オパール : その歴史と科学-2-

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In this part, recent results of optical and electron microscopic investigations of opals are reviewed, together with explanation of geological settings of opal localities, so as to throw light to the understanding of the origin of play of colour and for the elucidation of the genesis of opals. On the fracture surface of precious opal with play of colour, wavy or circular banding patterns are seen under the reflection optical microscope, and they are inferred to the phenomenon of optical diffraction due to regular packing of silica spheres. Such banding patterns are seen only where play of colour is seen. Simple experiments, however, demonstrated that the banding patterns correspond to the steps exposed on the fracture surface, and not to optical diffraction. Under the electron microscope, it is observed that they are steps formed between different lamellae consisting of silica spheres of nearly equal sizes. Electron microscopic observations of both fresh and etched fracture surfaces revealed that there are three different types of lamellae ; 1) those without voids, 2) those consisting of unit layer without voids and with three voids around silica spheres, 3) those consisting of unit layer with six voids and with three voids. Opal can be classified as shown in Table 2, depending on the existence and types of voids. The banding patterns vary depending on this difference. Based on these observations, the origin of play of colour is discussed. Geological occurrences of precious opals in Australia, Mexico, Czecoslovakia, and Japan are described in some details, and the processes of opal formation both in sedimentary and igneous environments are discussed.
宝石学会(日本)の論文
1975-03-15

オパール : その歴史と科学-2-

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