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Occurrence and Production of Terbium
Occurrence
Terbium is never found in nature as the free element, but it is contained in many minerals, including cerite (Ce9Fe(SiO4)6(SiO3)(OH)4), gadolinite ((Ce,La,Nd,Y)2FeBe2Si2O10), monazite ((Ce,LaTh,Nd,Y)PO4, which contains up to 0.03% of terbium), xenotime (YPO4) and euxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)2O6, which contains 1% or more of terbium). The richest current commercial sources of terbium are the ion-adsorption clays of southern China. The high-yttrium concentrate versions of these are about two-thirds yttrium oxide by weight, and about 1% terbia. However, small amounts occur in bastnaesite and monazite, and when these are processed by solvent-extraction to recover the valuable heavy lanthanoids in the form of "samarium-europioum-gadolinium concentrate" (SEG conccentrate), the terbium content of the ore ends up therein. Due to the large volumes of bastnaesite processed, relative to the richer ion-adsorption clays, a significant proportion of the world's terbium supply comes from bastnaesite.