Diatomite with variable chemical and mineralogical composition occurs in the Komnina Lignite Basin. The diatomite layers, which overly lignite seams of Upper Miocene age, are rich in silica or calcium. These layers correspond respectively to quartz-rich and calcite-rich layers. The diatomite mainly consists of opal-A, while other minerals are quartz, feldspars, calcite, smectite, illite, kaolinite, chlorite, cristobalite, and muscovite, The well-preserved nature of the diatom species indicates a weak silica diagenesis. This is also indicated by the mineralogical composition of diatomite, especially the absence of opal-CT. The organic material present in the lake acted as coating and prevented diatom dissolution. The shallow lake, where the diatoms were deposited, did not allow diagenesis of diatomite. Diatoms were deposited in the Komnina Lake as a result of the acidic pH, the warm-humid conditions, and the silica-rich environment that occurred due to volcanic activity in the nearby area (Aridea). Remains of volcanic ash have been identified in the lignite deposits of the wider Ptolemais area. The association of diatomite with lignite was brought about by tectonic movements that occurred in the wider area of Monastir-Florina-Ptolemais-Kozani-Elassona-Sarantaporo in the Upper Miocene, resulting to the development of individual small shallow lakes. The paleoenvironment changed to acidic from the previous alkaline pH, the vegetation died off, the silica produced from the volcanic activity and the calm water conditions allowed the deposition of diatoms. (c) 2006 Elsevier B.V. All rights reserved.