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P.23<br />

close association with albite, zircon, apatite, sphene, biotite. Pyrochlore occurs in both<br />

the metamict and crystalline state. It has quite a variety <strong>of</strong> names:<br />

Pyrochlore (Na,Ca,U,Ce,Y)2_x(Nb,Ta,Ti)206(OH,F);<br />

Betafite (U,Ca)2_x(Nb,Ti,Ta)206-x(OH)l+x, high Ti and U;<br />

Zirconolite (CaZrTi 20 7 );<br />

Microlite (Ca,Na) 2Ta20 6 (0,OH,F), high Ta;<br />

Djalmaite (Ca,Na,U) 2Ta20 6 (0,OH,F), high U relative to microlite;<br />

Obruchenite (Y,U,Ca)2_xNb 20 6 (OH), low Ti, high Y and U.<br />

The differences among minerals reflect only the amounts <strong>of</strong> U, Ti, Ta, Y relative to<br />

pyrochlore. Pyrochlore from carbonatites can have up to 4% ThO 2 . Hydration <strong>of</strong> pyrochlore<br />

leads to loss <strong>of</strong> mobile REE, Ca, Na and an increase in U (Vlasov 1966). Pyrochlore can have<br />

up to 19% U 30 8 and high Sr. Pyrochlore also occurs as a placer deposit-forming mineral.<br />

P.2.2.3 AB206--Nb-Ti-Ta Oxides<br />

Columbite Structure<br />

Columbite (Fe,Mn)(Nb,Ta) 20 6 (tantalite). Columbite can have up to 3% REE, little U.<br />

It is very abundant in acid rocks, e.g. (rarer) granite, granitic pegmatites, quartz<br />

veins; occurs in association with biotite, albite, zircon. Columbite-tantalite is a placer<br />

deposit-mineral and is insoluble in acids (Vlasov 1966). Furthermore, it is very resistant<br />

to weathering and accumulates in deluvial, eluvial, and alluvial placers, resulting from the<br />

weathering <strong>of</strong> columbite-bearing granite and pegmatite. In placers, it is associated with<br />

cassiterite, zircon, ilmenite and rutile. Columbite may be a good candidate for hosting<br />

lanthanide and actinide elements.<br />

Euxenite Structure<br />

Euxenite-polycrase Y(Nb,Ti) 2 (O,OH) 6--Y(Ti,Nb) 2 (0,OH) 6<br />

Delorenjite Y(Ta,Nb) 2 (0,OH) 6<br />

Fersmite (Ca,Ce)(Nb,Ti,Fe) 2 (0,OH,F) 6<br />

Thorium is in higher coordination in euxenite structures than in columbite structures.<br />

Th, U, and Ca can replace Y up to several percent, U up to 16% U0 2 , Th up to 8% ThO 2.<br />

Euxenites are widespread in granite pegmatites. Euxenite occurs as accessory mineral in<br />

granites and is also found in small amounts in placers. It is associated with ilmenite,<br />

monazite, xenotime, zircon, and garnet.<br />

Fersmite is found in nepheline-syenite and carbonatite massifs in association with<br />

columbite, apatite, calcite, fluorite. It is typical <strong>of</strong> rocks <strong>of</strong> intermediate composition<br />

(for weathering and alteration see below).<br />

Priorite Structure<br />

Priorite-Aeschynite (Ce,Nd,Th,Y)(Ti,Nb) 20 6<br />

Polymignite (Ca,Fe,Ce)(Zr,Ti,NbO 20 6<br />

Sinicite (Ce,Nd,Th,U)(Ti,Nb020 6, high U.

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