554 



UNITED STATES MINERAL RESOURCES 



Table 117. — Identified resources ' of the rare earths and some potential rare-earth deposits of the world — Continued 



^Identified resources: Specific, identified mineral deposits that may or may not be evaluated 

 may or may not be profitably recoverable with existing technology and economic conditions. 

 2 Monazite assumed to contain 60 percent and euxenite 15 percent rare-earth oxides. 



to extent and grade, and whose contained minerals 



gether with data on their reserves or resources, 

 where such information is available. For some de- 

 posits the rare-earth tonnage has been derived in- 

 directly from data in the cited references. The total 

 resource potential, more than I8V2 million tons, 

 does not include the resources of rare earths in the 

 sedimentary phosphatic rocks but does include the 

 igneous apatite of the Kola Peninsula. 



Only a fraction of this rare-earth potential can 

 likely be recovered. Some deposits, such as many of 

 the monazite placers in the Southeastern United 

 States, either are too small to be worked or are on 

 land too valuable for other purposes to be mined. 

 Where rare earths are a potential byproduct, as in 

 deposits mined for uranium, niobium, or phosphate, 

 there will be no recovery without suificient demand. 



PROSPECTING TECHNIQUES 



The discovery of rare-earth deposits is likely to 

 be an accidental adjunct to the search for radioac- 

 tive materials. This was true of the Mountain Pass 

 bastnaesite deposit, where a local radioactivity 

 anomaly due to thorite attracted the attention of 

 prospectors. Numerous other rare-earth occurrences, 

 for the most part uneconomic, were found during 

 the period when the use of portable radiation- 

 detecting instruments became commonplace. Many 

 of these discoveries were made because of the radio- 

 activity of monazite, which normally contains sev- 

 eral percent thorium. Bastnaesite, however, is not 

 itself appreciably radioactive, and where it is largely 

 unaccompanied by thorium or uranium minerals, as 

 in the Gallinas Mountains, N. Mex. (Perhac and 

 Heinrich, 1964), radioactivity is of little value in 

 prospecting. 



Lanthanide ions — notably those of neodymium, 

 praseodymium, erbium, and holmium — produce ab- 

 sorption bands in the spectrum of light transmitted 

 by many of the minerals in which they are present. 

 These bands are easily seen with a hand spectro- 



scope or microspectroscope when the mineral is 

 illuminated by a strong source of white light 

 (Adams, 1965). Samples can be examined in the 

 field using sunlight, but they can be examined more 

 effectively in the laboratory. The mineral grains 

 must be sufficiently translucent to transmit some 

 light, as is commonly true of bastnaesite, monazite, 

 and xenotime. The use of this technique in the 

 examination of panned concentrates was suggested 

 by Mertie (1954). 



The recognition of cerium subgroup minerals by 

 the use of unfiltered light from an ultraviolet lamp 

 has been described by Murata and Bastron (1956). 

 The emerald green shown by the minerals is due 

 to the absorption of certain wavelengths of light 

 by neodymium and praseodymium and is a very 

 useful preliminary test for bastnaesite, monazite, 

 and some other nonopaque rare-earth minerals. 



Many rare-earth-bearings materials are not rec- 

 ognizable by mineralogical examination, particularly 

 when these elements are present in carrier minerals 

 such as apatite, sphene, zircon, or thorite. For such 

 rocks some analytical method such as X-ray fluores- 

 cence or emission spectrography (Parker and Ba- 

 roch, 1971) must be used. 



PROBLEMS FOR RESEARCH 



Although known resources of rare earths are 

 large, they undoubtedly can be increased by con- 

 tinued investigation of geologic materials in all 

 parts of the world. Some potential domestic sources 

 are not evaluated, and many others remain to be 

 discovered. Resources in the apatite of phosphorites 

 and iron deposits are impressive but are of little 

 value until economic methods of recovery are de- 

 veloped, and research in this field is needed in 

 anticipation of their eventual use. This is also true 

 for the resources in thorite veins, where the rare 

 earths may be an important coproduct. (See "Thor- 

 ium" in chapter on "Nuclear fuels.") 



