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3.212 Petrographic changes

 The megascopic and the microscopic apperance of coals alters considerably

during coalication. Megascopically, the visibility of plant fragments as well as

the colour, density, lustre and stratication channge. n soft and dull brown coals,

pieces of wood, seeds, leaves and resins may gelication !vitrini"ation# ahomogeni"ation of plant fragments ta$e place, the colour of the coal changes

from dar$ brown to blac$, and a distinct lustre and a pronounced stratication

developed. The processes, probably due to the onset of bitumini"ation, ta$e

place at the ran$ stage of sub%bituminous &'( coals and represent the most

stri$ing petrographic change during the whole coalication process. n the range

of bituminous coals, changes of hardness and fribility !grindability# ta$e place

!)igure 3.22# which are related to increasing bitumi"ation and the progressive

devolatili"ation of the newly formed bitumen !*debimtumini"ation* of +evine,

13#. -nthracites are distinguished by being very hard and having a bright

lustre, the latter attaning a metallic, yellowish appearance in non%myloniti"edmeta%anthracites.

ith re/ected light microscopy the di0erent coalication behaviour of the

three maceral groups and even single macerals can be observed. Most stri$ing

are *coalication umps* and the generation of *secondary macerals* which are

formed during coalication of primary macerals.

n thin section of average thic$ness !1%1 um# there is a very clear

decrease of translucency of vitrinites and liptinites with rising ran$. n low ran$

bituminous coals, vitrinites are red in transmitted light, whereas liptinites appear

yellow and inertinites are blac$ !opa4ue#. +iptinities attain a red colour at higherran$ stages. +ow ran$ coals, up to the boundary between high%volatile

bituminous - coals and the medium%volatile bituminous coal stage !1.15 6r#,

become tranlucent at thic$ness of about 1 um. Thin sections of higher%ran$

coals have to be made thinner, down to um. -s the absorption inde7 increases

with increasing ran$, it eventually becomes impossible in practice to achieve

translucency.

n polished sections, an increase of re/ectance is most characteristic

during progressive coalication of huminites'vitrinites. -s )igure 3.18 shows, the

rate of re/ectance increase di0ers in di0erent ranges of ran$. The rate ofincrease is low at low ran$ stages up to abaout 15 6r. n this range the

relectance values are relatively scattered due to variations in the original

composition of humic plant materials. -t the stage of higher ran$ bituminous

coals and especially during antharaciti"ation, the relectance of vitrinites rises

more rapidly. n metaanthracites ma7iumum vitrinite re/ectance e7ceeds even

fusinite re/ectance !-lpern 9 +emos de :ousa, 1;#

:$yes et al. !18# ddiscused the implications for ran$ assessments on

lignites which still carry ulminites - !dar$# and & !light#. -ccording to (ameron

!11#, :as$atchewan lignites show di0erences of up to .285 in 6r%values

between ulminites - and & within the same coal.

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primary constituensts li$e cellulose and'pr hydrogen%rich material li$e resin, wa7,

cutin or suberin in various 4uantities.

-ccording to the &eer%)resnel e4uation !see :ection ;.=21# vitrinite

re/ectance depends on the refractive and the absorption indices. The refractive

inde7 !n# is related to the ultrane structure and other thisgs being e4ual,increases with the degree of aromati"ation. The absorption inde7 !$# depends on

the number of delocali"ed electrons, since their mobility facilitates absorption of