K-Na exchange equilibria between muscovite-paragonite solid solution and hydrothermal chloride solutions

Three ion exchange equilibrium isotherms between muscovite-paragonite solid solution and 2molal KCI-NaC1 aqueous solutions have been studied at (1) 420 ~ 1 kbar, (2) 420 ~ 2 kbar, and (3) 550 ~ 2 kbar. The AG ~ (Joules) + 2a of the ion exchange reaction are: AG ~ (1) = 17 259_+ 686, AG ~ (2) = 18 268 _ 560, AG ~ (3) = 16 018 _ 336. The excess mixing parameters ('subregular solution') of the solid solution (at 1 bar) have been calculated: Wvaras.(J/mol ) = 5996 _ 1816 (420 ~ 6348 _ 624 (550 ~ W~ ...... (J/mol) = 14781 _ 1448 (420 ~ 16 785_ 554 (550 ~ The corresponding binodal compositions are (muscovite mol fraction): 12-56 % at 420 ~ 1 bar and 15-51% at 550 ~ I bar. The compositions of micas in equilibrium with perthites (high structural state) at 400, 500, 600 ~ and 2 kbar are respectively: Xmu s = 91, 86, and 82%. The mixing properties of the solution were estimated using the speciation of two molal chloride solutions calculated from the dissociation constants of NaC1 and KC1 in aqueous solution. Although NaC1 appears to be substantially more dissociated than KCI, the resulting excess free energy of mixing of the hydrothermal (Na,K)CI solution was found less than 500 J at temperatures above 400 ~ and pressures up to 2 kbar. The difference in Gibbs free energy of formation (from the elements at 25 ~ 1 bar) between NaC1 and KC1 in two molal aqueous solutions is proposed: AG~cl-AG~ci = 18 152t9A1 T Joules (at 2 kbar). KEY W O R D S: muscovite, paragonite, ion exchange, chloride solutions. IN a preceding work (Pascal and Roux, 1982) we reviewed the exper imental da ta on equi l ibr ium between hydro the rma l (Na, K)CI solut ions and sodi-potassic silicates. This enabled us to propose an est imate of the difference in Gibbs free energy of format ion (from the elements at 25 ~ 1 bar) between KC1 and NaC1 in aqueous solut ion of molal i ty > 0.5, between 400 and 800 ~ and 1 and 2 kbar. The set of exper imental da ta studied was Copyright the Mineraloffical Society found to be remarkab ly self-consistent. However, the need has emerged for more accurate da ta on the equi l ibr ium between pa ragon i t e -muscov i t e solid solut ions and hydro the rma l (Na,K)C1 solutions. In this paper we present three new isotherms at 420 ~ 1 and 2 k b a r and 550 ~ 2 kbar . These new data, a long with ano the r i so therm obta ined by Chat ter jee and Flux (pers. comm.) at 7 kba r and 620 ~ cover large P, T ranges (limited by the stabil i ty of the mica solid solution) where the dissociat ion cons tan t of NaC1 and KCI are known. We have therefore a t t empted to improve the t he rmodynamic t r ea tment of these ion-exchange equil ibria taking into account the dissociat ion of the salt, and es t imat ing the effect on the macroscopic propert ies of the hydro the rma l solutions. Exper imen ta l procedure A gel on the join muscovite-paragonite and a (Na,K)CI-H20 solution were sealed in a gold capsule. The solids crystallized and equilibrated with the solutions during the run in a conventional cold-seal pressure vessel. After the run the solutions and solids were recovered quantitatively, by freezing the gold capsule in liquid nitrogen (to prevent loss of solution), cutting it open, and flushing its content into a plastic vial. The solution was then diluted and analysed by flame photometry for K and Na. For the 550 ~ isotherm, the bulk composition of the mica was also determined by flame photometry, taking care of A1 interferences. When the solid was not analysed, its composition was calculated from that of the solution after the experiment, assuming that the total number of alkali elements in the hydrothermal solution did not vary during the experiment (e.g. due to the solubility of the mica in the hydrothermal solution), from the relations: N'.n' + X'.x ' = N".n" + X".x" (1) n' = n" (2) x' = x" (3) where X, N, are the mole fractions of the K end-member in the solid and solution respectively, x and n the number of moles of alkali in the solid and solution respectively. (') stands for starting material, (") for final products. 516 M. L. P A S C A L A N D J. R O U X Table I summarizes the experimental data (composition, and weights of starting and run products) for each isotherm. Because special care was taken to ensure an accurate recovery of the solution, it was possible to make a direct check of the validity of relation (2) which was found to be satisfied within analytical uncertainty for most of the runs. For those runs where the solid was analysed there was a good agreement between the analysed and calculated compositions of the solid. The run durations were 3 weeks for the 550 ~ isotherm and 3 to 5 months for the 420 ~ isotherms. There was evidence that the equilibrium between the solid and the solution was achieved: (a) the runs using different starting materials (muscovite or paragonite gels) gave identical TABLE I. Muscovite-Paragonlte ion-exchange isotherms : experimental data Composition of the starting products: KS: K mol/Kg of starting solution Nas: Na mol/Kg of starting solution ms: weight of starting solution (mg)