Electron Microscope Studies of the Structure of Opal

Abstract
Replication electron microscopy has confirmed that the microstructure of precious opal consists of spherical particles of amorphous silica, SiO2, packed in a cubic close‐packed array. Twinning relationships have been observed obeying the spinel twin law. Regions related by twinning correspond to the bands or striations observed under the light microscope. A specimen of common opal displayed a unique topography upon etching.

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