US EPA

6878059 

Journal Article 

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

Zimmermann, R; Knop, E; Heinrich, W; Pehlke, I; Franz, G; , 

1997 

Yes 

European Journal of Mineralogy
ISSN: 0935-1221 

E SCHWEIZERBART'SCHE VERLAGS 

STUTTGART 

97-114 

WOS:A1997WE78200010 

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(ag)in the system K-Na-Ca-Mg-Si-O-H-Cl by hydrothermal cation exchange experiments at 700 degrees C/2 kbar.In both reactions the product amphiboles do not Lie on the binary joins. Instead they form chemically inhomogeneous ternary or quatenary 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 X(Ca) = 2Ca/(2Ca+Na). Microprobe analyses of the run products show a compositional gap on the binary richterite-tremolite in the range tremolite(approximate to 80)-richterite(approximate to 20) to tremolite(approximate to 40)-richterite(approximate to 60), interpreted as a miscibility gap at 700 degrees 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.