| 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.
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Occurrence