In the present work, mineralogical analysis of fly ashes produced from the brown coal burning power plants of Agios Dimitrios, Kardia, Ptolemais, LIPTOL, Amynteon, and Achlada-Meliti (Western Macedonia, Greece) was performed, with the aim of characterizing the ashes on the basis of their quantitative mineral phase composition and asses their variability at different time periods. The fly ashes from the Agios Dimitrios, Kardia, and Ptolemais power plants were found to have nearly the same mineralogical composition, consisting mainly of feldspars, lime, anhydrite, quartz, calcium silicates, and high amounts of amorphous phases. The fly ashes from Amynteon were slightly different, having lower content of lime and higher content of feldspars, whilst those from LIPTOL had a relative variable quantitative composition. The fly ashes from the Meliti-Achlada power plant consisted mainly of amorphous phases (very high amounts), mullite, feldspars, and quartz. The mineralogical composition of the ashes produced in all the power plants, except from these of LIPTOL, did not fluctuate significantly over time. An assessment of the hydraulic (cementitious) or pozzolanic character of the ashes is proposed, introducing the use of triangle diagrams A-B-C, which represent the total fraction of the phases with hydraulic or pozzolanic (A), inert (B) character, and the amorphous phases (C).