Zw. Han et al., Classification of torbanite and cannel coal I. Insights from petrographic analysis of density fractions, INT J COAL, 38(3-4), 1999, pp. 181-202
Torbanite and cannel coal are considered to be coals because of their low m
ineral content and overall physical morphology. However, the texture and co
mposition of the organic matter in torbanite and cannel coal are similar to
the kerogen occurring in oil shales and lacustrine source rocks. Therefore
, understanding the nature and origin of organic components in torbanite an
d cannel coal is of significance in the study of kerogen and petroleum form
ation. in this research, a set of torbanites and cannel coals from differen
t locations throughout the world were petrographically characterized and pr
ocessed using a density gradient centrifugation (DGC) technique. Microscopi
cally, the torbanite and cannel coal are composed of coarser maceral partic
les set in a fine-grained to amorphous groundmass. The groundmass is a mixt
ure of more than one type of substance and accounts for 10 to 80% (by volum
e) of the torbanites and cannel coals. Botryococcus-related alginite is the
most characteristic component of the torbanite. While sporinite typically
is the main phytoclast in the cannel coals, in most cases the,groundmass is
volumetrically the dominant component, determining the overall character o
f the sample. This observation calls into question the traditional practice
of classifying such coals using the alginite to sporinite ratio. Variation
s in composition, texture and fluorescence permits the recognition of three
different types of groundmass: lamalginitic, bituminitic and vitrinitic. H
igh purity alginite, sporinite, vitrinite and varieties of groundmass were
separated using the DGC technique. The distribution of density fractions cl
osely reflects the petrographic composition of the various torbanites and c
annel coals. Distinct peaks on the density profiles represent the major org
anic components and peak magnitudes are functions of the percentage of the
components, demonstrating that the density gradient profiles can be used to
distinguish the different types of torbanite and cannel coal. The separati
on data also indicate a gradual shift towards higher density from lamalgini
tic to bituminitic to vitrinitic groundmass. (C) 1999 Elsevier Science B.V.
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