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Minerals 2015, 5(4), 693-706;

Monazite Alteration in H2O ± HCl ± NaCl ± CaCl2 Fluids at 150 ºC and psat: Implications for Uranium Deposits

GeoRessources lab., Université de Lorraine, CNRS, CREGU, Boulevard des Aiguillettes B.P. 70239, F-54506 Vandoeuvre-lès-Nancy, France
Author to whom correspondence should be addressed.
Academic Editor: Mostafa Fayek
Received: 23 July 2015 / Accepted: 9 October 2015 / Published: 16 October 2015
(This article belongs to the Special Issue Uranium Minerals: From Resources to Environmental Impact)
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Spectacular alteration of monazite by diagenetic/hydrothermal brines is well documented in some Proterozoic sedimentary basins in close relationship with high-grade uranium (U) deposits. Hence, monazite has been proposed as a viable source for some U deposits. However, monazite alteration remains enigmatic with regard to its high stability in relatively low temperature hydrothermal conditions. Here, the results of batch experiments in which 10 mg of natural monazite grains were reacted with 15 mL of Na-Ca-Cl (6 molal Cl) solutions as well as in pure water at 150 ºC and saturated vapor pressure (psat) for one and six months are reported. The influence of pH (pH = 1, 3, 7) and relative molar proportions of Na and Ca (Na/(Na + Ca) = 0, 0.5, 1), were tested. Discrete alteration features (etch pits and roughened surfaces) appear in a minority of the one month experiments and are more developed in the six months experiments, especially at pH = 1 and 3. Although spectacular alteration of monazite, as seen around U deposits, could not be reproduced here, this study shows that monazite is unstable in the presence of fluids analogous to acidic deep basinal brines. View Full-Text
Keywords: monazite; hydrothermal; alteration; uranium; experimental monazite; hydrothermal; alteration; uranium; experimental

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Richard, A.; Montel, J.-M.; Leborgne, R.; Peiffert, C.; Cuney, M.; Cathelineau, M. Monazite Alteration in H2O ± HCl ± NaCl ± CaCl2 Fluids at 150 ºC and psat: Implications for Uranium Deposits. Minerals 2015, 5, 693-706.

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