Abstract
On the wall of a tunnel dug in sandy tuff of Fukuda formation of Neogene system southwest of Yoshimi hill in Yoshimi Town, Hiki County of Saitama Prefecture, sulfate minerals have crystallied on the surface due to weathering of rocks. Amount of crystallization changes considerably between dry and wet periods, and distribution and mineral species also vary. Minerals confirmed with results of XRD as surface crystals are mostly alunogen, jarosite and gypsum, accompanied with halotrichite, epsomite, thenardite, tamarugite, and sodium alum as well. Crystallization of these minerals, particularly of alunogen, on the weathering process of rocks indicates that this weathering is advancing in strongly acidic environment compared with that of ordinary weathering. Alunogen is a mineral that is originally formed in concern with water vaporization in a strongly acidic environment as a secondary mineral mainly of volcanic sublimate, sinter and sulfide deposit. At this site of study, alunogen crystallizes at ordinary temperature by covering rack surface in skinny shell, and in paragenesis with jarosite. It is regarded by observing formation process of the crystallization and its distribution that alunogen crystallized with action of water strongly acidified by the formation of jarosite and oxidation of pyrite and H2S.
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