Infiltration of clay into pores and cavities of Neogene basalts from Eastern Sayans

Clay tonsils mark permeable porous boundaries of basaltic flows. Poor penetration of water is expressed in local occurrence of smectite-sepiolite tonsils with admixtures of halloysite and polygorskite. Increasing water infiltration that results in complete filling of pores and cavities in basalts by the same clay minerals is accompanied by the entry of a goethite admixture. Mature weathering crust on basalts is characterized by smectite-goethite composition without sepiolite. The cryptic clay mineralization of permeable basalt layers might result in overestimating ages in K-Ar system and distortion of primary geochemical signatures of volcanic rocks.