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. Extraction of inclusions has often been motivated by a lack of experimental probes to measure the inclusions in situ in their host diamonds. However, developments in technology mean that a wide range of spectroscopies is now available that can be used on inclusions in situ to offer rapid initial phase identification (e.g., Raman), and to determine certain aspects of chemical composition. Fourier transform infra-red spectrosocopy (FTIR) can be used to determine OH contents of minerals, synchrotron Mössbauer and µ-EXAFS to determine details about element oxidation states and structural environments in inclusions. None of these methods, however, provide a definitive identification of the mineral phase in the inclusion, nor the details of its crystallographic structure, nor its stress state. These require diffraction measurements. Single-crystal X-ray diffraction has been used since the work of Mitchell and Giardini (1953) to measure inclusions in diamonds. The presence of the diamond presents some challenges to obtaining the highest quality data and therefore the precision with which scientific conclusions can be derived from the data. But, by using methods developed over several decades to study crystals held in diamond-anvil pressure cells (DACs) and exploiting rapid developments in diffractometer technology and intensity of X-ray sources, diffraction measurements of inclusions can be of similar quality to those of crystals at ambient conditions, if the measurements are performed with care. In this chapter we describe the methods that we have developed as best practice that, when used, will enable the reader to obtain the highest-quality data from most inclusions by using standard laboratory diffractometers. We focus on details of data collection and analysis methods and point out the pitfalls that can arise from poor set-up of the experiment. We show with examples how the crystallographic data measured from inclusions still buried deep within their diamonds can be used to provide simultaneously three key pieces of information to reconstruct the history of the diamond–inclusion pair. Intensity data collection and structure refinement confirms the identification of the mineral phase in the inclusion and allows the composition of the mineral to be determined, and the environment in which the host diamond grew to be characterized. Determination of the crystallographic orientations of inclusions with respect to their diamond hosts has recently provided new fundamental insights into how diamonds nucleate and grow in the mantle. The developments in methods to measure the stress state in inclusions and the theory of elastic geobarometry allows the depth of formation of diamonds to be inferred on the basis of a physical process that is independent from chemical equilibrium. The same principles and methods also apply to those inclusions that today are too small to be measured with laboratory X-ray sources and must still be measured at synchrotron beamlines. And they can be applied equally to all inclusions in diamond, whether the diamond hosts are believed to be lithospheric or sub-lithospheric (so-called super-deep diamonds). Indeed, the identification of the mineralogy of the inclusions and their stress state is the major determinant of the depth the formation of the diamond host.