Mica-feldspar equilibria in supercritical alkali chloride solutions |
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| Authors: | W D Gunter H P Eugster |
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| Institution: | 1. Institut für Kristallographie und Petrographie, ETH-Zentrum, CH-8092, Zürich, Switzerland
2. Department of Earth and Planetary Sciences, Johns Hopkins University, 21218, Baltimore, Maryland, USA
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| Abstract: | The equilibrium position of the reaction $$\begin{gathered} 1.5 KAlSi_3 O_8 + HCl = 0.5 KAl_3 Si_3 O_{10} (OH)_2 \hfill \\ + 3SiO_2 + KCl \hfill \\ \end{gathered} $$ has been located at 1 and 2 kb pressure and temperatures between 600° and 670° C using the Ag-AgCl buffer. These data can be combined with information on the dissociation of KC1, HC1 and H2O to determine species abundances in supercritical aqueous fluids in equilibrium with muscovite — K-feldspar — quartz assemblages. Chloride species become increasingly associated with increasingT, increasing total molality, (m tot or \(m_{Cl_{tot} } \) ), and decreasing \(P_{H_2 O} \) . Master variable diagrams indicate that the pH of the solutions may vary from near neutral to quite acid. Published data on the paragonite-albite-quartz reaction and exchange reactions involving feldspars and micas were included to calculate speciation in mica-feldspar-NaCl-KCl-HCl-H2O fluids at 2kb pressure and temperatures between 300° and 600° C. The data are not accurate enough to distinguish different feldspar structural states. Concentration gradients were calculated for individual species between K-feldspar+quartz, muscovite+quartz and andalusite+quartz assemblages at 500° C, 2 kb. Assuming that the proton diffuses most rapidly and that there are no H+] gradients, the molality of the solution must vary 30-fold, with feldspar+quartz at the more concentrated side. The data on mica-feldspar-chloride equilibria are used to interpret the spacial distribution of micas, feldspar and quartz in microfolds. This distribution can be accounted for by pressure solution, due to the fact that non-hydrostatic pressure affects congruently dissolving minerals, auch as quartz, differently from minerals which dissolve incongruently, such as micas and feldspars. We postulate, that during folding at constant \(P_{H_2 O} \) ,T and \(m_{Cl - } \) , gradients in KC1 and SiO2 are created by stress differences between hinge and limb of a microfold, such that both migrate to the hinge area where quartz precipitates and muscovite is converted to K-felspar, thus accounting for the observed mineral distribution. |
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