Original paper

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

Zimmermann, Rolf; Knop, Erich; Heinricrf, Wilhelm; Pehlke, Inge; Franz, Gerhard

European Journal of Mineralogy Volume 9 Number 1 (1996), p. 97 - 114

32 references

published: Dec 30, 1997
manuscript accepted: Sep 16, 1996
manuscript received: Jun 18, 1996

DOI: 10.1127/ejm/9/1/0097

BibTeX file

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Abstract We have investigated the cation exchange equilibria (1) richterite (synth.) + CaCl2(aq) = tremolite + 2NaCl(aq), and (2) richterite (synth.) + KCl(aq) = K-richterite (synth.) + NaCl(aq) in the system K-Na-Ca-Mg-Si-O-H-Cl by hydrothermal cation exchange experiments at 700°C/2 kbar. In both reactions the product amphiboles do not lie on the binary joins. Instead they form chemically inhomogeneous ternary or quaternary solid solutions containing tremolite and/or Mg-cummingtonite as additional components. The synthetic starting and product richterites are generally free of triple- and quadruple-chains. In contrast, the tremolitic reaction products from reaction (1) are structurally disordered chain-silicates with abundant chain multiplicity faults. Diopside, talc or forsterite occur as minor additional synthetic phases and force the remaining bulk compositions away from the binary amphibole systems. Therefore the experimental results give only qualitative information on the Na-K- and the Na-Ca-distribution between amphiboles and the coexisting fluid phase. The equilibrium for reaction (1) is strongly shifted to tremolite + 2NaCl(aq). Richterite is only stable in an aqueous solution with a very low Xca = 2Ca/(2Ca+Na). Microprobe analyses of the run products show a compositional gap on the binary richterite-tremolite in the range tremolite^o-richterite^o to tremolite^o-richterite^o, interpreted as a miscibility gap at 700°C/2 kbar. In reaction (2) Na and K distribute about equally between the solid and the fluid phase. The chemical composition of the amphiboles, results of SEM and HRTEM investigations, and the observed chemical inhomogeneities are explained as a result of a dissolution-fractional crystallization process. The product amphiboles grow epitactically on the dissolving endmember reactants, and HRTEM investigations suggest that they are infected by any structural defects of the latter


amphibolescation exchangekineticsreaction mechanism