CHARACTERIZATION OF DIAMONDS AND OTHER GEM MINERALS WITH FIB/TEM

Microstructure such as grain size, dislocation density, micro- and nanoporosity and inclusions in gem minerals even on a nanometre scale can be investigated with TEM using focused ion beam (FIB) prepared electron transparent foils. Such foils are prepared from any mineral or solid state material by sputtering atoms out of the target material using Ga-ions accelerated to 30 keV⁽¹⁾. One major advantage of FIB sample preparation is that only a very small volume (< 3x10³ μm³) of the gem will be removed. The sputtering site is almost invisible under a binocular microscope. Therefore, it can be considered a non-destructive method. Inclusions in gem minerals may tell us something about the origin and the formation conditions of gems. Nano-inclusions in gem minerals can influence the colour. This is demonstrated with three examples. The blue colour of quartz is caused by significant amounts of submicron-sized (1000 nm – 100 nm) and even nanometre-sized (< 100 nm) inclusions of mica, ilmenite and rutile. Rayleigh scattering of light by the nano-inclusions generates the blue colour⁽²⁾. The black appearance of carbonado diamond is due to its polycrystalline nature with grain size < 10 μm, and its partially open grain boundaries thus causing internal total reflection of light. Cloudy inclusions are the main cause for colouration in sapphires. FIB/TEM studies of a rough sapphire from Ilakaka, Madagascar identified a nanocrystalline (5-40 nm) Ti-rich phase with α-PbO₂ structure as major constituent of the clouds⁽³⁾. Nanometre-sized inclusions in diamond allow conclusions from which depth the diamond has originated. The nanoinclusions suite observed in FIB sputtered TEM foils suggest that Lower mantel diamonds have grown from a carbonatitic melt whereas the chemical composition of nanoinclusions in upper mantle diamonds, the majority of all diamonds, indicate that they have grown from a high-density fluid (HDF).

⁽¹⁾Wirth, R., (2004) European Journal Mineralogy, 16, 863-876.

⁽²⁾Seifert et al., (2011), Mineralogical Magazine, 75, 2519-2534.

⁽³⁾Shen A. & Wirth R., (2012) Gems and Gemology, 48, 150-151.