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

P.47
uranium or thorium. Reports of radioactive columbites are almost certainly mixtures of col-
umbite and metamict microlite (Lima-de-Faria 1964).. A number of the phases (bastnaesite and
all hydrated phases) are alteration products. The primary phases that consistently occur
in crystalline form, even with high concentrations of uranium or thorium, are indicated by
asterisks.
P.4.3 Rate of Metamictization
The rate of metamictization of a given mineral to a first approximation, depends on:
1) the inherent stability of its structure and 2) the alpha particle flux resulting from the
presence of uranium, thorium and their unstable daughter nuclides (Pabst 1952).
Pabst calculated that a minimum of 110,000 years is required for gadolinite, 0.4% Th,
to become metamict. This figure, which could be low by a factor of 1000 (Ueda 1957, Lipova
1966, Hurley and Fairbain 1953), was obtained by assuming that all of that alpha decay
energy was spent in disordering the structure and that this energy was measurable by DTA
(Pabst 1952).
Most zircons become metamict upon receiving a radiation dose of about 1016 a/mg
(Holland and Gottfried 1955). Using this dosage criterion, the following table gives esti-
mates of the time required for some radioactive zircons to become metamict.
Initial radionuclide content Estimate time (yrs)
1% Th 1.4 x 109
1% U 3.3 x 108
10% U 3.2 x 107
1% Pu 236 (does not exist in nature) 2.0
There are, however, zircons and thorites (thorite has 'the zircon structure and is
expected to show similar radiation effects) which show anomalous radiation effects. Some
zircons that have had radiation doses of only 2.8 x 1015 a/mg are metamict (Krasnobayev et
al. 1974). On the opposite extreme is a report of a non-metamict thorite containing 10%
uranium that is at least 1.2 x 108 years old (Hutton 1950). If this age is correct, then
the thorite specimen has withstood a radiation dose of about 9 x 10 16 a/mg. These data
suggest that factors other than structural stability and alpha particle flux are important
in determining the rate of metamictization.
P.4.4 Alteration Effects
Minerals that occur in the metamict state are often severely altered, either as a
result of hydrothermal alteration or surface weathering. The resulting complicated composi-
tional variates are in part responsible for the very complex mineral nomenclature. Most of
the available data on alteration effects pertains to various Nb-Ta-Ti oxides (Ewing 1975,
Wambeke 1970) and zircon, (Zr,U)SiO 4 . In both cases alteration may be extensive and fol-
lowed by recrystalliztation of phases quite different from the original pre-metamict phase
(Ewing 1974).