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

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P.29<br />

Xenotime contains a high amount <strong>of</strong> yttrium rare earths. It is widespread in granites,<br />

pegmaties and metamorphic gneisses (Vlasov 1966). When granites weather, xenotime accumulates<br />

in placers (e.g. in New Zealand and USSR). Xenotime is very stable under surface<br />

conditions.<br />

P.2.5.1 Iodine Minerals<br />

P.2.5 Iodine Hosts<br />

Iodine is a relatively rare element in rocks and minerals. It.occurs in both the I-<br />

and I 5 + valence states. Iodine is easily oxidized to the 5-valent state and appears in many<br />

<strong>of</strong> its natural compounds as the iodate, 103 ion. These are:<br />

Lautarite Ca(IO 3 ) 2<br />

Bellingerite Ci(I0 3) 2.2/3H 20<br />

Salesite Cu(IO 3 )OH<br />

Schwartzembergite Pb 5 (I0 3 )C1 30 3<br />

Dietzeite Ca 2 (IO 3 ) 2CrO 4.<br />

The above compounds are at least slightly soluble in water, and all are soluble in solutions<br />

with low pH. The iodate minerals are found in evaporite deposits or as weathering products<br />

<strong>of</strong> ores in very dry environments.<br />

Marshite, Cul, iodargyrite, AgI, and their solid solution, miersite, occur in nature<br />

and might be stable in a bedded salt type <strong>of</strong> repository but in general no natural mineral<br />

<strong>of</strong> iodine hints <strong>of</strong> very long-term stability.<br />

P.2.5.2 Framework Structures for Iodine<br />

Two candidate minerals that are composed <strong>of</strong> three-dimensional frameworks contain<br />

cavities sufficiently large to house the I- ion: sodalite and the boracite family.<br />

Sodalite, Na 4A1 3Si30 12C1, is a member <strong>of</strong> the feldspathoid group. It is a three-<br />

dimensional framework and the essential Cl" is locked in cage-like interstices. Iodine can<br />

be substitued for C1-1 and maintained in this structure.<br />

Boracite, Mg 3B 70 13C1 is a three-dimensional framework <strong>of</strong> B-O tetrahedra with the Cl"<br />

locked in cage structure. Other minerals <strong>of</strong> the boracite family are ericaite,<br />

(Fe,Mn) 3B 70 13C1, and chambersite, Mn 3B70 13C1. However, a very large number <strong>of</strong> synthetic<br />

materials with the boracite structure have been synthesized. Many <strong>of</strong> the synthetics contain<br />

I' rather than Cl-. They are stable under hydrothermal conditions.<br />

P.2.5.3 Lead Oxyhalides<br />

There exists a small group <strong>of</strong> minerals composed <strong>of</strong> the oxy- or hydroxy-halides <strong>of</strong> lead.<br />

These materials usually appear as oxidation products on lead-zinc ores which is evidence for<br />

their stability in the surface environment. The list includes:

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