Natural occurence. Minerals (excluding silicates)  Th : Thorium : System number 44 

 - 8. ed. 
 -  1990 
 - XV, 391 S. : zahlr. graph. Darst. 
 - Ditz, Reiner; Sarbas, Barbel .
<br/><br/><label>ISBN: </label>3540936114 0387936114 
<br/><br/><label>Universal Decimal Class. No.: </label>546