Abstract
Heulandite was synthesized without any natural seed from amorphous solids of the Na4K4Al8Si28O72·Aq-Ca4Al8Si28O72·Aq system (KH system) and the Na3.3K3.3Al6.6Si29.5O72·Aq-Ca3.3Al6.6Si29.5O72·Aq system (KF system). 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 coexisted with mordenite, analcime or wairakite in the range from 275 to 295°C. The compositional ranges of synthesized heulandites including those obtained previously in the clinoptilolite-epistilbite system are Si/Al=3.0-4.5 and Ca/(Ca+NaK)=0.4-1.0, and these ranges are not contradictory to those of natural heulandites.
The synthesized heulandites tend to change morphologically from prismatic to platy habit with increase of the Ca content. The result of heating treatment of the heulandites indicates that their thermal stability increases gradually with increasing the NaK or Si contents. The relationship between chemical composition and thermal behavior of the synthetic heulandites is similar to that of natural heulandites, and the above-mentioned thermal behaviors of the synthetic heulandites raise adoubt about the classification of heulandites based on their thermal behaviors such as heulandite type 1 and type 2 proposed by Alietti (1972).
