Clarkeite: New chemical and structural data
- Univ. of New Mexico, Albuquerque, NM (United States)
Clarkeite crystallizes during metasomatic replacement of pegmatitic uraninite by latestage, 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), c = 17.73(1) {Angstrom} (Z = 3). The structure of clarkeite is based on anionic sheets of the form [(UO{sub 2})(O,OH){sub 2}]. The sheets are bonded to each other through interlayer cations and H{sub 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}) {Sigma}{sub 0.88} [(UO{sub 2}){sub 0.942}O{sub 0.918}(OH){sub 1.082}] (H{sub 2}O){sub 0.069} Na predominates and the Pb is radiogenic. The general formula for clarkeite is ((Na,K){sub p}M{sub q}{sup 2+} M{sub r}{sup 3+} M{sub s}{sup 4+} Pb{sub x})[(UO{sub 2}){sub 1-x}O{sub 1-y}(OH){sub 1+y}] (H{sub 2}O){sub z} where Na >> K and p > (q + r + s). The number of O{sup 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{sub 2})O(OH)](H{sub 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 woelsendorfite or curite. 45 refs., 5 figs., 7 tabs.
- OSTI ID:
- 543160
- Journal Information:
- American Mineralogist, Journal Name: American Mineralogist Journal Issue: 5-6 Vol. 82; ISSN 0003-004X; ISSN AMMIAY
- Country of Publication:
- United States
- Language:
- English
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