The morphological characteristics, occurrence and weathering features of opaline silica particles of the volcanic ash soils from three different climatic regions in Japan were studied.
1) Organic matter and free iron oxides were removed from the samples which were fractionated. Opaline silica particles in each fraction were examined electron optically.
2) Four types of opaline silica particles which are extremely thin, were distinguished circular, elliptical, rectangular and rhombic. They show fine-grained uneven surfaces. The weathered opaline silica particles appear to be very porous suggesting that the particles are composed of extremely fine silica spheres.
3) Opaline silica particles occur only in the 0.2 to 5μ fractions, and are most abundant in the 0.4 to 2μ fractions. Of the four types the circular and elliptical types predominate. The former is the most common in the coarser fraction, and the latter the most numerous in the fine fraction.
4) Compared with B- or C-horizons, the A-horizon of a profile is relatively rich in opailne silica particles. They are abundant in the recent volcanic ash solis particularly in Hokkaido soils less than 500 years old. Soils in this district more than 7, 000 years old contain opaline silica particles in small amounts, whereas they are rare in Kanto soils less than 6, 000 years old and in Kyushu soils less than 4, 000 years old. The weathering of opaline silica particles in relation to the age and to the genetic horizon of volcanic ash soils and also to the climatic conditions is considered.