Petrography of American lignites

Abstract

The extensive deposits of low-rank coal in the United States are receiving more than usual attention as a result of declining reserves of competitively more favorable coals and the development of important nonfuel uses of coal. Use of domestic lignite for solvent extraction of montan wax to replace foreign supply cut off from American industry is an example.This paper describes briefly occurrences of lignitic coals in the United States and includes discussion of terminology, petrographic composition, coalification, and rank classification of these coals.Certain tests are described and data presented showing results of petrographic investigation of the source of wax and resin in lignite. Purpose of the investigation was to determine the components in lignite contributing to the solvent extraction products, and the phyteral source of waxy, resinous, and elements in the extracts.Certain varieties of attrital lignite proved to be the most important source material of waxy solvent extracts. Pure attrital lignite of known high solvent yield was broken down by a method of chemical maceration which removed humic matter and freed various ingredients of the attritus, mostly yellow translucent matter. Phyteral components in the residue were spores, pollen, particles of lump-type resin, round, light and dark bodies, dark resinous rodlets, remains of petioles and cuticular matter.A microscope hot stage was used to observe the effect of heat on these various ingredients and their solvent products. Information resulted which indicated that, for attrital lignites tested, the wax in solvent extracts is derived mainly from cuticular remains, resin from lump-type resin and round bodies, and asphaltic material probably from dark resinous bodies. The discovery of wax associated with cuticular remains is in keeping with the known fact that in living plants wax is found exclusively in cuticular coverings of leaves, petioles, and reproductive organs, and in suberin. Origin of attrital and xyloid lignites is discussed.