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

P.11
Using Table P.1.6 as a guide, one can see that Cs + 1 is large and most like K+1 and pos-
sibly Ba + 2 . There is only one mineral in which Cs is essential, and that is pollucite
(Cs 2A12Si40 1 2 * nH20), a member of the analcime (Na 2A12Si40 1 2 * nH 2O) family of minerals.
The fact that it is acting in the role of Na I suggests that other Na + 1 and K + 1 phases may
act as hosts for Cs + 1 . Other possible examples include the feldspars (K,Na,Ca) (Al, Si) 40 8,
feldspathoids, (K,Na,Ca) (Al,Si20304_6, zeolite, (K, Na, Ca) Al, Si)m02m * nH20 and micas,
(K,Na,Ca) 2 (A1,Mg,Fe) 4 _6(AI,Si) 80 20 (OH) 4 . Traces of cesium are known to occur in each of
these minerals.
The next element, Sr+ 2 , is found in many compounds in nature. Often it shows substitu-
tional relations with Ba + 2 and sometimes with Ca+ 2 . It may also occur in many of the same
phases as indicated for Cs + 1 above.
Iodine exists in nature both as I in two compounds and as 103 in several other
phases. Its crystal chemistry is similar to the halogens; it behaves most similarly to Br-
and possibly Cl-, although the radii are markedly different. Very few synthetic iodine com-
pounds have bromine of chlorine isostructural counterparts. Ways to tie iodine up in the
crystalline state are discussed later.
Technetium is chemically most similar to manganese and rhenium. There are no known
technetium compounds in nature, and there is little knowledge of its crystal chemistry. It
is discussed separately below.
The rare earth elements are all very similar in ionic size, although the heavier ones
are small enough to cause them to form different series of compounds in some instances from
the larger ones. For example, the large lanthanides behave similarly to Ce+ 3 and commonly
substitute for it. The smaller lanthanides tend to substitute for Y 3. Rare earths are
also known to substitute for Th + 4 and Zr + 4 in many of their minerals.
The actinides show some similarities in size and commonly follow Y+3, Th+4, Zr+4, U+4
and Ce+ 4 . There are enough differences between uranium chemistry and actinide chemistry to
make casual geochemical reasoning suspect and specific research is needed. Uranium readily
oxidizes in nature and is commonly found as U+ 6 uranates and as uranyl, UO+ 2 . Plutonyl and
Neptonyl can be made and may substitute for uranyl.
Crystalline Sollutions
Because of the ease of substitution of ions for other similar ions, it is common for
solid solutions to occur. A solid solution is a compound in the crystalline state in which
one or more ions have replaced other similar ions in the crystal structure without disrup-
ting the atomic arrangement. Substitutions may be complete (e.g., Fe-Mg in olivine (Mg,
Fe) 2 Si0 4 ), or limited, (e.g., K-Na in nepheline (Na,K) AlSi04) between two end member
compositions.
Natural compounds are rarely pure end members, as solid solution is very common in
minerals. Some minerals may have several substitutions and thus extreme variability in
chemical compositions occurs. The amphibole family, which has four different sites that
allow substitution, is an extreme example. Partial solid solution may actually be desirable