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

P.19
aluminosilicate framework structures with larger cages and cage openings than do the felds-
pathoids, and all zeolites show exchange properties of the nonframework cations. This property
is undesirable in a repository compound unless the radionuclide can be stabilized in
the structure.
Both Cs and Sr zeolites have been synthesized, and one Sr zeolite occurs in nature, the
mineral brewsterite, SrA12Si60 16*5H 20. It is found in volcanic basalts in gas cavities as a
very late-formed mineral.
Zeolites can be synthesized by gel and by hydrothermal methods. They contain consider-
able water, which helps keep the framework open and which can be driven off by heat. Some
structures collapse at relatively low temperatures, even as low as 100 0 C; but may retain
their structural integrity as high as 800 0 C. The exchangeability of the cation, however,
suggests that the zeolites in general will not desired cations for sufficient times under
various conditions to be effective repository compounds.
Rare earths have been exchanged in some of the zeolite phases. In particular the
faujasite series may be synthesized with a Ce:Ca ratio of 6:4 (Olsen et al. 1967). The
faujasites have one of the more open zeolite framework structures. Considerable research
is needed to determine the suitability of zeolite structures as waste repositories; they
cannot be dismissed summarily.
P.2.11 Borosilicate Minerals
Because boron forms a very stable oxyanion, both as BO3 and B04 coordination polyhedra,
many borosilicates are quite stable mineral structures. Beryllium as Be04 coordination
polyhedra also forms quite stable minerals with silicates. Many minerals of this type are
known to contain rare earth elements (REE) either as essential elements or in solid solution
to significant levels. Table P.2.1 lists the most important of these minerals. These min-
erals are considered possible repository phases.
The borosilicates and berylosilicates are primarily found in rare-earth bearing pegma-
tites, both granite and nepheline syenite types. The affinity for rare-earth elements is
indicated by their formation. The stability of these phases under repository conditions is
unknown. Considerable experinentation is needed to determine their suitability.
P.2.1.12 Zirconosilicate and Titanosilicate Minerals
Interest in the zirconosilicate and titanosilicate minerals arises from the known sub-
stitution of rare-earth elements and actinides for both Ti- and Zr. Usually, the quantities
are small. The known minerals are listed in Table P.2.2. Both the zirconosilicates and
titanosilicates are formed in pegmatite deposits. They are commonly associated with other
rare-earth bearing minerals. Evidence suggests that many of them may be quite resistant to
weathering and zircon and titanite are known to survive as heavy minerals in placer
deposits.