Sequential formation of natrolite-group zeolites in amygdules of basaltic lavas

Natrolite-group minerals were found in amygdules in Middle Eocene basaltic lavas in northern central Anatolia and have been evaluated in detail by petrographical, mineralogical, and chemical studies. A sequential formation of zeolites, which occurred in response to the interaction of the host rock with heated water, was determined by examination of the mineral assemblage. The compounds necessary for the formation of secondary minerals might have been derived from alteration of volcanic glass and Ca-rich pyroxenes in the host rock as well as from the associated marine sediments. Formation of fibrous zeolites on the walls of the amygdules began with a Na-rich zeolite, followed by a progressive increase in Ca content, and ended with a Ca-rich zeolite. The growth of natrolite as the earliest phase starts around an unidentified nucleus. It is followed by mesolite, which forms epitaxial overgrowths with natrolite. Scolecite is the last zeolite in the crystallization sequence. A continuous increase of the Si/Al ratio is observed from the earlier phase towards the later ones. The latest phase occupying the cavities within the amygdules is calcite.