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Reversed Na-K exchange data between mica and a 2 molal aqueous(Na,K)Cl fluid (Flux & Chatterjee, 1986) have been employedto model the thermodynamic mixing behaviour of muscovite-paragonitecrystalline solutions on the basis of the Redlich-Kister equation.For these binary micas, Gexm may be expressed as
where A=11222+1.389 T+0.2359 P, B=–1134+6.806 T–0.0840 P, and C=–7305+9.043 T, with T in K, P in b, Gexm, A, B, and C in joules/mol. Gmex is well constrained between 450 and 620?C, and may be extrapolatedbeyond that range with caution. The calculated solvi are skewedtoward the paragonite end member. In the range up to 15 kb,the critical temperature, Tc and the critical composition, Xcmay be expressed as a function of P by the relations:
and
with P indicated in bars. Calculated phase relations of muscovite-paragonite crystallinesolutions have been depicted in terms of the system KAlSi3O8-NaAlSi3O8-Al2O3-SiO2-H2O.These data may be applied to appropriate assemblages involvingmica, alkali feldspar, an Al2 polymorph, and quartz to estimateP, T and aH2O conditions of their equilibration. In principle,the muscovite limb of the solvus may be used to obtain geothermometricdata for coexisting muscovite-paragonite pairs, provided theequilibrium pressure is independently known. However, such applicationmust be restricted for the present to micas on the ideal muscovite-paragonitejoin. Mica-alkali feldspar-Al2SiO5-quartz or mica-plagioclase-Al2SiO5-quartzassemblages may be used to deduce aH2O in the coexisting fluid,if P, and T of equilibrium are independently known. Examplesof such geological applications are given. 相似文献
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Single-phase 2M1 muscovite-paragonite crystalline solutionsin the range 0?00–0?10 and 0?70–1?00 Xms have beensynthesized by hydrothermal treatment of gels of appropriatecompositions at 600–700?C, and 7 to 18 kb PH2O. The molarvolumes of these micas may be expressed as V(J/b?mol) = 13?1845+1?463Xms+0?0160 Xms2–0?1679 Xms3 (?0?005), which translateto a substantial positive excess molar volume of mixing. Na-K ion exchange experiments between presynthesized 2M1 micacrystalline solutions and 2 molal aqueous (Na,K)Cl fluids failedto proceed to completion despite 98 day runs at 500–600?C,6 kb Ptotal. Results of analogous exchange experiments provedencouraging however, when a much finer-grained 1M mica was usedas starting material. Applying the tie line rotation technique,reversal of ion exchange experiments could be achieved in the2-phase fields, not, however, in the 3-phase field of the ms-pg-NaCl-KClreciprocal ternary. Using gels as starting material, reversalexperiments were eventually successful both in the 2-phase andthe 3-phase fields; the results of reversal experiments withinthe two-phase fields being identical to those obtained earlierusing 1M micas. Four isobaric-isothermal sections through the ms-pg-NaCl-KClternary were reversibly determined at 450?C/5 kb, 550?C/6 kb,550?C/15 kb, and 620?C/7 kb. At 450?C, the coexisting mica compositionsin the 3-phase field (2 micas plus 1 fluid) are 0?10 and 0?77Xms, at 550?C they are 0?10 and 0?60 Xms, and finally, at 620?Cthese are 0?12 and 0?51 Xms. To the extent that internal equilibriumwas accomplished between the coexisting micas, these data wouldindicate a wide solvus at 450?C, narrowing gradually with increasingtemperature to 620?C. The critical temperature will be wellin excess of 620?C, although the mica at the critical conditionwill prove to be metastable with respect to the assemblage alkalifeldspars+corundum+H2O. The companion paper by Chatterjee & Flux (1986) presentsa thermodynamic analysis of the above experimental data. 相似文献
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