OPALINE SILICA OF RECENT VOLCANIC ASH SOILS IN JAPAN

Abstract

SummarySoil samples were investigated from three different climatic regions of Japan. The soil profiles were well drained and consisted of a number of buried soils. Organic matter and free iron oxides were removed from the samples which were then fractionated. Laminar opaline silica particles occurring in the 0.2 to 5 μm fractions were examined electron‐optically following ultrasonic and cold 5 per cent sodium carbonate treatment.Four types of opaline silica particles were distinguished: circular, elliptical, rectangular, and rhombic. The opaline silica particles are extremely thin. They show fine‐grained uneven surfaces and weathered and alkali‐treated opaline silica particles appear to be very porous suggesting that the particles are actually composed of extremely fine silica spheres. Opaline silica panicles occur only in the 0.2 to 5 μm fractions and are most abundant in the 0.4 to 2 μm fractions. Of the four types the circular and elliptical types predominate. The elliptical type is the most common in the fine fraction and the circular type in the coarse fraction.Compared with B‐ or C‐horizons, the A‐horizon of a profile tends to be relatively rich in opaline silica. Opaline silica particles are abundant in recent volcanic ash soils, particularly in Hokkaido soils < 500 years old. Soils of this area more than 7000 years old contain opaline silica particles whereas they are rare in Kanto soils > 6000 years old and in Kyushu soils > 4000 years old.Hokkaido has much lower mean annual temperatures and rainfall than Kanto or Kyushu. This may have caused slower weathering of opaline silica in the Hokkaido soils.Conditions for formation of opaline silica may be less favourable in subsoils because supersaturation of silica and concentration of bases is less likely to occur in subsoils than in surface soils.