Abstract
The pyroclastic deposit in Kusu formation in Kusu, Oita prefecture was surveyed. Average particle sizes in the collected five areas range from 11 to 150μm. Particle size of the upper level in the pyloclastic deposit was smaller than that of the lower level. Main constituents of minerals in the samples were volcanic glass, feldspar, hornblende and quartz. Two kinds of upper level samples were separated by elutriation method to three size fractions of 5-10μm (S), 10-20μm (M) and 20-45μm (L), of average particle diameters of 9μm, 18.5μm, 28μm, respectively. Particle densities of samples were 2.35 g/cm3 in average. The separated samples were heat-foamed for a few seconds at 1,000 °C in a furnace that was designed for the heat-treatment of small size particles. Expanding ratios of the partially foamed S samples after heat treatments were determined to be from 1.11 to 1.21, and the particle densities from 1.95 to 2.12 g/cm3. Expanding ratios of the foamed L samples ranged from 2.65 to 2.66, and the particle densities from 0.88 to 0.89 g/cm3. Floating part in water of the foamed S samples was smaller in mass-ratio of less than 2 %；however, that of the foamed L samples was relatively large, from 56.8 % to 61.5 %. The shapes of the foamed particles were almost spherical.
