Beulah (North Dakota) lignite and Elk Creek (West Virginia) high-volatile A bituminous coal were burned in both pulverized coal and coal-water slurry fuel forms to study the effect of the modes of occurrence and composition of aluminosilicates and silicates on the particle size distribution and composition of ash. The dominant mechanism for ash formation in the Beulah pulverized coal was fragmentation of mineral particles, such as quartz and pyrite, during combustion. By contrast, the main mechanism for determining the Beulah CWSF ash particle size distribution was coalescence and agglomeration of inherent aluminosilicates and silicates during combustion. The particle size distribution of the inorganic phases formed during combustion of the Elk Creek fuels is slightly coarser than the original mineral matter, due to coalescence of inherent aluminosilicates and silicates during combustion. The slurry ash is slightly coarser than the pulverized coal ash as a result of the larger agglomerate formed on atomization of the Elk Creek slurry. The larger slurry agglomerate increases the number of mineral particles in proximity to one another and increases the time required for char burnout. In turn, the increased char burnout time increases the time interval during which mineral particles can coalesce, as evident by changes in the particle size distribution and composition of silicates and aluminosilicates. The result is enhanced coalescence and agglomeration of the mineral particles in the Elk Creek slurry compared to the pulverized coal. The results emphasize the importance of determining the size distribution and occurrence of inorganics in a fuel and the effect of changing either of these two parameters for a particular mineral group as a result of fuel form.