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

P.48
For metamict, AB 2 0 6 -type, Nb-Ta-Ti oxides (A = REE, Fe + 2 , Mn, Ca, Th, U, Pb; B = Nb,
Ta, Ti, Fe +3 ) primary hydrothermal alteration causes a consistent increase in calcium con-
tent, generally a decrease in the uranium and thorium content, a decrease in total rare-
earth concentrations, a slight decrease in B-site cations, and an increase in structural and
absorbed water. Secondary alteration caused by weathering is similar in effect but produces
a decrease itnt h:content, an increased leaching of A-site cations and a relative increase
in B-siite cations. Refractive index, specific gravity and reflectance decrease with both
types; off a&lteration,, but VHN50 remains approximately constant. It is important to note that
al.thou gi alteration effects in these natural materials have been carefully documented, there
arei no experimental data on hydrothermal alteration effects, solubility as a function of
degree' of metamictization, or the kinetics of these reactions.
There is an abundant literature on metamictization and alteration effects observed in
zircon, (Zr,U)SiO 4 , a phase commonly used by geologists in U/Pb radiometric dating. A sum
mary of this literature is beyond the scope of this Appendix, but it should be the subject
of future research. Discordant ages reported for metamict zircons indicate that the U/Pb
rations can be changed or slightly disturbed by alteration (Krogh and Davis 1975). Labora-
tory experiments involving zircon have demonstrated that altered regions are more rapidly
dissolved by 48% hydrofluoric acid. There are some data which suggest that zircons that
have become metamict are susceptible to attack by solutions that can cause alteration (Krogh
and Davis 1975; Larsen et al. 1953). However, Mumpton and Roy (1961) have recrystallized
numerous metamict zircons by hydrothermal treatment at temperatures of 500*C and above, and
found that the Zr:Si ratio remained close to 1:1. This is an indication that neither ele-
ment was selectively dissolved. They also demonstrated that the water often found in meta-
mict zircons was molecular H 2 0 and not the result of H +
ion exchange leaching. The data are
stiill too limited to draw broad conclusions regarding the effect of metamictization on solu-
iiility, even for metamict zircons. Yet, at worst, this does. not seem to be a major problem.
Monazite, a mineral that apparently does not metamictize, was chosen as the lanthanide and
actinide synthetic mineral in the reference scenario (see Section 3.2.1.3).