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P.24
Priorite differs from euxenite by having cerium REEs and a high content of thorium and
Zr (little U). The REE have the same coordination as in the euxenite structure. Aeschynite
occurs as an accessory in some deposits related to nepheline-syenite and alkali-syenite mas-
sifs in association with zircon, biotite, corundum, muscovite, sphene, and fluorite.
The weathering and alteration of the AB 20 6 and A 2B 20 6 (0,OH,F) REE-Nb-Ti-Ta complex
oxides can be handled in one group. These oxides have pervasive alteration with a usual
weathered crust surrounding fresher oxides (Ewing 1975a). The results of weathering are
leaching of the A-site cations (i.e., U, REE) and introduction of H 20 or OH" or 0= into the
oxide. The B cation remain basically unchanged (Ewing 1975a, Wambeke 1970).
In weathering, up to 40% decrease in the REE content is possible, although the REE dis-
tributions remain nearly the same (Ewing 1975a). For example, a priorite from the Kibara
Mountains, North Katanga, had a fresh inner zone (black) with -0.075 cerium atoms and 0.95 U
atoms per 5.58 0 atoms. Wambeke (1970) gives the relative leaching rate of A cations as
110 REE atoms, 120 Na atoms and 40 U atoms per 100 atoms of Ca leached out. There are lit-
tle hard data on the kinetics or solubility of these complex oxides; these should be
obtained. It seems that columbite might be a good candidate among this group for Ce dis-
posal, since it can be very resistant to alteration. Euxenite is the candidate for the U
elements.
P.2.2.4 ABO. Oxides
Fergusonite Structure
A = Y, REE, U, Ca, Th
B = Nb, Ta, Ti.
Solid solution: YNbO 4 - YTaO 4
fergusonite formanite
The REE in fergusonite are mostly the yttrium rare earths (Vlasov 1966). Fergusonite occurs
as a metamict mineral. Fairly abundant in granite pegmatites, it accumulates in small
amounts in placers and is found as an accessory mineral in granites. In pegmatites, it is
associated with zircon, monazite, xenotime and euxenite. A study of monazite-bearing allu-
vial deposits in Malaya (Flinter et al. 1963) showed fergusonite occurring with columbite,
Ta/Nb rutile, cassiterite and garnet. The samples were derived from a cassiterite-bearing
granite. It thus seems that fergusonite might be relatively stable as a host of REE and
actinides.
P.2.3.1 Rare Earth Fluorocarbonates
P.2.3 Carbonate and Sulfate Minerals
Carbonate minerals are compounds of some cations with the carbonate anion, CO- , often
with hydroxyls and waters of hydration. Of more than 70 naturally occurring carbonate com-
pounds, most are either water soluble or are easily decomposed. These include the simple and