Crystallographic Methods for Non-destructive Characterization of Mineral Inclusions in Diamonds

The mineralogy and chemical compositions of inclusions in diamonds are the primary source of information about the environment in which diamonds grow and help constrain the mechanisms of their growth. However, the vast majority of the information about inclusions has been gathered by extracting them from their diamonds, thus destroying all possibility of obtaining further information about the diamond-inclusion system as a whole with new experimental probes unavailable at the time of extraction. One such specific example is the recent discovery by X-ray tomography and in situ spectroscopy of the hydrous silicic fluid film that appears to be ubiquitous around silicate inclusions in lithospheric diamonds (Nimis et al. 2016); the films escaped detection in a multitude of analyses during more than 70 years of research involving the extraction of many thousands of such inclusions. Inclusions in diamond are under compressive stress as a result of their encapsulation at depth and ascent of the diamond to the Earth's surface; extraction also destroys this stress and thus prevents the depth of entrapment from being determined from the stress state by elastic geobarometry. The stress release on extraction can also lead to the phase changes and/or conversion of the inclusion to a powder (e.g., Joswig 2011). For inclusions from diamonds suspected as being from super-deep sources, extraction therefore risks the loss of rare or possibly unique samples. Non-destructive characterization of inclusions in diamonds should therefore be made in situ whenever possible.