EXPERIMENTAL REVERSAL OF THE NA-K EXCHANGE REACTION BETWEEN MUSCOVITE-PARAGONITE CRYSTALLINE SOLUTIONS AND A 2 MOLAL AQUEOUS (NA,K)CL FLUID

Abstract

Single-phase 2M1 muscovite-paragonite crystalline solutions in the range 0?00–0?10 and 0?70–1?00 Xms have been synthesized by hydrothermal treatment of gels of appropriate compositions at 600–700°C, and 7 to 18 kb PH2O. The molar volumes of these micas may be expressed as V(J/b?mol) = 13?1845+1?463 Xms+0?0160 Xms2?0?1679 Xms3 (?0?005), which translate to a substantial positive excess molar volume of mixing. Na-K ion exchange experiments between presynthesized 2M1 mica crystalline solutions and 2 molal aqueous (Na,K)Cl fluids failed to proceed to completion despite 98 day runs at 500–600°C, 6 kb Ptotal. Results of analogous exchange experiments proved encouraging however, when a much finer-grained 1M mica was used as starting material. Applying the tie line rotation technique, reversal of ion exchange experiments could be achieved in the 2-phase fields, not, however, in the 3-phase field of the ms-pg-NaCl-KCl reciprocal ternary. Using gels as starting material, reversal experiments were eventually successful both in the 2-phase and the 3-phase fields; the results of reversal experiments within the two-phase fields being identical to those obtained earlier using 1M micas. Four isobaric-isothermal sections through the ms-pg-NaCl-KCl ternary 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 compositions in the 3-phase field (2 micas plus 1 fluid) are 0?10 and 0?77 Xms, at 550°C they are 0?10 and 0?60 Xms, and finally, at 620°C these are 0?12 and 0?51 Xms. To the extent that internal equilibrium was accomplished between the coexisting micas, these data would indicate a wide solvus at 450°C, narrowing gradually with increasing temperature to 620°C. The critical temperature will be well in excess of 620°C, although the mica at the critical condition will prove to be metastable with respect to the assemblage alkali feldspars+corundum+H2O. The companion paper by Chatterjee & Flux (1986) presents a thermodynamic analysis of the above experimental data.