Structure of Opal

@article{Jones1964StructureOO,
  title={Structure of Opal},
  author={J. Bowen Jones and John V. Sanders and E. R. Segnit},
  journal={Nature},
  year={1964},
  volume={204},
  pages={990-991}
}
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… 
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References

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