Management of Commercially Generated Radioactive Waste - U.S. ...

Murdochite PbCu 6 (0,C1,Br) 8
Mendipite Pb 3C1 202
Penfieldite Pb 2Cl 3 (OH)
P.30
Yedlinite Pb6CrC1 6 (0,OH) 8
Phosgenite Pb2(C03 )C12 .
Little is known of the structures, solubilities, and ranges of stability of these materials.
The substitution of iodine for chloride in the lead oxyhalide structures should be
investigated.
P.2.6 Uranium Minerals
Uranium occurs in nature in both the U + 4 and U + 6 valence state. The U + 5 valence state
has been postulated, especially in U 30 8 and other oxides intermediate between U0 2 and U0 3,
but it has not really been verified. Its existence is not critical to our discussion.
P.2.6.1 U+4Minerals
Uranium occurs as U+ 4 in only a small group of minerals. The most important and best
known is uraninite, U0 2 , which has the fluorite, CaF 2 , structure. It is the principal min-
eral in most uranium deposits and is found in pegmatites, in sandstones and metasediments,
and as an accessory mineral in some granites. Natural UO 2 is rarely stoichiometric and is
better described as U 2+x where x ranges between 0 and 0.25. Most uraninite from older
sources is metamict and may be called pitchblende.
In sandstone deposits the uraninite has formed from circulating ground water by reduc-
tion of the U + 6 . In the reduced form it is very stable and is common in the placer deposits
of the Witwatersrand district in Africa. These uraninite grains were carried down streams
and deposited in energetic depositional environments without chemical breakdown because the
atmospheric conditions of the time were highly reducing. If uraninite could be maintained
in its U + 4 state it would be a good repository mineral. Unfortunately, it alters rapidly in
present-day atmospheres.
Uraninite is usually only uranium bearing in sandstone deposits, but in pegmatites it
may contain significant quantities of Ce and Th in solid solution. Actually, complete solid
solutions of these elements can be prepared under laboratory conditions.
Saoe of the other U + 4 minerals occur in quantities sufficient for them to be called ore
minerals. Coffinite, USiO 4 , brannerite, UTi20 6 , and ningyoite, CaU(PO 4 ) 2.1.5H 20, occur pri-
marily in sedimentary or metasedimentary environments probably as syngenetic minerals.
Other U + 4 minerals include lermontovite, (U,Ca,Ce...) 3 (PO 4) 4 -6H 2 0; sedovite, U(Mo04) 2 ;
uranopyrochlore, U 2Nb 20 6 (0,OH,F); cliffordite, UTe 30 8 , and ishakowaite, (U...)(Nb,Ta)O 4.
In addition U + 4 occurs as a minor element in many minerals, mostly replacing other group IV
elements or the rare earths. At the conditions existing at the earth's surface all these
U + 4 minerals readily alter by oxidation and weather by releasing the uranium into the ground
water system. The U+ 6 may be fixed immediately in new minerals or may migrate for long dis-
tances before being redeposited.