Association of uranyl-oxide hydroxy-hydrates and “calciolepersonnite” from the Krátka Dolina Valley (Slovakia)

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Marco E. Ciriotti
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Association of uranyl-oxide hydroxy-hydrates and “calciolepersonnite” from the Krátka Dolina Valley (Slovakia)

Messaggio da Marco E. Ciriotti » mer 24 ott, 2018 11:33

▪ Ferenc, Š., Biroň, A., Mikuš, T., Spišiak, J., Budzák, Š. (2018): Initial replacement stage of primary uranium (UIV) minerals by supergene alteration: association of uranyl-oxide hydroxy-hydrates and “calciolepersonnite” from the Krátka Dolina Valley (Gemerská Poloma, Gemeric Unit, Western Carpathians, Slovakia). Journal of Geosciences, 63, 277-291.

Mineral association with uranyl-oxide hydroxy-hydrates and uranyl carbonate-silicates was found in J-1 quartz vein containing U-Au mineralization at Krátka Dolina Valley (Rožňava district, Slovakia). The vein penetrated Lower Palaeozoic graphitic phyllites and metalydites (Vlachovo Fm., Gemeric Unit).
Among minerals identified at the site, becquerelite I is characterized by the highest Ca content and its composition is close to the ideal . On the other hand, becquerelite II is characterized by increased K at the expense of Ca. The average chemical composition of both types of becquerelite can be expressed by the empirical formulae: (Ca0.85K0.04Na0.01Fe0.02Zn0.02Ba0.01Pb0.01)Σ0.96[(UO2)6O4(OH)5.84]·8H2O (becquerelite I), and (Ca0.36K0.27Na0.01Fe0.02Zn0.02Pb0.01Bi0.01)Σ0.78[(UO2)6O4(OH)5.29]·8H2O (becquerelite II). Vandendriesscheite with unusual chemical composition imitates a transition phase between gauthierite and vandendriesscheite. Negative correlation of K vs. Pb indicates that in the studied mineral phase lead is partially replaced by potassium (and other cations). An average chemical composition of the studied vandendriesscheite can be expressed as: (K0.49Na0.02)Σ0.51(Pb1.20Fe0.05Zn0.04Ba0.03Sr0.02Al0.02)Σ1.36[(UO2)10O6(SiO4)0.05(PO4)0.02(OH)10.86]·11H2O. A leesite-like phase with an average composition (K0.72Sr0.01Ba0.02Fe0.03Zn0.01Pb0.02Al0.02)Σ0.83(H2O)2[(UO2)4O2(SiO4)0.01(OH)5,00]Σ11,01·3H2O, was found only rarely. An unnamed mineral phase, with chemical composition close to lepersonnite-(Gd), designated as “calciolepersonnite”, is younger at the studied site then uranyl-oxide hydroxy-hydrates. Compared to the ideal lepersonnite-(Gd) formula, there is a lower REE content at the cationic position, an increased Ca and there are also monovalent cations (especially K) entering the structure. An average “calciolepersonnite” chemical composition is: (K0.62Na0.09)Σ0.71(Ca2.08Mg0.04Sr0.02Ba0.02Fe0.05Zn0.05Pb0.03)Σ2.30(Y + REE)Σ0.92[(UO2)23.76{(SiO4)3.19(PO4)0.11(AsO4)0.02(SO4)0.02}Σ3.34(CO3)8(OH)26.37]·46.82H2O.
In the supergene zone of J-1 vein at Gemerská Poloma, three stages of development can be defined: (I) formation of uranyl-oxide hydroxy-hydrates that directly, partially or completely, replace UIV minerals; (II) formation of uranyl carbonate-silicates (i.e., “calciolepersonnite”) that replace uranyl-oxide hydroxy-hydrates, apparently indicating shift to relative acid environment (but still remaining alkaline to neutral pH) and (III) formation of uranyl phosphates/arsenates of the autunite group (“uranium micas”) that precipitated relatively far from accumulations of primary (UIV) minerals (in cracks and cavities of the gangue, or in the surrounding non-mineralized rocks). Their origin documents the change of alkaline-neutral to acidic environment, due the more advanced weathering of vein sulphides. Given the absence of Y + REE in older uranyl-oxide hydroxy-hydrates that directly replace brannerite, most of these elements required for “calciolepersonnite” formation were probably released from the host rocks and not from the primary, hydrothermal uranium minerals.
Marco E. Ciriotti

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