An electron microscopy study of exsolved phases in natural black Australian sapphire

Abstract

Natural Australian sapphire (α-Al 2O 3 with around 1% Fe and up to 0.03% Ti) occurs in many colours, generally with an abundance of exsolved phases. Such ‘silky’ stones, cut en cabochon, exhibit the well-known phenomenon of asterism. For stones with a well-defined ‘star’ needle-like precipitate phases can be seen under an optical microscope. The needle axes are perpendicular to the star axes and generally parallel to the prism a-axes. (For artificial star sapphire the precipitate needle axes are perpendicular to the prism axes, e.g. Phillips et al., 1980.) Many authors have assumed that these precipitate phases in natural sapphire were needles of rutile (TiO 2) only, as is the case for artificial sapphire (Phillips et al., 1980). Our studies on natural Australian black sapphire, however, confirm the existence of a variety of iron/titanium rich phases, as well as rutile. Sample specimens were prepared by illuminating a polished basal plane with a He-Ne laser to obtain the star directions and then cutting slices of the stone parallel to the star axes. The polished sections were then ion-beam thinned and examined in a JEOL 100CX (S)TEM, equipped with an energy dispersive X-ray spectrometer. Precipitate phases were identified by electron diffraction and from their X-ray spectrum.