Distinguishing opaline silica polymorphs from alpha-cristobalite in Gedikler bentonite (Usak, Turkey)


APPLIED CLAY SCIENCE, pp.80-86, 2012 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume:
  • Publication Date: 2012
  • Doi Number: 10.1016/j.clay.2012.03.007
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.80-86
  • Keywords: Silica polymorphs, alpha-cristobalite, Bentonite, Phosphoric acid digestion, NaOH dissolution, Thermal treatment, X-RAY-DIFFRACTION, CRYSTALLINE SILICA, CT
  • Dokuz Eylül University Affiliated: Yes


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 angstrom. Opaline silica polymorphs, opal-C and opal-CT, can be misidentified as alpha-cristobalite using standard XRD patterns of the raw bentonite samples. In order to distinguish opaline silica polymorphs from alpha-cristobalite, NaOH dissolution (0.5 M and 10 min.), phosphoric acid (H3PO4) digestion (240 degrees C, 15 min) and thermal treatment (1150 degrees 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 angstrom almost disappeared after NaOH dissolution and phosphoric acid digestion. Moreover, the crystal order and the diffraction maxima at 4.04 angstrom 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 add digestion residues, which consist mainly of opal-A, were transformed to alpha-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 alpha-cristobalite. The result of the phosphoric acid digestion method and the NaOH dissolution method yielded comparable results. (C) 2012 Elsevier B.V. All rights reserved.