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Structure of Opal


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 to apparently amorphous or near amorphous material. The X-ray diffractometer traces shown in Fig. 1 illustrate the variations encountered; it should be noted, however, that, of the diffuse bands visible on powder photographs, only that at about 4.1 Å is recorded in traces G and H. Most specimens which would be termed ‘common opal’ by mineralogists are crystalline. Rarely, these may be well crystallized, as in sample B (which may be compared with A, a synthetic α-cristobalite), or very poorly crystallized, as in F. Most samples, however, give traces similar to C, D and E. Most precious opals and their associated ‘poteh’, as well as hyalite, give patterns like G and H.

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  1. 1

    Flörke, O. W. N., Jahrbuch Mineralogie, Monatsh., 10, 217 (1955).

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  2. 2

    Jones, J. B., Segnit, E. R., and Nickson, N. M., Nature, 198, 1191 (1963).

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JONES, J., SANDERS, J. & SEGNIT, E. Structure of Opal. Nature 204, 990–991 (1964).

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