Feed coal and coarse ash particles (heated rock fragments and clinkers), produced from Sasol-Lurgi gasifier tests under different operating conditions, have been characterized by quantitative X-ray diffraction, electron microprobe analysis, and associated chemical techniques, as a basis for better understanding of the relations between the mineralogical and physical properties of the ash particles. Crystalline phases in the ashes include quartz particles inherited directly from the feed coal, as well as anorthite, mullite, and diopside, derived from solid-state reactions or crystallization of a silicate melt during the gasification process. Glass, cooled from the melt, is also abundant in the ash materials. The abundance of large particles of hard minerals in the coal or the ash, such as quartz, anorthite, pyrite, and diopside, has been correlated with a laboratory-determined abrasion index and may contribute significantly to wear on mechanical equipment during coal- or ash-handling operations.