Siliceous Phytogenic Particles in Volcanic Ash

Abstract

A great many examples of soils have been known to contain siliceous phytogenic particles called, also, plant opal, grass opal, phytolith, etc., which are originated from siliceous fillings of cells in plant grasses. The contents of the siliceous phytogenic particles in soils are in the positive relation to the humus contents.
The same siliceous particles as in soils are also found in the various Quaternary volcanic ash deposits in which zones of buried soils (dark zones or cracky zones) are more abundant in the siliceous particles.
The siliceous phytogenic particles are low (2.10-2.15) in specific density, very low (1.44-1.45) in refractive index, and are optically isotropic. They reveal only very broad diffraction near 4Å in X-ray diffraction diagram. Silica is predominant component of them. All above characteristics prove the particles to be opal. The sizes and shapes of the siliceous phytogenic particles seem to be controlled by the cells wherin they deposited. Thus, the fan-shaped ones are about 0.04mm. large, the rod-shaped ones are 0.02mm. wide with maximum length of 0.5mm., and the dumbell-shared ones are about 0.01mm. large (Fig. 1).
The rather common occurrence of the siliceous phytogenic particles in the Quaternary volcanic ash deposits calls our attention in the following two aspects.
First, the siliceous phytogenic particle is a possible indicator of a buried soil in the volcanic ash deposits with supplemental field and in-door evidences such as content of organic matter, degree of weathering, etc. (Fig. 2).
Second, they must suggest the presence of plant grasses which grew in situ on the subaerial ground where volcanic ash deposited. Although some promissing results are in hand, it is riserved for further precise study whether species or genus names of the plant grasses, otherwise only the family names can be identified by them.