Variation in maceral chemistry within and between coals of varying rank: An electron microprobe and micro‐Fourier transform infra‐red investigation

Summary Electron microprobe and reflectance micro‐Fourier infra‐red techniques have been used to document the evolution of maceral chemistry from lignite to anthracite as well as to determine chemical variations between macerals and within macerals in iso‐rank coals. This study shows that the evolution of elemental composition and functional groups in vitrinite differs from those of liptinite. In vitrinite, there is a decrease in the absorbance of both aliphatic stretching bands (2800–3000 cm —1 ) and the aromatic band (1600 cm —1 ) with an increase in rank from high‐volatile bituminous to anthracitic coals. The change in functional groups is associated with an increase in C content from about 86 up to 94% and a decrease in O content from 7 to 1%. In sporinite, carboxyl/carbonyl bands (1710 cm —1 ), distinct in sub‐bituminous and high‐volatile bituminous ranks, disappear at higher rank. The 1600 cm —1 band is relatively weak in liptinite and increases dramatically when liptinite and vitrinite coalification paths merge. These changes in sporinite functional groups are accompanied by an increase in C content from 73 to 88–89%, decrease in O from 14 to 4% and decrease in H from more than 9 to less than 4%. Variations in chemistry between macerals and within macerals depend on coal rank. Up to high‐volatile bituminous rank, liptinite macerals have higher absorbances of aliphatic stretching bands and lower absorbances of the aromatic 1600 cm —1 band than any other macerals. At medium‐volatile bituminous rank, aliphatic stretching bands are distinct in vitrinite and semifusinite and very weak to absent in other inertinite macerals. At anthracitic rank, variations both in elemental composition and in functional groups between maceral groups are of the same magnitude as within maceral groups.