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

P.22
Again, these minerals require considerable research to define their suitability as
repository phases. Their long-time stability must be defined particularly under hydrother-
mal conditions.
P.2.2.1 Perovskite structure--ABO 3 (CaTiO 3)
P.2.2 Oxide Minerals
A = Ca, REE, Na, Th, U radius .1.0 A
B = Ti, Nb, Ta, Fe 3 + , Mg, Zr radius %0.7 A
Knopite (Ca,Ce)(Fe,Ti)0 3
Dysanalyte (Ca,Ce,Na)(Ti,Nb,Fe)0 3
Loparite (Na,Ce,Ca)(Ti,Nb)0 3
Irinite (Na,Ce,Th)l_x(Ti,Nb)03_x(OH)x
Metaloparite (Ce,Ca)l_x(Ti,Nb) 3_x(OH) 2
Loparite, irinite and knopite are found as metamict minerals. Perovskite occurs as an
accessory mineral in basic igneous rocks, often in association with melflite, nepheline or
rare'earth apatite, as well as in metamorphosed calcareous rocks in contact with basic igne-
ous rocks. The B ion is mostly Ti with a little Nb and Fe 3 + in all the various minerals
above. The variety rich in rare earths, chiefly cerium, is knopite and is also high in
alkalis (Na), loparite, or its hydrate, metaloparite. Dysanalyte is high in Nb and irinite
is distinguished by its high thorium content.
Since Ca 2 + is in 12-fold coordination in perovskite, it is replaced preferentially by
0 0
the large light lanthanides, i.e. La and Ce (rLa3+ = 1.15 A, rCe 3 + = 1.11 A), rather than
the yttrium earths. Hydrothermal alteration of loparite leads to metaloparite with loss of
alkalis, assimilation of water and enrichment in the rare-earth elements (Vlasov 1966).
Thus it seems that loparite retains the REE in alteration. Loparite is also known to occur
as a placer deposit-forming mineral. Therefore, perovskite minerals are a possible host for
lanthanide and actinide elements.
We can calculate the conversion of perovskite to rutile by a weathering solution, i.e.
CaTiO 3 + 2H 2 + - Ca 2 + + H20 +TiO 2
K298 101814
Hence for pH = 6 aCa 2 + = 10 6 1 4 m;
pH = 8 aCa 2 + = 102.14.
Evidently the reaction, at equilibrium proceeds overwhelmingly to the right, which sug-
gests that loss of Ca (and maybe REE) would follow if equilibrium were.maintained. However,
the kinetics of the above reaction may be slow, and more work is needed to determine the
leaching rate.
P.2.2.2 Pyrochlore--A 2B 20 6(0,F,OH) or (Ca,Na,Ce) 2-x(Nb,Ti) 206(OH,F)
The pyrochlores are also characteristic of basic rocks and alkali rock massifs
(nepheline-syenites, alkali syenites, albitized granites and carbonatites) and occur in