Cation-exchange in zeolite crystals: experimental study and geological implications
|Location||International Geological Congress,oslo 2008|
|Holding Date||29 September 2008|
The ion-exchange phenomenon is well known for zeolites. Many investigations and publications are devoted to the experimental study of ion-exchange properties of zeolites and their industrial potential, but reactions mechanisms in each crystal and products of natural ion-exchange were not studied to recent time. In our experiments were used 7 zeolites with different framework density (natrolite, thomsonite, heulandite, paranatrolite, phillipsite, gmelinite, chabazite) from hydrothermal veins of Khibiny alkaline complex (Kola, Russia) and Siberian traps. In the experiments crystals (1-2 mm) of zeolites were placed to 1N aqueous solutions NaCl, KCl, CaCl2, SrCl2, BaCl2 and combined solutions, obtained from equal volumes 1N Na, K, Ca, Sr and Ba chlorides. NaK, KBa, CaSr, CaSrBa, NaKCaSrBa combinations were used. There were 3 experiments: t-20°C, duration 1.5 months; t-80°C duration 12 hours; t-150°C, duration 3 hours. The cation composition of minerals after the experiments was studied by electron-microprobe on cross-sections. Zeolites with high framework density (natrolite, heulandite, thomsonite) do not exchange cations in any solutions. Gmelinite exchange all cations in all solutions actively. Other zeolites (phillipsite, paranatrolite, chabazite) show their cation-exchange selectivity. The main cations, which enter into: chabazite - is K; phillipsite - Ba. Paranatrolite exchange cations in cases, when Na is absent in the solution. Sr is very rare element in natural zeolites. It does not enter into phillipsite and paranatrolite in the room conditions. In warm solutions it can slowly displace Na and K. We can see it in peripheral parts and in zones along cracks and other defects. In hot solutions the exchange process is quicker. Additional amounts of Ca do not enter into zeolites in the room conditions. It practically can not be displaced by other elements in cold solutions. When the temperature is increasing K, Ba and Sr can partly displace Ca. Natural cation exchange is very important process in the nature. For example: it can explain strong potassium enrichment of chabazite in alkaline complexes of Kola peninsula; harmotom forming from other phillipsites in each solution in nature which contain small amount of Ba.