Q. Show the Occurrence of the f-block element?
Apart from promethium which is unstable and is found in traces in uranium ores, all the lanthanides usually occur together. Although a large number of minerals are define to contain lanthanides, only three of them, viz., monazite, xenotime and bastnaesite are of commercial importance. Monazite and xenotime are a mixture of phosphates of thorium, lanthanum and lanthanides. Monazite is generally but sparsely distributed in many rocks, but because of its high density and inertness, it is concentrated by weathering into sands on beaches and river beds. Deposits of monazite occur in Southern India, Brazil and South Africa. Bastnaesite is a mixture of fluoride carbonates, LnFC03, of lanthanum and the lanthanides. Both bastnaesite and monazite are richer in the lighter lanthanides, i.e., the cerium earths, but with the difference that monazite also takes upto 30% Thus, which is absent' in bastnaesite. On the other hand xenotime is a valuable source of the heavier rare earths.
Eve known isotope 'of the actinide elements is radioactive and their half-lives are such that only 232, Th235U238 and possibly 244 Pu have survived during the very period of their existence. Only uranium and thorium are found in nature in amounts sufficient for practical extraction. Thorium constitutes 8.1 x 10-4% of the earth's crust and it is almost as abundant as boron. As define earlier, monazite is the most important source of thorium. Uranium comprises 2.3 x 10-4% the earth's crust and it is slightly more abundant than tin. Pitchblende or uraninite, U3O8, and carnotite, K2 (U02)2(V04)2 .3H20, are two important ores uranium.