Texture and structure of opal-CT and opal-C in volcanic rocks

Toshiro Nagase, Mizuhiko Akizuki

Research output: Contribution to journalArticle

34 Citations (Scopus)

Abstract

Textures and structures of opal-CT and opal-C in volcanic rocks from the Hosaka and Akase opal mines in Japan were studied using an optical microscope and scanning and transmission electron microscopes. Textures of the opal-CT are optically classified into anisotropic columnar and isotropic massive (opal-CTM) types. Both textures consist of thin and platy crystals showing two wide {101} faces of low cristobalite. Images obtained by high-resolution transmission electron microscopy show that the cristobalite structure is the fundamental component of volcanic-type opal-CT, and that many stacking faults are present randomly in the structure. Columns are produced by parallel growth of platy crystals; opal-CTM and lepispheres consist of criss-crossing aggregates of blades. Each blade is composed of roughly parallel aggregates of platy crystals. The variation of d101 values with textural and structural changes of the opal-CT was measured by an X-ray powder-diffraction method. With decrease of d101 value, highly ordered cristobalite domains develop surrounding the domains of disordered cristobalite in the crystal. The considerable difference in degree of order of stacking between the two types of domain implies changes in the growth process of the crystal.

Original languageEnglish
Pages (from-to)947-958
Number of pages12
JournalCanadian Mineralogist
Volume35
Issue number4
Publication statusPublished - 1997 Aug 1

Keywords

  • Cristobalite
  • Japan
  • Lepisphere
  • Opal mines
  • Opal-C
  • Opal-CT
  • Scanning electron microscopy
  • Transmission electron microscopy

ASJC Scopus subject areas

  • Geochemistry and Petrology

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