Synthesis and thermal properties of heulandites in the clinoptilolite-epistilbite system

Abstract

Heulandite was synthesized without any natural seed from amorphous solids of the clinoptilolite-epistilbite system which consists of the Na₃K₃Al₆Si₃₀O₇₂·Aq-Ca_4.5Al₉Si₂₇O₇₂·Aq join (CE subsystem) and the Na_2.9K_2.9Al_5.8Si_30.2O₇₂·Aq-Ca_4.8Al_9.6Si_26.4O₇₂·Aq join (CW subsystem). At PH₂o=1, 000 bars and reaction time =50 days, heulandite was formed as a single phase in the temperature range lower than 275°C and mordenite and analcime (or wairakite) coexisted with heulandite in the range from 275 to 295°C. The compositional range of the synthesized heulandites is Si/Al=3-4 and Ca/(Ca+NaK)=0.5-0.9, and these ranges are not contradictory to those of natural heulandites. The results of heating treatment and DTA of the synthesized heulandites indicate that their thermal stability increases gradually with increasing Si and NaK contents. The relation between chemical composition and thermal behavior of the synthetic heulandites is similar to that of natural heulandites, but a synthetic heulandite of the CE3 composition which is included in the compositional range of heulandite type 2 shows the thermal behavior close to heulandite type 1.