The shape and size of the crystals of mordenite, as well as the Si/Al ratio, have implications as regards industrial applications in hydrocarbon conversion and separation. The ratio between the incorporation yields of silicon and aluminum is inversely proportional to the alkalinity level. The size of mordenite crystals can be modified by a factor of 10 by appropriate aging of the silica source. The alkalinity of the synthesis system is an important factor affecting the morphology of the mordenite crystals, with flatter crystals being formed at low alkalinity.