 

Variations both or grain size and mineral density result in separation of minerals and rock fragments during aqueous and aeolian transport of sedimentary material. Such transport may affect lanthanide abundance patterns in the resulting sedimentary rock because of the widely variable patterns in the constituent minerals. The two most important effects are... 

Lanthanide abundances in various grain size fractions (and in accessory heavy minerals) were examined by Cullers et al. (1979) in an attempt to address this problem. They concluded that... 

The potential of heavy minerals to distort lanthanide patterns in sedimentary rocks is well recognised (see table 25 and fig. 42 for some typical lanthanide abundances and patterns in common heavy, or accessory minerals). An example from Archean metaquartzites from the Western Gneiss Terrain, Australia is instructive (Taylor et al. 1986). Enrichments of light lanthanides were observed in... 

Fig. 42. Lanthanide abundance patterns for accessory minerals. Note in particular the high abundances are light lanthanide enrichment of monazite and al-lanite, and the extreme heavy lanthanide enrichment of zircon. (Data are from table 24.)...

The lanthanide abundances in quartz-rich sedimentary rocks (quartzites, orthoquartzites, etc.) are typically very low (table 28, fig. 46). The shape of the pattern, however, is similar to that of typical shales. As discussed above, the role of heavy minerals is more important when sizeable clay fractions are absent (Cullers et al. 1979, Taylor et al. 1986, see also below). The most common effect is to cause enrichment of the heavy lanthanides (Gd-Lu). 

Fig. 46. Lanthanide abundance patterns for aikoses and quartzites. Note the overall lower abundances but amilar patterns to typical shales. Quartzites, with very low abundances and high heavy-mineral concentrations, can exhibit some heavy lanthanide enrichment. (Data are from table 28.)...

Because the lanthanides and yttrium are dispersed elements they are found mainly as trace constituents of common rocks. Within those rocks they occur partly as trace constituents of the major, rock-forming minerals and partly in accessory minerals in which the lanthanides are either essential constituents (e.g., monazite) or are concentrated (e.g., apatite). More than 100 different lanthanide and lanthanide-concentrating minerals are known. Most of these form from liquids that are highly differentiated chemically relative to common magmas. Such liquids are rich in a wide variety of elements that are trace elements in terms of their natural abundances but major constituents of the final dregs of liquid when magmas freeze. 

Lanthanide-concentrating minerals can also be selective, depending on which lanthanides more readily substitute into their structures. The true extent of their selectivity is less easily inferred from compositons of natural minerals than for the lanthanide minerals proper. Their lanthanide abundances tend to reflect those of their parent liquids, which are usually not known. Those minerals that do not show a consistent pattern of domination by light lanthanides, heavy lanthanides, or even middle lanthanides are usually listed as complex. Presumably, whatever selectivity they have is insufficient to overcome variations in composition caused by variations in parent liquid composition. Because many of these minerals form late in a crystallization sequence, the lanthanide distributions of their parents may differ considerably from those of the original magmas or those of the rocks in which the minerals occur. 

Yttrium (j Y) is often confused with another element of the lanthanide series of rare Earths— Ytterbium ( Yb). Also confusing is the fact that the rare-earth elements terbium and erbium were found in the same minerals in the same quarry in Sweden. Yttrium ranks second in abundance of all 16 rare-earth, and Ytterbium ranks 10th. Yttrium is a dark silvery-gray hghtweight metal that, in the form of powder or shavings, will ignite spontaneously. Therefore, it is considered a moderately active rare-earth metal. 

The ores from which rare-earth elements are extracted are monazite, bastnasite, and oxides of yttrium and related fluorocarbonate minerals. These ores are found in South Africa, Australia, South America, India, and in the United States in Cahfomia, Florida, and the Carolinas. Several of the rare-earth elements are also produced as fission by-products during the decay of the radioactive elements uranium and plutonium. The elements of the lanthanide series that have an even atomic number are much more abundant than are those of the series that have an odd atomic number. 

Of all the 17 rare-earths in the lanthanide series, terbium is number 14 in abundance. Terbium can be separated from the minerals xenotime (YPO ) and euxenite, a mixmre of the following (Y, Ca, Er, La, Ce, Y, Th)(Nb, Ta, Ti O ). It is obtained in commercial amount from monazite sand by the ion-exchange process. Monazite may contain as much as 50% rare-earth elements, and about 0.03% of this is terbium. 

Thulium was discovered in 1879 by Cleve and named after Thule, the earliest name for Scandinavia. Its oxide thulia was isolated by James in 1911. Thulium is one of the least abundant lanthanide elements and is found in very small amounts with other rare earths. It occurs in the yttrium-rich minerals xenotime, euxenite, samarskite, gadolinite, loparite, fergusonite, and yttroparisite. Also, it occurs in trace quantities in minerals monazite and... 

The lanthanides (Ln) include lanthanum (La) and the following fourteen elements—Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu— in which the 4f orbitals are progressively filled. These fifteen elements together with scandium (Sc) and yttrium (Y) are termed the rare-earth metals. The designation of rare earths arises from the fact that these elements were first found in rare minerals and were isolated as oxides (called earths in the early literature). In fact, their occurrence in nature is quite abundant, especially in China, as reserves have been estimated to exceed 84 x 106 tons. In a broader sense, even the actinides (the 5f elements) are sometimes included in the rare-earth family. 

The actinides (U, Th, Pu), alkaline earths (Be, Mg, Ca, Sr, Ba), lanthanides (elements La - Lu), Al, and the elements in groups 3b (Sc, Y), 4b (Ti, Zr, Hf), and 5b (V, Nb, Ta) of the periodic table are refractory lithophile elements. The refractory lithophiles are 5% of the total mass of the rock in solar composition material. Aluminum Al, calcium Ca, and titanium Ti are the three most abundant refractory lithophiles, and they form minerals that are the host phases for most of the less abundant refractory lithophile elements such as the actinides, lanthanides, and transition elements in group 5b of the periodic table. Some of the less abundant refractory lithophiles - the group 4b elements Zr, Hf, and the group 3b elements Y and Sc - condense as oxides before any Ca, Al, Ti-bearing minerals form [9], But the rest condense into the more abundant host phases. 

Cerium is the most abundant member of the lanthanide, or rare earth, elements. It has two stable valence states, Ce (cerous) and Ce " (ceric). It is found as a trace element in several minerals, but only two, bastnasite, LnFCOs, and monazite, (Ln, Th)P04 (where Ln = a lanthanide element, such as lanthanum, praseodymium, neodymium, or cerium), which are approximately 30 percent and 22 to 25 percent cerium, respectively, are the principal sources of this element. 

In the earth s crust, thorium is about three times as abundant as uranium. In minerals, thorium is frequently associated with other quadrivalent species (Zr, Hf, Ce, and U), but also with tervalent lanthanides. Table XII-1 gives a list of common thorium... 

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. 

Quantitative modelling has been less successfully applied to rocks cff more felsic composition, such as granodiorites, dacites, granites and rhyolites. This is principally due to the ubiquitous presence in these evolved rocks of minor mineral phases, such as sphene, allanite, apatite and zircon, whose lanthanide contents may account for a substantial fraction of the total rock budget. Thus Gromet and Silver (1983) found that sphene and allanite, in a granodiorite from the Peninsular Ranges, California, contained 80-95% of the lanthanide content of the total rock. Distribution coefficients are not well known for these phases and the abundances of these trace minerals are difficult to determine accurately. 

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