Unit Cell Parameter as a Compositional Proxy using In-situ Micro XRD and Portable Terra XRD for Diamond Exploration

English Abstract:

Chromite-rich spinels, with the general formula (Fe,Mg)[Cr,Al]₂O₄, and with high Cr and very low Ti contents, are typically used as a vector in diamond exploration. Exploration for new diamond deposits relies on the increased footprint produced by the dispersal of resistant kimberlite indicator minerals (KIMs), including chromite. KIMs have, until recently, predominantly been analyzed in lab facilities, but, with the advent of portable integrated X-ray diffraction and fluorescence technology, it will be possible to acquire crystal structural parameters in the field. Chromite grains from non-kimberlite and kimberlite sources were analyzed using a D8 Discover in-situ micro X-ray Diffractometer and preliminary results are also reported using a portable Terra XRD; The unit cell parameter was obtained from least squares unit cell refinement of the diffraction data. The unit cell parameter can, under suitable conditions, discriminate chromite in the diamond inclusion field, which has values ranging from a_o = 8.31-8.33 Å. The micro XRD work enabled the creation of a calibration data set for use with the field portable equipment. The Terra XRD, adapted from the Mars Science Laboratory Curiosity Rover, using the powder matrix method, can identify grains in the field by XRD and collect unit cell data. The X-ray fluorescence capability of the Terra demonstrates a potential method to increase the applicability of the unit cell as a proxy in a field setting. This study demonstrates our powder matrix method and the use of unit cell for diamond exploration. In addition it provides a proof-of-concept for the determination of the aluminum content of natural spinels in the field using unit cell parameter as a compositional proxy. Crystal structural measurements with a portable XRD would enable screening of samples in the field, reducing the amount of material that must be shipped to for laboratory analysis, which could in turn reduce overall exploration costs.