Pegmatoid segregations containing three polymorphous Al2SiO5 modifications have been revealed in metamorphic rocks of the Tsel block localized in the Hercynian belt on the southern flank of Mongolian Altay. Petrographic study showed a successive substitution of polymorphs in the sequence: andalusite–kyanite–fibrolite–sillimanite. Estimated parameters of the host-rock metamorphism indicate that the mineral assemblage of pegmatoid veins formed at two successive stages of metamorphism: andalusite-sillimanite and kyanite-sillimanite. It is suggested that the transformation of Al2SiO5 from one to another polymorphous modification occurs by the ion exchange mechanism with the participation of muscovite.

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