550 



UNITED STATES MINERAL RESOURCES 



rock types from the data of Haskin and Frey (1966). 

 Even this limited sampling illustrates that the 

 amount of rare earths present can vary widely 

 within a single rock type. Rare-earth contents 

 greater than those shown in the table have been 

 found in both sedimentary and igneous rocks; ma- 

 rine phosphorites, for example, commonly have a 

 high rare-earth content, and some granites and 

 syenites may contain more than 1,000 ppm. 



A significant part of the rare-earth content of 

 rocks may be present in "carrier" minerals such as 

 apatite, sphene, zircon, and the ferromagnesian 

 minerals. Plagioclase feldspar may be enriched in 

 europium (Haskin and Frey, 1966, p. 313). These 

 minerals, as well as the specific rare-earth minerals, 

 contain preferential rare-earth assemblages which 

 may vary with their geologic environment (Flei- 

 scher and Altschuler, 1969). 



When rare-earth minerals are liberated from their 

 host rocks by weathering, they may accumulate in 

 the soil or be transported from their original site 

 by running water. Heavy, resistate minerals such 

 as monazite may accumulate in fluviatile or sea- 

 beach placers or become incorporated in sediments 

 depositing offshore. As the minerals are largely 

 insoluble under surface-weathering conditions, there 

 is very little release of rare-earth ions for the de- 

 velopment of secondary minerals. 



An interesting geochemical cycle for monazite 

 was proposed by Overstreet (1960, 1967) to ex- 

 plain its relative abundance in highly metamor- 

 phosed rocks, and its virtual absence in those where 

 the grade of metamorphism was low. He suggested 

 that detrital monazite in the original sediment be- 



comes unstable during initial metamorphism and 

 breaks down, the rare earths entering other struc- 

 tures. As the intensity of metamorphism increases, 

 monazite is again formed and reaches its maximum 

 abundance in rocks that have undergone the most 

 intense metamorphism. Monazite commonly con- 

 tains 3-8 percent of thorium, and, as noted by 

 Overstreet, the thorium content is also influenced 

 by the degree of metamorphism, being greatest in 

 monazite from the most highly metamorphosed 

 rocks. 



MINERALOGY 



The rare earths are essential constituents of over 

 100 mineral species and are present through sub- 

 stitution in many more. Only a few, however, have 

 been found in sufficient concentration to be used 

 solely as rare-earth ores ; some others are recovered 

 primarily for one or more other elements, with rare 

 earths as a byproduct. 



A partial list of rare-earth-bearing minerals and 

 their general compositions is given in table 116. 

 Of these, monazite and bastnaesite are the most im- 

 portant by virtue of their abundance, high rare- 

 earth content, and amenability to processing and 

 are the two basic ore minerals used by industry. 

 Xenotime, the yttrium phosphate, is also used, but 

 to a minor extent. Byproduct rare earths have been 

 produced from apatite in Finland, euxenite in the 

 United States, loparite in the U.S.S.R., and from 

 uraninite and brannerite in Canada. 



Monazite is a common accessory mineral in some 

 igneous and metamorphic rocks and in ancient and 

 modern placers. Most economic concentrations of 



Table 116. — A partial list of rare-earth-bearing minerals 



Flu 



Cerianite Ge02. 



orides : 

 Flu 



■ite (Ce.La)F3. 



Fluorite, cerian (yttroeerite) .CaFs + dominantly Ce subgroup. 

 Fluorite, yttrian (yttrofluorite) -CaFi- + dominantly Y subgroup. 

 Carbonates and fluocarbonates : 



Ancylite ( Ce,La)j (Sr.Ca)3(C03)7(0H) i-3H:0. 



Bastnasite CeFCOa. 



Doverite CaY(C03)=F. 



Parisite 2CeFC03CaC03. 



Synchisite CeFC03-CaC03. 



AUanite (Ca,Ce,Th):( Al,Fe,Mg)3Si30i2- (OH) . 



Cenosite Ca-CCeYjsSiiOnCOsHsO. 



Cerite (Ce.Ca) iSi (0,0H)5. 



Gadolinite BejFeYiSiiOio. 



Huttonite ThSi04. 



Stillwellite (Ce.La.Ca) BSiOs. 



Thalenite YjSiiOv. 



Thorite _ ThSiOj. 



Thortveitite (SciYjiSiaOv. 



Phosphates: 



Apatite > Car. ( PO4 ) sF. 



Brockite (Ca.Th.Ce) P04-H20. 



Florencite Ce.A' P0i) = (0H)8. 



1 May contain rare-earth elements. 



^ Cerian varieties known. 



^ Cerian and thorian varieties known. 



Mineral 



Composition 



Phos phates — continued : 



Monazite (Ce,La,Th,Y)P0j. 



Rhabdophane (Ce,Y)P0iH20. 



Weinschenkite YP0r2H=0. 



Xenotime YPO4. 



Multiple oxides containing 



columbium (niobium), 

 tantalum, and titanium: 



Brannerite (U.Ca.Pe.Th.YjsTisOie. 



Eschynite-priorite series: 



Eschynite (Ce,Ca,Fe,Th) (Ti,Nb)206. 



Priorite ( Y,Er,Ca,Fe,Th) (Ti,Nb)20e. 



Euxenite-polycrase series: 



Euxenite ( Y,Ca,Ce,U,Th) (Nb,Ta,Ti)208. 



Polycrase ( Y,Ca,Ce,U,Th) (Ti,Nb,Ta)206. 



Fergusonite-formanite series: 



Ferjusonite (Y,Er,U.Th) (Nb,Ta,Ti)04. 



Formanite (Y,Er,U,Th) (Ta,Nb)04. 



Loparite (Ce,Na,Ca) (Ti.NbjaOe. 



Perovskite '' CaTiOs. 



Pyrochlore-microlite series: 



Betaflte (U,Ca) (Nb,Ta,Ti)309-nH20. 



Microliter (Na,Ca)2Ta206(0,0H,F) . 



Pyrochlore NaCaNb206F. 



Samarskite ( Y,Er,Ce,U,Fe,Th) (Nb.TajaOe. 



Yttrotantalite (Fe,Y,U,Ca) (Ta,Nb,Zr,Sn) O4. 



