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

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

Yılmaz, Hatice / Kaçmaz, Hülya

Abstract X-ray diffraction (XRD) patterns of the Gedikler (Eşme-Uşak) bentonite samples show the presence of silica polymorphs which have a characteristic XRD reflection at ca. 4.04Å. 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 (0.5M and 10min.), phosphoric acid (H3PO4) digestion (240°C, 15min) and thermal treatment (1150°C, 48h) 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. 4.04Å almost disappeared after NaOH dissolution and phosphoric acid digestion. Moreover, the crystal order and the diffraction maxima at 4.04Å 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.

Highlights ► This study was performed to distinguish silica polymorphs in Gedikler bentonite. ► NaOH dissolution, H3PO4 digestion and thermal treatment methods were used. ► Opal-CT, opal-A and minor amounts of α-cristobalite were determined. ► H3PO4 digestion and NaOH dissolution methods were compared each other.