Procedure for hydrothermal transformation of natural clinoptilolite at 160°C is described. The synthesis is carried out in the system Na2O-Al2O3-SiO2-H2O. Products of 24–168 hr runs are characterized by x-ray diffraction,ir spectroscopy, energy dispersive analysis of x-rays (edax), and electron microscopy. Presence of analcime-type and faujasite-type zeolites in the final products is established. Si/Al ratio of the final mixed phase is estimated as 2·91. Spheroid crystals of analcime, fibrous crystals of mordenite (on analcime), and well developed polyhedral crystals of analcime-faujasite mixed phase are illustrated and described. It is shown that crystals grow by two-dimensional nucleation mechanism.