Distinguishing opaline silica polymorphs from α-cristobalite in Gedikler bentonite (Uşak, Turkey)

Abstract X-ray diffraction (XRD) patterns of the Gedikler (Esme-Usak) bentonite samples show the presence of silica polymorphs which have a characteristic XRD reflection at ca. 4.04 A. Opaline silica polymorphs, opal-C and opal-CT, can be misidentified as α-cristobalite using standard XRD patterns of the raw bentonite samples. In order to distinguish opaline silica polymorphs from α-cristobalite, NaOH dissolution (0.5 M and 10 min.), phosphoric acid (H 3 PO 4 ) digestion (240 °C, 15 min) and thermal treatment (1150 °C, 48 h) were applied. After these applications, samples were examined by XRD and Fourier transform infrared (FTIR) spectroscopy. It was observed that the 101 diffraction maximum at ca. 4.04 A almost disappeared after NaOH dissolution and phosphoric acid digestion. Moreover, the crystal order and the diffraction maxima at 4.04 A increased dramatically after thermal treatment. Besides, FTIR spectra of the samples are similar to untreated ones after NaOH dissolution, whereas they are comparable to opal-A after phosphoric acid digestion. Furthermore, the phosphoric acid digestion residues, which consist mainly of opal-A, were transformed to α-cristobalite during the thermal treatment. Based on these studies, it was concluded that the Gedikler bentonite samples contain mainly opal-CT, opal-A and minor amounts of α-cristobalite. The result of the phosphoric acid digestion method and the NaOH dissolution method yielded comparable results.