Thermal Stability of Woolly Erionite-K and Considerations about the Heat-Induced Behaviour of the Erionite Group

The thermal behavior of a woolly erionite-K sample (Lander County, NV, USA), chemical formula (Ca2.03Na0.73K2.52Mg0.26)[Al8.22Si27.78O71.80]center dot 35.94H(2)O, was investigated in the 303-1173 K thermal range by in situ X-ray powder diffraction. Present data suggest a general thermally-induced volume contraction whose magnitude increases as Si/Si+Al ratio becomes smaller. An inverse correlation between Si/Si+Al ratio and T-dehydr is observed because higher Si/Si+Al ratio values are associated to lower dehydration temperatures. A positive dependence exists between Si/Si+Al ratio and T-break. A higher Si content results in a greater thermal stability, in agreement with the general trend observed in zeolites. On the contrary, no correlation has been found between T-break and weighted ionic potential (Z/r)(wt) as suggested by reference data. Heating produces a general depletion of the Ca1, Ca2, Ca3, and K1 sites, which is counterbalanced by an increase of the K2 site scattering, even though the latter is not populated at RT. No internal ion exchange mechanism was apparently acting in the present sample differently from other erionite samples analysed in the past. At 303 K approximately 20 e(-) allocated at the OW H2O sites might be assigned to (extra-framework) EF cations. Such fraction increases due to their migration from the extra-framework cation sites following the same mechanism reported in reference data.