Ontogenic types of zonality in crystals of pyrochlore from carbonatites and their diagnostics

摘要

Specific features of pyrochlore crystal structurepredetermine its isomorphic capacity and possibility of ion-exchange processes in it. It is resulted in wide expansion of zonalpyrochlore crystals and differences in the nature of their zonality.The classification of types of pyrochlore zonality in carbonatites isgiven, it includes primary zonality of growth and secondaryzonality of substitution. The first one is presented by roughevolutional zonality(zonality of the first kind) and fine oscillationzonality (zonality of the second kind); the second one is subdividedinto endogenic (arising during autometasomatic changes ofpyrochlore-containing rocks) and exogenic one (developed whilethe carbonatite weathering). Primary zonality is caused by thechange of cation composition in various structural positionsaccording to the principle of isomorphism. Secondary zonality isdeveloped according to the ion-exchange mechanism andassociates with substitution of cations in structural position A andmaintaining of carcassforming cations. Examples andcharacteristic diagnostic features of different types of pyrochlorezonality are described in the paper.