1981 年 15 巻 Special 号 p. 223-238
Clay minerals are often found in the ejecta from volcanos which have been active for 20 years. Mineralogical and chemical properties of these clay minerals are described. It was found that these ejecta had been already altered by various geological actions before their erupting and their alteration products are different from each other in the different places and conditions. Opal is formed in the acid condition, allophane and halloysite in the neutral condition and montmorillonite in the basic condition. The writers have come to be able to get many inf ormations about volcanic activities by having a good knowledge of clay minerals contained in the ejecta from volcanos.