ALTERATION OF VOLCANIC GLASS OF RECENT TOWADA ASH IN DIFFERENT SOIL ENVIRONMENTS OF NORTHEASTERN JAPAN

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Abstract
The alteration of silicic volcanic glass of recent Towada ash in the different soil environments was studied by means of electron-microprobe analysis. Two types of glass alteration (tentatively named a and b types) were established and were distinguished on the basis of K and Ca contents of the glass samples. The a type glass alteration was characterized by a very large gain of K and a large loss of Na. This alteration was explained by the ion exchange reaction between Na+ and H+ plus K+ without significant change in the glass structure. The b type alteration characteristically showed a significant loss of K and a large gain of Ca and some other elements. The behaviors of alkalies and alkaline earths in the b type glass could not be interpreted by the stoichiometry of exchange processes of these elements. Changes of glass structure during weathering were suggested for the b type glass. There were close relationships between the mode of glass alteration, clay mineralogy of ash-derived soils, and soil environments. Only a type glass alteration was observed in the 2:1 mineral soils that showed the very low pH, very low contents of exchangeable bases, but relatively high content of exchangeable K. In contrast the b type glass alteration was the most intense in the allophanic Ando soil, which showed a relatively high content of exchangeable bases and a high pH.