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Abstract An unusual sample of gem opal has been found to contain silica spheres of two different sizes, mixed in various proportions. Electron micrographs show extensive areas of long-range order. The structures of two different ordered phases, corresponding to compounds AB2 and AB13, have been deduced from the micrographs. Their calculated densities are found to be greater than that of the two separate components in a close-packed regular arrangement.
J. V. Sanders (Mon,) studied this question.