Studies of Small Inclusions in Synthetic Diamonds by Optical Microscopy, Microradiography and Transmission Electron Microscopy

A combination of low-resolution and high-resolution techniques was applied to establish the composition, crystallography and distribution of small inclusions, average diameters ~1 μm, dispersed within large synthetic diamonds. Both metallic and silicate phases were present among included bodies, often lying adjacent in the same cavity within the diamond matrix. Monochromatic X-ray microradiography, achieving 1 μm resolution with Cu Kα radiation, demonstrated that transition-metal phases constituted a minor fraction of the total volume of inclusions present. In a specimen rich in dispersed inclusions, the sizes of 1483 inclusions were individually measured. In aggregate they occupied ~4 × 10⁻⁴ of the diamond volume. High local inclusion concentrations were associated with high local concentrations of substitutional nitrogen impurity in the diamond matrix. The electron-microscopic techniques included convergent-beam electron diffraction, energy-dispersive X-ray spectroscopy and electron-energy-loss spectroscopy. Detailed crystallographic identifications of individual particles included a b.c.c. Fe–Co alloy (a₀ ≃ 2.86 Å), a garnet of andradite variety [a₀ = 11.95 (5) Å] and a clinopyroxene with composition corresponding to that in the augite-ferroaugite division of pyroxenes.