A major consideration in the development of a coal-fueled turbine engine is to minimize the deleterious effect of products of combustion (POC) on the engine components. Characterization of the POC phases is important for understanding and regulating combustion, emissions, deposition, corrosion and erosion behavior. POC phases deposited from the combustion of beneficiated bituminous coal in turbine simulators were characterized using x-ray techniques and electron microscopy. The phase assemblages include plagioclase feldspars, feldspathoids, spinels, anhydrite, hematite and noncrystalline material. The deposits had the form of a partially sintered powder and a slag. Differences in the phase composition of the powder and slag were determined by xray analysis and observed in the feldspar/spinel lattice parameters. The morphology, phase associations, compositional differences of the powder and slag, and heat treatment experiments indicate that the slag is largely formed by fusion of the powder. The hypothetical fusion process decomposes the CaSO4 in the powder and the released calcia is incorporated into the plagioclase solid solution powder phase. The additional alumina required for charge neutrality in the plagioclase comes from the powder's Al-rich spinel which transforms to an Al-poor spinel. These phase characterization results are useful for assessing and optimizing deposit remediation methods.