Cation and oxygen isotope exchange between plagioclase and aqueous chloride solution.

The exchange reaction 2NaAlSi₃O₈ + CaCl₂ = CaAl₂Si₂O₈ + 4SiO₂ + 2NaCl is one of the important equilibria governing feldspar-fluid interactions. It is characterized by large volume changes in the plagioclase feldspars, which result in significant differences in the mechanism and kinetics of cation and oxygen isotope exchange. Hydrothermal experiments at 700oC and 2 kbar indicate two different exchange mechanisms: 1) a solution-precipitation process in the An- producing direction, resulting in very fast exchange rates and a one-to-one correlation between cation and oxygen isotope exchange, and 2) slow diffusional exchange through an armouring Ab-rich surface layer in the Ab-forming reaction, with O diffusing faster than coupled cations.-R.A.H.