Opaline is an important phase associated with bentonites. The present study is devoted to the structural characterization of opal from various bentonites in Turkey. The goal is to characterize the relation between d-spacings and widths of the main opal peak (4 Å band) based on X-ray diffraction (XRD) diagrams, and to interpret the results in terms of genetic aspects of the bentonites and their associated opal phases. A linear relation was observed between the d-spacing and full width at half maximum (FWHM) values of the 4 Å peak of opal in the bentonite samples studied. The d-spacing and FWHM values of the 4 Å peak of the bentoniteopals range from 4.022 to 4.090 Å and from 0.110 to 0.555 ° 2 θ, respectively. Most of the bentonite-opals from northwest, west and central Turkey (Edirne, Istanbul, çanakkale, Balikesir and çankiri areas) show both lower d-spacing and FWHM values than those of the samples from other regions. Therefore, these bentonite-opals can be described as opal-C (opal-cristobalite) and also low-cristobalite, whereas the bentonite-opals from other regions are opal-C and opal-CT (opal-cristobalite/tridymite). The mineralogy of opal phases and associated bentonite and their formation are mainly controlled by regional geology and host rock composition. The opal structures of the studied bentonites in Edirne, Istanbul, çanakkale, Balikesir and çankiri areas may be considered as diagenetic products in general, and those from other regions have both hydrothermal and diagenetic origins. Furthermore, values of d-spacing of the 4 Å peak of bentonite-opals from this study and from the literature were found to be significantly lower than those of the opals in non-bentonitic rocks (bentonite-opals < 4.09 Å < non-bentonite-opals). © 2013 E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany.