NANOPARTICLES IN NATURAL BERYLLIUM-BEARING SAPPHIRES

Trace element chemistry of gemstones have long been a focus of gemological research, as it not only helps reveal the geographic origin, but also determines their colors. Elements not typically found in natural corundum, such as Be, are particularly important to understand, since they can be artificially diffused into natural sapphires and rubies during heat treatment to improve their colors. Natural Be in corundum is always correlated with heavy high field strength elements (HHFSEs) such as Nb, Ta and W, which are hypothesized to be incompatible with the corundum structure due to their high charges.

To better understand the incorporation and distribution of Be and HFSEs in natural corundum, two natural Be-bearing sapphire crystals from Afghanistan (metamorphic) and Nigeria (magmatic) are studied using atom probe tomography (APT). Reconstructed three-dimensional maps reveal precipitates and clusters ~10 nanometers in diameter. The trace element fractionation by atomic mass, along with the coherent boundaries observed in TEM images, indicates the secondary nature of the nanoparticles, which proves that HHFSEs can actually be incorporated into the corundum structure. Sector zoning in the metamorphic sapphire from Afghanistan suggests the high field strength elements were disequilibrated by growth kinetics, which allows precipitation to occur not only through slow cooling, but also by heating. Be is attracted by the HHFSEs on the growth surface of corundum and incorporated into the structure, resulting in extremely strong correlation between their concentrations across the growth zones. The milky growth zones in the magmatic Nigerian sapphire indicates the Be and/or HHFSEs inhibits the precipitation of TiO₂ in corundum. Comparison with previous transmission electron microscope studies implies that the same process of incorporating beryllium and HHFSEs also applies for other natural corundums from different localities.