RARE-EARTH ELEMENTS 



551 



monazite are in placers, but a few have been found 

 in carbonatites and veins. Monazite, in addition to 

 having been a major source of rare earths, has been 

 the principal source of thorium. Its ThO, content is 

 variable but is commonly in the 5-10 percent range. 

 Although dominantly a mineral of the light rare 

 earths, monazite generally has a higher content of 

 yttrium and the heavy lanthanides than does bast- 

 naesite. 



Bastnaesite is a relatively rare mineral, and its 

 use as an ore is largely the result of its concentra- 

 tions in a very few large deposits, the most im- 

 portant of which is the carbonatite body at Moun- 

 tain Pass, Calif. (Olson and others, 1954). Bast- 

 naesite may also occur in veins, contact-metamor- 

 phic zones, and pegmatites, and as a rare accessory 

 mineral in igneous rocks. 



Xenotime, the yttrium phosphate, is found in the 

 same environments as monazite, but is usually much 

 less abundant. 



Apatite, though not a rare-earth mineral, may 

 incorporate these elements through substitution, 

 commonly to the extent of less than half a percent 

 of the total oxides but occasionally much higher. 

 The light rare earths are usually dominant, al- 

 though some apatites contain yttrium in excess of 

 cerium (Cruft, 1966). The relationships between 

 the rare-earth distribution in apatite and its geo- 

 logic environment have been discussed by Fleischer 

 and Altschuler (1969). Apatite is important as a 

 potential source of rare earths, inasmuch as millions 

 of tons of apatite-bearing rock are processed annu- 

 ally for the production of fertilizer materials. 



A large proportion of rare-earth minerals is 

 known only from minor occurrences, chiefly in peg- 

 matites, but almost any one of these minerals may 

 become important through the discovery of some 

 new or long-unrecognized deposit. 



TYPES OF DEPOSITS 



Primary concentrations of rare-earth-bearing 

 minerals are found in a wide variety of geologic 

 environments, such as veins, gneisses and migma- 

 tites, skarns, pegmatites, and alkalic rock complexes 

 and related carbonatites. Only a very few of these 

 primary deposits have been mined successfully for 

 rare earths alone. Notable exceptions are the vein 

 deposits in Burundi in Central Africa, which pro- 

 duced more than 2,300 tons of bastnaesite between 

 1947 and 1956, and the bastnaesite deposit in car- 

 bonatite at Mountain Pass, Calif., which is probably 

 the largest known rare-earth concentration. Some 

 primary deposits have been exploited for other com- 

 modities, with rare earths as a byproduct. These 



include the monazite-apatite vein at Steenskamps- 

 kraal in the Republic of South Africa (Pike, 1958), 

 mined primarily for thorium, and apatite deposits 

 in the U.S.S.R. and Finland, where rare earths are 

 recovered from apatite processed for its phosphate 

 content. 



Unconsolidated secondary deposits — including sea- 

 beach placers, river valley (fluviatile) placers, and 

 deltaic deposits — represent by far the largest num- 

 ber of minable concentrations of rare-earth minerals 

 and include many of the world's important sources. 

 Monazite is the most common rare-earth mineral 

 found in the placers, where it is invariably asso- 

 ciated with other stable heavy minerals, such as 

 zircon, garnet, magnetite, ilmenite, rutile, sphene, 

 and xenotime. Sea-beach placers, notably those of 

 Brazil and the west coast of India, have been the 

 largest monazite producers. In the United States, 

 fluviatile placers in the Southeastern States and in 

 Idaho have been mined for monazite. A deltaic de- 

 posit at the mouth of the Nile has been worked 

 sporadically. Some placers may be mined for mona- 

 zite alone, but in many operations monazite is a 

 byproduct or coproduct of placers that are worked 

 primarily for zircon or for the titanium minerals 

 ilmenite or rutile. Rare-earth minerals other than 

 monazite generally are not recovered from placers. 

 In the period 1956-59, however, the multiple-oxide 

 mineral euxenite was mined, together with monazite 

 and other heavy minerals, from fluviatile placers at 

 Bear Valley, Idaho, primarily for niobium, but with 

 an important amount of yttrium and heavy lan- 

 thanides as byproducts. Recovery of xenotime, an 

 yttrium phosphate mineral, has recently been initi- 

 ated as a byproduct of the tin placer mining in 

 Malaysia. 



Most consolidated sedimentary rocks have a very 

 low rare-earth content, but with some important 

 exceptions — phosphorites, fossil placers, and certain 

 ancient conglomerates. The phosphorites, such as 

 units in the Phosphoria Formation in the Western 

 United States, represent vast resources of rare 

 earths that are contained largely in apatite. Fossil 

 placers — for example, those found in Upper Cre- 

 taceous sandstones in several Western States — com- 

 monly contain monazite, together with ilmenite, zir- 

 con, and other heavy minerals. The conglomerate 

 type, as exemplified by the Huronian conglomerates 

 of the Blind River area, Ontario, are mined exten- 

 sively for uranium contained in the minerals bran- 

 nerite and uraninite; monazite is also present in the 

 ore. Recovery of yttrium oxide was begun in 1965 

 as a byproduct, of uranium processing and continued 

 as late as 1970. Although the ores have been esti- 



