The geological and chemical compositional heterogeneties of the two different types of bentonite occurrence, from the Hoylu and Caltumar areas in Fatsa of Ordu province (NE Turkey), were discussed in terms of silica polymorph structure. The Hoylu occurrence was formed by means of in situ alteration: the Caltumar bentonite was deposited within a volcano-sedimentary sequence and it has undergone diagenetic process. The samples of both areas contain smectite and opal (cristobalite/tridymite) as the main minerals. Feldspar, quartz and calcite are the other minerals observed in the Caltumar samples while the latter two minerals are not observed in the Hoylu samples. The cristobalites in Caltumar samples have strongly ordered microstructure and high crystallinity. The d-spacings of 101 line (4Å band) of these cristobalites are lower than 4.07Å and these silica polymorphs were identified as opal-C. But the opals from the Hoylu samples have disordered microstructure and probably rich in trydimite layers. The d-values of 4Å band of these opal-CT are higher than 4.08 Å.