Experimental Study of RbAlSi3O8–BaAl2Si2O8 Solid Solutions: Cation-Exchange Equilibria with Fluid

The cation exchange reaction 2RbAlSi3O8 + BaCl2(aq) = BaAl2Si2O8 + 4SiO2 + 2RbCl(aq) was studied under hydrothermal conditions (T = 650°C and P = 2 kbar). It was shown that Ba partitions preferentially into (Rb,Ba)-feldspar relative to fluid over the entire compositional range. The unit-cell parameters of (Rb,Ba)-feldspar solid solutions were refined. The excess mixing volume is negative and can be described by the two-parameter Margules model: WV1 = –2.11 ± 0.05 and WV2 = –1.85 ± 0.06 cm3/mol. Based on the distribution of Rb and Ba between feldspar and fluid, the excess mixing energy of the solid solution was estimated: WG1 = 6.82 ± 0.54 and WG2 = 12.25 ± 0.81 kJ/mol. The calculated excess mixing functions were compared with those of other feldspar solid solutions.