Consideration on the Weathering Process of Volcanic Ash on the Basis of Clay Mineralogy

Abstract

The principal clay minerals in the volcanic ash soils in Pleistocene age in Japan are allophane in the younger volcanic ash soils and hydrated halloysite in the older.Examining clay mineralogical properties of the volcanic ash soils by means of X-ray, thermal analysis and electron-micrographic observation, the writers postulate a sequence of crystallization process in the course of weathering of volcanic ashes, and confirm that the clay mineralogical investigations are useful to decide the stratigraphical features of volcanic ash soils.The change from allophane to hydrated halloysite may take place throughout the following several stages of crystallization process.The initial stage is characterized by the forming of allophane with the decomposition of volcanic glass, which corresponds to the weathering process of the upper-most part of the volcanic ash bed in Pleistocene age, e. g. the Tachikawa Loam member in the Kanto Loam consisting of volcanic ash.The second stage is remarkable in the presence of allophane associated with small amounts of the lower crystalline hydrated halloysite, showing the fine rounded grain on the morphological features. As the results of the writers' studies it is no doubt that hydrated halloysite is crystallized out from allophane. This stage is recognized at the weathering process of the younger volcanic ash soils such as the upper part of the Musashino Loam member of the Kanta Loam.In the third stage, large amounts of allophane crystallize out to hydrated halloysiet, revealing morphologically the chestnut-shell shape as pointed out by Sudo.The final stage is characterized by the formation of pure hydrated halloysite which has the higher degree of crystallinity than that of hydrated halloysite in the second or third stage. The particles of pure hydrated halloysite are generally elongated tubular in shape. The final stage progresses in the older volcanic ash soils, e. g. the Tama Loam member in the Kanto Loam.Generally speaking, the crystallization process of volcanic glass may be intimately related to the geological time of volcanic ash soils. It does not, however, necessarily follow that crystallization at the same geological time advances at the similar grade.The black band (presumed as ‘the fossil soil’ in volcanic ash) intercalated into brown soils in the Tachikawa Loam member shows the slightly more progressive stage than the crystallization stage in the brown soil.The marine deposits of volcanic ash have a peculiar clay mineralogical feature as compared with that of aeolian deposits. They generally include random mixed layer minerals of halloysite and hydrated halloysite which is formed by imperfect dehydration of hydrated halloysite.Taking the above mentioned two results into consideration, the writers intend to emphasize the influence of the environments wherein volcanic ashes are deposited.The relation between the starting material and the crystallization process will be lastly discussed.The crystallization process of the pumice soil is harder than that of the brownish volcanic ash soil.This fact may be concerned with the follwing factors: (a) mineral component of pumiceous materials, and (b) their grain size.In the light of above discussion, the writers are in the present step utilizing the results of clay mineralogical studies of volcanic ash soils for the purpose of the relative correlation among several volcanic ash beds.