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

P.18
and Fe +3 are known to substitue for Al, and Cs and Sr may substitute for the cation. A
SrAl 2Si 20 8 phase can be synthesized, which is analogous to BaA12Si 20 8 , but the level of Sr
in natural feldspars is rarely 0.5 wt%. The level of Cs is never greater than 0.005 wt%.
Feldspars weather slowly to clay minerals under surface ambients but are very stable in
rocks.
P.2.1.9 Feldspathoid Minerals
The feldspathoid minerals form when alkali-rich aluminosilicate compositions have
insufficient SiO 2 to form free quartz. The minerals usually coexist with feldspar, particu-
larly the one with the corresponding alkali ion. The important feldspathoids are nepheline,
(Na,K) 4A1 4Si 40 16 ; leucite, KAISi 20 6 ; analcime, NaAISi 20 6 H 20; soldalite, NagA1 6Si 60 24 C1 2,
and cancrinite (Na,K,Ca)6_8 (Al,Si) 120 24 (Cl,So 4 ,CO 3 ) 1 .5-2. 0 nH20. Scapolite,
(Na,Ca,K)4A13Ial,Si) 3Si 60 24 (Cl,SO4 ,C03 ), may also be considered here because it resembles
sodalite and cancrinite in behavior although it is not formally considered a feldspathoid.
Nepheline is a stuffed derivative of tridymite (Si02) and can accept alkali ions in the
framework to charge compensate the Al that substitutes for Si. The cages are just large
enough to accept K (ionic radius = 1.38 A) and actually prefer some Na (ionic
radius = 1.02 A) to relieve some of the strains on the framework linkages. To accept larger
cations such as Cs and Sr would be too much strain on the structure. Cs and Sr are gener-
ally not reported in any nepheline analyses.
Leucite and analcime have similar crystal structures with identical frameworks. The
cages are larger than in nepheline and Cs will substitute freely in the analcime to form the
only Cs mineral in nature. Pollucite, CsA1Si20 6 0.5H 20, forms readily from its components,
and is the leading candidate as a repository phase for Cs (Komarnini et al. 1978). Consid-
erable study has already been made on pollucite for this purpose. The possiblity of a Sr
analog also exists, but it does not occur in nature.
Sodalite, cancrinite and scapolite may have two uses as potential waste minerals
although considerable research is needed to verify their potential. All three minerals may
have Cs and Sr analogs, where these elements substitute for Na, Ca, or K, as in leucite-
analcime. The framework cages are larger than in leucite and analcime, but because of this
increased size the alkali cations are easily exchanged and hence easily leachable. Another
interesting aspect of these structures is the trapping of large anions in the cages. All
three minerals are known to have significant quantities of C1-, SO = and CO in the struc-
tural cages, and sodalite often has S . This behavior immediately suggests the possibility
of trapping I- inside the cages. If the structure can be grown around the I- before the
iodine volatilizes, it may be effectively caged because its radius (2.20 A) is considerably
larger than the cage opening (1.40 A). Much research is needed on this potential.
P.2.1.10 Zeolite Minerals
The zeolites are a large group of industrially important compounds, many of which exist
as minerals. Their properties have been surveyed by Breck (1959). They have