AbstractThermal behaviour of natural heulandite crystals collected from Pashan area near Poona, India, is studied by employing static weight loss method (SWL), X‐ray diffraction, infrared spectroscopy, TG and DTA. The combination of these techniques provide better understanding of the structural modifications that occur in heulandite during thermal treatment. It is observed that heulandite structure is transformed from its ‘A’ phase to heulandite ‘B’ phase between 250–400 °C, and forms another intermediate metastable phase heulandite ‘C’ before the structure starts degrading above 500 °C.