Abstract

Clarkeite crystallizes during metasomatic replacement of pegmatitic uraninite by late-stage, oxidizing hydrothermal fluids. Samples are zoned compositionally: Clarkeite, which is Na rich, surrounds a K-rich core (commonly with remnant uraninite) and is surrounded by more Ca-rich material; volumetrically, clarkeite is most abundant. Clarkeite is hexagonal (space group R3m) a = 3.954(4), b = 17.73(1) Aa (Z = 3). The structure of clarkeite is based on anionic sheets of the form [(UO 2 )(O,OH) 2 ]. The sheets are bonded to each other through interlayer cations and H 2 O molecules. The empirical formula for clarkeite from the Fanny Gouge mine near Spruce Pine, North Carolina, is: {Na (sub 0.733) K (sub 0.029) Ca (sub 0.021) Sr (sub 0.009) Y (sub 0.024) Th (sub 0.006) Pb (sub 0.058) } (sub Sigma 0.880) [(UO 2 ) (sub 0.942) O (sub 0. 918) ](OH) (sub 1.082) ](H 2 O) (sub 0.069) . Na predominates and the Pb is radiogenic. The general formula for clarkeite is {Na,K) p M (super 2+) q M (super 3+) r M (super 4+) s Pb x } {(UO 2 ) (sub 1-x) O (sub 1-y) (OH) (sub 1-y) ](H 2 O), where Na>>K and p>(q+r+s). The number of O (super 2-) ions and OH groups in the structural unit is determined by the net charge of the interlayer cations (except Pb): y = 1-(p+2q+3r+4s). This suggests that the ideal formula for ideal end-member clarkeite is Na[(UO 2 )O(OH)]H 2 O) (sub 0-1) . The structural sheets are destabilized as U decays to Pb (increasing x), and Pb enters vacant interlayer cation sites. Clarkeite eventually recrystallizes to lead uranyl oxide hydrates such as wolsendorfite or curite.

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