Original paper

Nanostructures and microinfrared behavior of black opal from Gracias, Honduras

Viti, Cecilia; Gemmi, Mauro

Abstract

This paper reports nanostructural and microinfrared data of a black opal from Gracias, Honduras. The opal is hosted in a volcanoclastic rock, as the cement among glass shards and as the filling material of steam vesicles (100-400 μm in diameter). It is amorphous (opal-A), inclusions-free and consist of silica spheres, 450 nm in diameter, arranged in ordered close-packed fcc lattice, producing intense play of color effects. The observation of thin sections under the polarizing light microscope reveals that the Honduran opal is characterized by pleochroism and sharp birefringence, together with striae, "twins" and zoning.Electron microscopy revealed that all these features are due to sphere packing mistakes. "Twins" and striae are due to changes in the close-packed planes stacking, whereas zoning is related to a preferential concentration of point defects (i.e., vacant silica spheres) at the opal/glass boundary.Microinfrared spectrometry indicates that the Honduran opal is closely associated with minor CO2, possibly infiltrating among silica spheres.