Structure of Opal

  title={Structure of Opal},
  author={J. Bowen Jones and John V. Sanders and E. R. Segnit},
SINCE the work of Flörke1 it has generally been accepted that natural opaline silicas fall into two broad categories: (1) specimens which give X-ray patterns indicative of an obviously, although in some cases poorly, crystalline structure; (2) specimens which give X-ray patterns showing only a few diffuse bands. Earlier work by some of us2 supported these conclusions, and a comprehensive re-examination of natural opals has confirmed that the structure can vary from almost perfect α-cristobalite… 
Raman, FT-IR and XRD investigation of natural opals
Opals are naturally occurring hydrous silica materials (SiO2*nH2O), characterized by different degrees of crystallinity and crystal structure. Because of their optical properties, opals have been
XRPD patterns of opals: A brief review and new results from recent studies
A new classification of opals through X-ray powder diffraction (XRPD) methodology, by analysing 75 new samples of opal came from different worldwide areas, is introduced. A brief historical summary
Thermal Crystallization Kinetics of an Opal-like Biogenic Silica
A gray-colored sediment from Erzurum/Turkey zone was characterized by scanning electron microscopy, X-ray diffraction, chemical analysis, and thermal analysis techniques. The raw sample contains
Tem and X-Ray Diffraction Evidence for Cristobalite and Tridymite Stacking Sequences in Opal
In an attempt to resolve the structure of opal-CT and opal-C more precisely, 24 opal samples from bentonites, Fuller’s Earths, zeolite tuffs, biogenic silicas and silicified kaolins have been
Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography
Cristobalite and tridymite are distinct forms of crystalline silica which, along with quartz, are encountered in industrial operations and industrial products. Because the International Agency for
Summary Experimental investigation shows that amorphous silica is converted into quartz through cristobalite under hydrothermal conditions. The rate of transformation, essentially dependent on the
Opals from Slovakia (“Hungarian” opals) a re-assessment of the conditions of formation
Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A),
Investigation on the gemological, physical and compositional properties of some opals from Slovakia ("Hungarian" opals)
The physical and compositional properties of some “Hungarian” opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, AvaSpec
Australian sedimentary opal-A and its associated minerals: Implications for natural silica sphere formation
Abstract The vast majority of precious opal on the world market comes from opal fields in the Great Artesian Basin of Australia pointing to very special prerequisites for amorphous silica to


Differential Thermal and X-Ray Analysis of Opal
PUBLISHED differential thermal analysis curves of opal show a marked rounded endotherm with the peak near 130° C, although one example shows a very small peak at this temperature1. Curves we have