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

P.4 METAMICTIZATION
P.44
Metamict minerals are a special class of amorphous materials which were initially crys-
talline (Broegger 1893). Although the mechanism for the transition is not clearly under-
stood, radiation damage caused by alpha particles and recoil nuclei is certainly critical
to the process (Graham and Throber 1974, Ewing 1975). The study of metamicitzation of natu-
rally occurring materials allows for the evaluation of the long-term effects that result
from this type of radiation damage, particularly changes in physical properties. Comparison
of metamict and non-metamict crystalline phases addresses the question of the susceptibility
of different bonding and structure types to radiation damage and provides useful insights
into defining radiation damage experiments.
P.4.1 Properties
The list below is an amplified tabulation of metamict mineral properties listed by
Pabst (1952).
1. They are generally optically isotropic but may show varying degrees of anisotropy.
Reconstitution of birefringence with heating is common.
2. Metamict phases lack cleavage. Conchoidal fracture is characteristic.
3. Some mineral species are pyronomic, that is, they glow incandescently on heating.
In many cases, however, recrystallization may occur without observable glowing.
4. Crystalline structure is reconstituted by heating. The metamict material recrystallizes
to a polycrystalline aggregate with a concomitant increased resistance
to attack-by acid. During recrystallization several phases may form, the particu-
lar phase assemblage is dependent on the conditions of recrystallization (e.g.,
temperatue and type of atmosphere). In many cases the original pre-metamict phase
may not recrystallize due to compositional changes causet by/post-metamict
alteration.
5. Metamict minerals contain U and Th, alto&ugt contents may be quite variable (as
low as 0.41% Th02 in gadolinite from Yttery, Norway . Rare-earth elements are
also common (in some cases over 50 wt%). Water of hydration may be high (up to
70 mole%).
6. They are x-ray amorphous. Partially crystalline metamict minerals display distinct
line broadening and decreased line intensities. A shift of lines to lower
values of two-theta is observed in specimens with a reduced specific gravity.
7. Some phases occur in both the crystalline and metamict state, and in these cases
there is little chemical difference.
The most common methods of analysis of the metamict state are x-ray diffraction analy-
sis of annealed material (Berman 1955, Lima-de-Faria 1964, Mitchell 1972, Ueda and