Transmission X-ray diffraction as a new tool for diamond fluid inclusion studies

Transmission X-ray diffraction is demonstrated as a new tool for examining daughter minerals within sub-micrometer fluid inclusions in fibrous diamond. In transmission
geometry, the X-ray beam passes through the sample, interacting with a volume of material. We analysed fibrous diamonds from the Democratic Republic of Congo, Wawa, the Ekati Mine, and Jericho kimberlite using X-rays from a high-brilliance lab source and a synchrotron source. Identified daughter minerals include celadonite mica, sylvite, halite, dolomite and other carbonates. This represents the first positive identification of halide mineralogy in fibrous diamond. Mineral inclusions like forsteritic olivine and pyrope garnet were also found. Unexpectedly, daughter minerals were identified in only 10 out of the 38 diamonds analysed, despite their concentrations being above experimentally proven detection limits. The presence of significant amounts of amorphous or dissolved material appears unlikely, but cannot be ruled out. Alternatively, the results may indicate a wide variety of related daughter minerals, such that most phases fall below the detection limits. Transmission X-ray diffraction should be applied cautiously in the study of fibrous diamond, as it provides an incomplete account of fluid inclusion mineralogy.