Lignite samples from the Southern lignite field of the Ptolemais basin, have been heated in air at 100°C intervals, in the temperature range from 600°C through 1000°C. The minerals detected are: calcite, dolomite, quartz, hematite, anhydrite, meta-kaolinite, spinel, wollastonite, spurrite, periclase, lime, åkermanite, gehlenite, merwinite, calcium ferrite and magnesioferrite. Anhydrite, hematite and periclase are present in all ashing temperatures. Calcite spinel, wollastonite and spurrite appear only in the 600°C ash sample. Dolomite and metakaolinite are present up to 700°C, quartz up to 800°C, while lime appears in the 700°C and 800°C ash samples. Åkermanite, gehlenite, merwinite, calcium ferrite and magnesioferrite appear from 700°C through 1000°C. Calcite, dolomite, quartz and partly hematite are constituents of the starting lignite. Anhydrite is formed from bassanite, but anhydrite and hematite form also from calcitepyrite reaction. Meta-kaolinite is formed from kaolinite, spinel from illite, while wollastonite from calcite-quartz reaction. Lime and periclase are formed from the decomposition of carbonates, while spurrite from calcite-wollastonite reaction or even directly from tilleyite. Carbonate-silicate ± spinel reactions are responsible for the formation of åkermanite, gehlenite and merwinite. Magnesioferrite forms from periclase-hematite reaction, while calcium ferrite from lime - hematite or even magnesioferrite caicite reactions.