Abstract
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 growth-controlling process for the Ostwald ripening, which occurred during the late stages of illitization. However, the normalized particle-size distribution curves fitted to a log-normal distribution curve better than the theoretical profile calculated for a screw-dislocation model by Chai (1975), as found by Eberl et al. (1990). The ripening illite crystals changed in morphology from laths to hexagons while they grew at a constant ratio of particle diameter to thickness (approximately 40), as was estimated from the mean volume and area of the particles. The ratio is consistent with the measured ratio of step separation to step height of the growth spirals. This morphological evolution forms a trend on a plot of mean diameter vs. mean aspect ratio that is distinct from a trend delineated for illitic minerals from diagenetic shales and bentonites. The difference in trends may be related to differences in the growth rate of illite crystals during ripening among various rock types. The most likely factor controlling the growth rate of illite crystals during ripening might be variations in the level of supersaturation with respect to illite in the pore solution by advective solute flux from outside of the system. In the rocks associated with a high-porosity, fluid-dominated system, such as hydrothermally altered pyroclastics from Kamikita, a percolating flow may facilitate growth of larger illite crystals during ripening, which would result in a coarsely skewed particle size distribution, as found in this study.
