Disequilibrium in cation exchange experiments between Na-K-Ca amphiboles and aqueous Na-K-Ca chloride solutions; effects of fractional crystallization

The cation exchange equilibria richterite (synth) + CaCl (sub 2) (aq) = tremolite + 2NaCl(aq) and richterite (synth) + KCl(aq) = K-richterite (synth) + NaCl(aq) in the system K-Na-Ca-Mg-Si-O-H-Cl have been investigated by hydrothermal experiments at 700 degrees C/2 kbar. The product amphiboles do not lie on the binary joins, but form chemically inhomogeneous ternary or quaternary solid solutions containing tremolite and/or Mg-cummingtonite as additional components. The richterites are free of triple and quadruple chains, but the tremolites are structurally disordered chain silicates with abundant chain multiplicity faults. Diopside, talc or forsterite occur as minor additional phases, forcing the remaining bulk compositions away from the binary amphibole systems. Richterite is only stable in an aqueous solution with a very low X (sub Ca) = 2Ca/(2Ca + Na). EPMA results show a compositional gap on the binary richterite-tremolite in the range Tr (sub 80) Rc (sub 20) to Tr (sub 40) Rc (sub 60) , interpreted as a miscibility gap at 700 degrees C/2 kbar. The chemical compositions of the amphiboles, the results of SEM and HRTEM, and observed chemical inhomogeneities are explained as a result of a dissolution-fractional crystallization process; the product amphiboles grow epitactically on the dissolving end-member reactants, and may be infected by any structural defects of the latter.