Variations in the morphology of illitic clay minerals were observed in the Kamikita hydrothermal system for illitized smectite. Particle sizes and microtopographies were measured by transmission electron microscopy using Pt shadowing and Au decoration. Particle size distributions normalized to the modes gave steady-state profiles skewing toward larger

values both for equivalent diameters and for thicknesses, which were independent of the formation ages and temperatures. Growth spirals were observed commonly on the basal surfaces of illite crystals. These data indicate that spiral growth operated as a growthcontrolling process for the Ostwald ripening, which occurred during the late stages of illitization.