Abstract
Metastable intermediate Na–K mica represents a product of hydrothermal alteration in volcanic rocks from the alteration halo of the Waterloo massive sulfide deposit, Australia. The XRD pattern of this solid solution between paragonite and muscovite is characterized by a rational series of basal reflections with d values intermediate between the end members. Transmission electron microscopy revealed that the intermediate Na–K mica forms thick stacks that belong to a two-layer polytype. Na-rich intermediate Na–K mica typically occurs together with paragonite whereas K-rich intermediate Na–K mica is intergrown with muscovite. The intermediate Na–K mica is interpreted to have formed as a result of the incomplete transformation of K-rich mica to Na-rich mica through dissolution and recrystallization processes driven by compositional changes of the hydrothermal fluids interacting with the volcanic rocks. Alteration must have proceeded under non-equilibrium conditions because the composition of the solid solution falls into the miscibility gap separating paragonite and muscovite.
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Acknowledgements
We express our gratitude to K.P. Becker for carrying out the EMP analysis and to U. Kempe for help provided during the SEM investigations. We gratefully acknowledge the analytical expertise of S. Köhler. We like to thank D.R. Peacor and J.B. Gemmell for valuable comments on the TEM investigations and the geology of the Waterloo deposit, respectively. We are especially grateful to K.J.T. Livi and an anonymous reviewer who helped us to improve an earlier version of the manuscript. The field work at Waterloo was supported by RGC Exploration Ltd. T.M. thanks the German National Merit Foundation for financial support and P.M.H. acknowledges funding by the Leibniz Program of the Deutsche Forschungsgemeinschaft. The XRD investigations were acquired under DFG grants KL 1286/1-1 and 1286/1-2. The TEM facility was financially supported by the Italian National Research Program in Antarctica (PNRA).
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Giorgetti, G., Monecke, T., Kleeberg, R. et al. Intermediate sodium–potassium mica in hydrothermally altered rocks of the Waterloo deposit, Australia: a combined SEM-EMP-XRD-TEM study. Contrib Mineral Petrol 146, 159–173 (2003). https://doi.org/10.1007/s00410-003-0487-5
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DOI: https://doi.org/10.1007/s00410-003-0487-5