The Alteration of Uraninite to Clarkeite

ABSTRACTThe oxidative alteration of uraninite by alkali-bearing, hydrothermal (200-400° C) solutions in granitic rocks produces the rare mineral clarkeite. The general formula for Na-clarkeite is{(Na,K)6-y-z(M2+PM3+qM4+r)y(□1+z-xPbx)}[UO2)7-xOt0]·nH2O. z ≈ p+2q+3r.The terms in square brackets designate the sheet structure; the other elements and vacancies (0) occur in interlayer sites. Clarkeite can accommodate Ca, Sr, Ba, Y, Th, and lanthanides. Thus, clarkeite is a potential actinide and fission product host. The replacement of uraninite by clarkeite occurs in the solid state and results in the loss of one-half of the uranium from uraninite. As U decays to Pb, the radiogenic Pb enters interlayer vacancies in clarkeite. This destabilizes the structure and clarkeite eventually decomposes to wölsendorfite, (Pb,Ca)2U2O7·2H2O, or curite, Pb3U8O27·3H2O.Clarkeite from Spruce Pine, North Carolina is zoned compositionally, with a K-rich core (atomic ratio K:Na:Ca = 1.0 : 0.11 : 0.12) surrounded by Na-clarkeite (Na:K:Ca = 1.0 : 0.03 : 0.03), rimmed and veined by Ca-clarkeite (Ca:Na:K = 1.0 : 1.25 : 0.05). Volumetrically, Na-clarkeite is the most important constituent. The chemical zoning reflects the disparate chemistries of K+, Na+, and Ca2+. Na-clarkeite and Ca-clarkeite are structurally similar, but have different sheet structures. The solubility of K in Na-clarkeite is less than five mole percent and is due to the different sizes of Na+ and K+ ions. The K-phase may not be related structurally to clarkeite.Exposed to ground water at low temperatures, clarkeite alters to the uranyl silicates, uranophane and kasolite. The alkalis are leached by ground water. The fate of Th, Y, and lanthanides from clarkeite is uncertain.