RARE-EARTH ELEMENTS 



549 



rare earths and an importer of the heavy rare 

 earths, although the latter role is one of economics 

 rather than one of necessity, inasmuch as large 

 undeveloped resources of yttrium are present in 

 domestic deposits, particularly in apatite-magnetite 

 bodies and western phosphate rocks. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



The lanthanides and yttrium are lithophilic ele- 

 ments and are most commonly found as phosphates, 

 carbonates, or silicates. In the geologic environment 

 they are commonly trivalent vi'ith the exceptions of 

 cerium, which may be in a Ce+* state, and europium, 

 which may be present as Eu+-. The lanthanides are 

 unique in their electronic arrangement in that with 

 increase in atomic number, electrons are added to 

 an inner (4f ) level rather than to outer levels. This 

 arrangement results in the general chemical similar- 

 ity of elements in the group, but it also gives them 

 some unusual physical properties. One consequence 

 of inner-level backfilling is the so-called lanthanide 

 contraction, a progressive decrease in the ionic 

 radius of the trivalent lanthanide ions from lantha- 

 num to lutetium. Ionic-radius relationships are very 

 important in rare-earth geochemistry in that they 

 are fundamental to natural mechanisms by which 

 the elements in this rather cohesive group may be- 

 come separated from one another. These mechan- 

 isms, which depend on ionic size, basicity differences, 

 coordination effects, solubility characteristics, and 

 stability of complexes, are able, individually or col- 

 lectively, to effect a natural fractionation of the 

 rare earths to a remarkable degree (Adams, 1969). 



The actual abundance and distribution patterns 

 of the lanthanides and yttrium in terrestrial and 

 extraterrestrial materials have been the subject of 

 much recent research facilitated by the use of 

 activation-analysis techniques (Haskin and others, 

 1966; Herrmann, 1970). From the analytical data 

 many attempts have been made to estimate the 

 rare-earth content and distribution both in the 

 whole earth and in its several crustal units, of which 

 the continental crust has the greatest significance 

 when we are considering the resources and geo- 

 chemistry of the elements in the accessible parts of 

 the earth. Taylor's (1964) abundance data for the 

 rare earths in the continental crust are plotted in 

 figure 64 to show their distribution pattern. Judg- 

 ing from Taylor's estimates and those of Lee and 

 Yao (1965), the average rare-earth content of the 

 continental crust is somewhat less than 200 ppm 

 (parts per million). 



60 



5 50 



-I 



UJ 2 



l9 

 H _|40 



Ui c 

 < tt- 



2S20- 



2z 



I- ~ 



D 



? 10 





INCREASING ATOMIC NUMBER — > 



Figure 64. — Crustal abundance of the rare-earth elements. 

 Even-odd pairs of elements are arranged in increasing 

 atomic number along the abscissa of the graph, and 

 abundance, in parts per million, is plotted along the 

 ordinate. The upper curve connects lanthanides of even 

 atomic number; the lower, their odd-numbered neighbors. 

 Yttrium has been arbitrarily placed with dysprosium and 

 holmium because of their similarity in ionic radii. 



The rare-earth content and distribution pattern 

 of the different rock types vary considerably. Has- 

 kin and Frey (1966, p. 310) noted that the absolute 

 rare-earth content of intrusive rocks generally in- 

 creases from ultramafic rocks, through mafic and 

 intermediate rocks, to the silicic granites. Various 

 sedimentary rock types, although they may differ 

 appreciably in their total rare-earth content, show 

 mutually similar distribution patterns, which indi- 

 cates that fractionation is minimal during sedimen- 

 tation. 



Table 115 shows the rare-earth content of various 



Table 115. — Rare-earth content of various rock types 



tFrom Haskin and Frey, 1966] 



Number Rare-earth 



Rock type of samples content (ppm) 



North American shales Composite 235 



of 40. 



Ocean sediments 8 102-271 



Graywackes 5 70-195 



Carbonate-bearing sedimentary 8 16-159 

 rocks. 



Sandstones 5 52-126 



Basalt Composite 174 



Granite (G-1)' ( = ) 334 



Russian granites 3 225-475 



Gabbros 3 28-123 



^ Wesberly, K.I. 



' Number of samples not known. 



