Laves phase observed by transmission electron microscopy in a natural Brazilian opal

The bidisperse structure of a natural Brazilian opal was revealed using transmission electron microscopy (TEM). Since TEM enables observation through the transparency of a few layers of silica spheres, the lamellar observation produced a complex image suggesting a flower-like repeating pattern generated by superimposed discs more or less opaque to the electron beam. Analysis of this disc combination revealed that the stacking structure of this opal was a hexagonal Laves phase of MgZn 2 type. The cutting plane is parallel to (11 2 0). A simulation over the thickness of the lamella matched the TEM image very well. The synthesis of such binary colloids is the aim of many experimental and theoretical studies with a variety of industrial applications. This study aims to prove that determining the structure of ordered natural opals can be performed by analysing TEM images.