Altered volcanic ashes in coal and coal-bearing sequences: A review of their nature and significance

Volcanic ashes in coal may serve as chronostratigraphic markers to identify and correlate coal seams, and can also be used for radiometric age determination. The ashes may have provided terrigenous materials that served as substrates for peat development; and may have terminated peat accumulation if volcanic ash in large quantities fell into the peat swamp; they can also be used to indicate the geodynamic processes of coal formation; and perhaps explain mass extinction events. Admixed volcanic ash may lower the quality of the coal if not removed in the preparation plant. From a practical viewpoint, alkali volcanic ashes may be significantly enriched in rare earth elements, Y, Nb, Ta, Zr, Hf, and Ga, which have potential economic significance. This paper reviews the distribution, geochemical and mineralogical compositions, and the significance and applications of volcanic ashes in coal and coal-bearing sequences.