Two small grains, about 50 × 60 × 70 μm in size, occurring in the chromitite associated with the altered harzburgite of Loma Peguera (Dominican Republic) have been investigated by electron microprobe analyses, X-ray computed tomography, and X-ray diffraction. Electron microprobe data indicate the grains to be composed of Ru, Os, Ir, Fe, and O. The obtained composition is similar to those reported from podiform chromitites worldwide and often reported as evidence for the existence of PGE-bearing oxide minerals. X-ray computed tomography and X-ray diffraction data, provided for the first time in this study, reveal that the Loma Peguera grains consist of a fine intergrown of ruthenium and magnetite not detectable at the scale of the electron microprobe. We believe that the investigated grains were originally laurite, ideally (Ru,Os)S2, crystallized during the magmatic stage. During alteration processes at low temperature, the mineral lost its original S, which transformed the laurite to ruthenium. The newly formed ruthenium was probably porous and, during the final stage of the alteration process, oxidizing fluids enriched in Fe filled the cavities leading to the subsequent crystallization of magnetite.

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