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P.21<br />

The stability <strong>of</strong> this group <strong>of</strong> minerals under repository conditions deserves more<br />

+1 +2<br />

study. They may actually prove to accept Cs + and Sr + in some <strong>of</strong> their structures for Ca,<br />

Na, or Ba. One Sr phase, lamprophyllite, is known.<br />

P.2.1.13 Rare-Earth Silicate Minerals<br />

A large number <strong>of</strong> minerals are essentially rare-earth silicates with or without other<br />

essential elements. These compounds must all be considered potential repository phases for<br />

both the lanthanides and actinides. Some <strong>of</strong> the phases have demonstrated stabilities, hav-<br />

ing formed in granites or pegmatites and then survived the sedimentary cycle to be deposited<br />

in Placers. Alanite is one exmple; it was discussed with the epidote minerals. Thorite,<br />

huttonite, and cheralite are other examples.<br />

Most <strong>of</strong> the minerals in Table P.2.3 are formed in pegmatites. The lanthanide (Ln)<br />

families <strong>of</strong> Ln 2 Si 20 7 and Ln 2 SiO 5 phases are easy to prepare synthetically. Many <strong>of</strong> them<br />

show severalstructural modifications, but they have high melting or decomposition tempera-<br />

tures. Some <strong>of</strong> the minerals such as c<strong>of</strong>finite, USiO 4 , may be synthesized at 100°C. These<br />

minerals form in sedimentary rocks from circulating ground waters.<br />

TABLE P.2.3. Rare-Earth Silicate Minerals<br />

Rare-Earth<br />

Silicates Formula<br />

Allanite (Ce,Ca,Y) 2 (Fe,A1 3 )(Si0 4 ) 3 (OH)<br />

Ashcr<strong>of</strong>tine KNaCaY 2Si60 12 (OH) 10-4H 20<br />

Britholite (Ca,Ce) 5 (Si0 4 ,PO 4 ) 3 (OH,F)<br />

Cheralite (Ca,Ce,Th)(P,Si)0 4<br />

C<strong>of</strong>finite U(SiO 4 )l-x(OH) 4 x<br />

Ekanite (Th,U)(Ca,Fe,Pb) 2Si 80 20<br />

Huttonite ThSiO 4<br />

limorite Y 5 (SiO 4 ) 3 (OH) 3<br />

Kainosite Ca 2 (Ce,Y) 2 Si 40 12 (CO 3 )H 20<br />

Miserite K(Ca,Ce) 4 Si 50 13 (OH) 3<br />

Nordite (La,Ce)(Sr,Ca)Na 2 (Na,Mn)(Zn,Mg)Si 60 17<br />

Phosinaite H 2Na 3 (Ca,Ce)SiO 4PO 4<br />

Sazhinite Na 3CeSi60 15 *6H20<br />

Soddyite (U0 2 ) 5Si 20 9.6H 20<br />

Thalenite Y 2Si 2O 7<br />

Thorite ThSiO 4<br />

Thorosteenstrupine (Ca,Th,Mn) 3 Si40 11F*6H 20<br />

Thirtveitite (Sc,Y) 2Si 20 7<br />

Tombarthite Y4 (SiH4)4012-x(OH)4+2 x<br />

Tornebohmite (Ce,La) 3Si 2 0 8 (OH)<br />

Umbozerite Na 3Sr 4ThSi 8 (0,OH) 24<br />

Uranophane Ca(U0 2 ) 2 (SiO 3OH) 2<br />

Weeksite K 2 (U0 2 ) 2Si60 15 -4H20<br />

Yttrialite (Y,Th) 2Si 20 7

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