Coal Petrology and Petrographic Analysis

Coal petrology and petrographic analysis 3.1

INTRODUC DUCTION

Close examination of coal in hand specimen generally shows it to be composed of different layers. Under the microscope, these layers in turn are seen to be composed of mixtures of discrete entities, each class of which is distinguished by having different optical characteristics. Coal petrology is the study of the origin, composition and technological behaviour of these different materials, while the systematic quantification of their proportions and characteristics under the microscope is sometimes known as ‘coal petrography’. he different layers or entities occurring in a single coal may possess quite different physical and chemical  properties, and hence their relative abundance and manner of admixture is vital in determining the overall characteristics of a coal seam or mined coal product. Coal Coal petr petrog ogra raphy phy has has been been wide widely ly appl applie iedd to the the sele select ctio ionn and and blen blendi ding ng of coal coalss for for prod produc ucti tion on of  metallurgical coke, ke, and is one of the ma!or  cons consid ider erat atio ions ns in rese resear arch ch dire direct cted ed towa toward rdss coal coal liquefaction operations. he techniques of coal petrology are also also used used in geo geolog logica icall invest investiga igatio tions ns aimed aimed at assessing the potential of rocks and sedimentary basins as sources of petroleum. 3.2 MEG!CO"IC##$ RECOGNI%&#E CON!TITUENT!

' large part of the terminology used in coal petrology is derived derived from the work of &topes $()()% in recogni*ing recogni*ing four basic ‘ingredients’ of banded bituminous coal that can can be dist distin ingu guis ishe hedd in hand hand spec specim imen ens. s. hes hesee constituents, regarded in current usage as ‘lithotypes’, were identified by &topes, a palaeobotanist, as follows+ $a% Vitrain $. vitrurn, glass% -itrain is the black, glassy, vitreous material that is  probably the most striking component of bituminous coals. t occurs as thin bands, commonly less than / or 0 mm in thickness and is usually very closely !ointed. -itrain tends to be more brittle than other megascopic coal coal con consti stitue tuents nts,, often often breaki breaking ng with with a con concho choida idall fraction $"ig. 1.(a%. $b% Clarain $. clarus, bright% his lithotype is represented by bright to semi#bright  bands of finely laminated coal. Clarain generally exhibits an overall silky lustre, and commonly contains fine fine vitra vitrain in ban bands ds altern alternati ating ng with with a duller duller attri attrital tal groundmass $"ig. 1.(b%. $c%  Durain $. durus, hard%

2urain occurs as grey to black bands with a dull to slightly slightly greasy greasy lustre. lustre. he material material is relative relatively ly hard he he petr petrol ology ogy of coal coal may be stud studie iedd at eith either er a compared to other lithotypes, and tends to break into megascopic or a microscopic scale. "rom a megascopic large large,, blocky blocky fragm fragment ents. s. 2urain 2urain may someti sometimes mes be  point of view, coal may be classified into two broad confused with impure coal or carbonaceous shale, which groups, the banded or ‘humic’ coals and the non#banded are also often dull and hard, but it can be distinguished $massive% or ‘sapropelic’ coals. he humic coals are  by its lower density density $"ig. 1.(c%. visibly stratified, consisting of layers or bands of organic material of varying appearance, with individual layers usually no more than a few centimetres in thickness. $d%  Fusain $. fusus, a spindle% &uch coals are derived from a heterogeneous mixture of  a wide range of plant debris. he sapropelic coals, on the n "rench, the word  fusain means charcoal, which at one other hand, are homogeneo homogeneous, us, tough materials, materials, often often time was made from the wood of the spindle tree. he displaying a marked conchoidal fracture. hey are made suffix 3ain’ was adopted for the other lithotypes. 4here up of specif specific ic kinds kinds of fine fine graine grainedd organi organicc matter matter,, it is unminerali*ed, fusain is a soft, friable material that closely resembles the charcoal from which it takes its notably masses of spores or algal material. name. &oft, or unminerali*ed, fusain easily disintegrates int into a bla black fibr ibrou ouss po powd wder er,, bu butt hard ard fusai usain, n, 3.2.1 3.2.1 #ithot #ithotype ypess in in 'and 'anded ed 'it()i 'it()ino( no(ss coal coalss impregnated with mineral matter, may be found in some coals coals as well. well. "usain "usain usuall usuallyy occurs occurs as thin thin lenses lenses,, seldom more than a few millimetres thick, and is only a very minor constituent of most bituminous coal seams on a volumetric basis $"ig. 1.d%. he terms duroclarain $Cady ()56% and clarodurain

Coal Petrology and Petrographic Analysis

$a%

,c-

dark in colour with a dull to greasy lustre and typically displa displayy a marke markedd con concho choida idall fractu fracture re $"ig. $"ig. 1.6a%. 1.6a%. &apropelic coals may occur as layers or plies within seams of banded or humic coal, often at the roof. hey also occur as seams made up mainly of homogeneous, non#banded material in their own right. he two ma!or types of sapropelic coal are ‘cannel coal7, composed largely of spores or fine organoclastic detritus, and ‘boghead coa8 composed largely of algal material. hese are, however, effectively end#members of a range of materials representing mixtures of these two two type typess of comp compon onen ents ts,, and and tran transi siti tion onal al or  inte interm rmed edia iate te form formss such such as cann cannel el## bo bogh ghea eadd and and  boghead#cannel may be recogni*ed as well. 9ogheads may grade laterally or vertically into oil shales. 4hen viewed under the microscope, cannel coal can  be distinguished from boghead coal both by the abundance of spores and by the presence of a regular  microstratification. he materials are, however, almost impos impossib sible le to distin distingui guish sh from from each each other other in han handd specimen.

$b%

,d-

"ig. 1.( ithotypes in banded bituminous coal, $a% -itrain -itrain in polished surface. Aote that the three  bands of vitrain vitrain at the top of of the block are more more highly !ointed $cleated% than the rest of the coal $xB1.0%. $b% Clarain in polished surface. he lower three quarters of the block are composed mainly of clarain the upper quarter consists of two durain  bands $grey% and a vitrain band $black% $black% $x?.0%. $c% 2urain in hand specimen $x?.1%. $d% "usain in bedding surface of hand specimen $x?.6%.

3.2.3 3.2.3

have been added to this list by some workers to extend the number number of terms terms availa available ble for megasc megascopi opicc coal coal description. hey represent material that is intermediate in character between clarain and durain.

:ricr-it

Ul;,lll-ll-ll 4t -< 4ll

$vit vitrain rain and clar larain% ain% and du dull ll components compon component entss $durai $durainn and fusain fusain%% is most most appa appare rent nt in coal coalss of bitu bitumi mino nous us rank rank.. n anthracites, however, the lithotypes all tend to develop a relatively bright lustre, and the contrast between them decreases. 3.2.2. #ithotypes in sapropelic 'it()ino(s coals

Unli Unlike ke the the band banded ed or hu humi micc coal coals, s, whic whichh were were deposited as peats made up of large to small fragments of plan plantt debr debris is,, the the sapr saprop opel elic ic coal coalss repr repres esen entt accumulations of fine organic mud containing concen# trations of algae or spore remains. &apropelic coals are charac character terist istica ically lly fine fine graine grained, d, faintl faintlyy bed bedded ded to homogeneous, massive materials. hey are generally

*iel *ield d desc descri ript ptio ion n o+ coal coal sea sea)s )s

'lthough the terms vitrain, clarain, durain and fusain are are wide widely ly used used for for the the desc descri ript ptio ionn of indi indivi vidu dual al specimens or discrete hori*ons within a coal seam, a number of difficulties arise with their employment in routine logging of seam sections in bore cores or coal exposures $2avis ()=0a%. ;ne ma!or problem is that these four terms refer to quite different kinds of units within within the coal. coal. -itra -itrain in ban bands ds repres represent ent coalif coalified ied fragments of wood or bark and are generally no larger  than an individual tree trunk in si*e. Clarain and durain, on the other hand, are usually more extensive units, each possibly representing a aepositional environment within the peat swamp. n a rather exaggerated analogy, the vitrain bands might be compared to an individual  pebble, while the clarains and durains are like the conglomerate in which the pebbles occur. 'nother disadvantage associated with use of &topes’ terminology lies in the fact that the individual layers or  lenses of the four lithotypes may be very thin, usually only only some some milli millimet metres res in thickn thickness ess.. >ven >ven with with the accept accepted ed minimu minimum m layer layer thickn thickness esses es of differ different ent countries $1#(? mm%, strict application of the &topes terms terms can result result in extremely extremely detailed descriptions descriptions.. @any @any fiel fieldd desc descri ript ptio ions ns of coal coal seam seams, s, ho howe weve ver, r, especially those of very thick seams, are based, for  expediency expediency,, on sub#divis sub#division ion into a relatively relatively small number of megascopically distinct units. he

75

Coal Petrology and Petrographic Analysis

$a%

,c-

dark in colour with a dull to greasy lustre and typically displa displayy a marke markedd con concho choida idall fractu fracture re $"ig. $"ig. 1.6a%. 1.6a%. &apropelic coals may occur as layers or plies within seams of banded or humic coal, often at the roof. hey also occur as seams made up mainly of homogeneous, non#banded material in their own right. he two ma!or types of sapropelic coal are ‘cannel coal7, composed largely of spores or fine organoclastic detritus, and ‘boghead coa8 composed largely of algal material. hese are, however, effectively end#members of a range of materials representing mixtures of these two two type typess of comp compon onen ents ts,, and and tran transi siti tion onal al or  inte interm rmed edia iate te form formss such such as cann cannel el## bo bogh ghea eadd and and  boghead#cannel may be recogni*ed as well. 9ogheads may grade laterally or vertically into oil shales. 4hen viewed under the microscope, cannel coal can  be distinguished from boghead coal both by the abundance of spores and by the presence of a regular  microstratification. he materials are, however, almost impos impossib sible le to distin distingui guish sh from from each each other other in han handd specimen.

$b%

,d-

"ig. 1.( ithotypes in banded bituminous coal, $a% -itrain -itrain in polished surface. Aote that the three  bands of vitrain vitrain at the top of of the block are more more highly !ointed $cleated% than the rest of the coal $xB1.0%. $b% Clarain in polished surface. he lower three quarters of the block are composed mainly of clarain the upper quarter consists of two durain  bands $grey% and a vitrain band $black% $black% $x?.0%. $c% 2urain in hand specimen $x?.1%. $d% "usain in bedding surface of hand specimen $x?.6%.

3.2.3 3.2.3

have been added to this list by some workers to extend the number number of terms terms availa available ble for megasc megascopi opicc coal coal description. hey represent material that is intermediate in character between clarain and durain.

:ricr-it

Ul;,lll-ll-ll 4t -< 4ll

$vit vitrain rain and clar larain% ain% and du dull ll components compon component entss $durai $durainn and fusain fusain%% is most most appa appare rent nt in coal coalss of bitu bitumi mino nous us rank rank.. n anthracites, however, the lithotypes all tend to develop a relatively bright lustre, and the contrast between them decreases. 3.2.2. #ithotypes in sapropelic 'it()ino(s coals

Unli Unlike ke the the band banded ed or hu humi micc coal coals, s, whic whichh were were deposited as peats made up of large to small fragments of plan plantt debr debris is,, the the sapr saprop opel elic ic coal coalss repr repres esen entt accumulations of fine organic mud containing concen# trations of algae or spore remains. &apropelic coals are charac character terist istica ically lly fine fine graine grained, d, faintl faintlyy bed bedded ded to homogeneous, massive materials. hey are generally

*iel *ield d desc descri ript ptio ion n o+ coal coal sea sea)s )s

'lthough the terms vitrain, clarain, durain and fusain are are wide widely ly used used for for the the desc descri ript ptio ionn of indi indivi vidu dual al specimens or discrete hori*ons within a coal seam, a number of difficulties arise with their employment in routine logging of seam sections in bore cores or coal exposures $2avis ()=0a%. ;ne ma!or problem is that these four terms refer to quite different kinds of units within within the coal. coal. -itra -itrain in ban bands ds repres represent ent coalif coalified ied fragments of wood or bark and are generally no larger  than an individual tree trunk in si*e. Clarain and durain, on the other hand, are usually more extensive units, each possibly representing a aepositional environment within the peat swamp. n a rather exaggerated analogy, the vitrain bands might be compared to an individual  pebble, while the clarains and durains are like the conglomerate in which the pebbles occur. 'nother disadvantage associated with use of &topes’ terminology lies in the fact that the individual layers or  lenses of the four lithotypes may be very thin, usually only only some some milli millimet metres res in thickn thickness ess.. >ven >ven with with the accept accepted ed minimu minimum m layer layer thickn thickness esses es of differ different ent countries $1#(? mm%, strict application of the &topes terms terms can result result in extremely extremely detailed descriptions descriptions.. @any @any fiel fieldd desc descri ript ptio ions ns of coal coal seam seams, s, ho howe weve ver, r, especially those of very thick seams, are based, for  expediency expediency,, on sub#divis sub#division ion into a relatively relatively small number of megascopically distinct units. he

75

76 

Chapter 3

&top &topes es syst system em was was no nott desi design gned ed for, for, and and is no nott this is summari*ed in able 1.6. "usain bands or lenses  particularly effective effective in, this kind of usage. thicker than about 7 mm are recorded separately, as are Decogni*ing these and other difficulties inherent in non#coal non#coal bands or partings, partings, for which conventional conventional the &topes &topes termin terminol ology ogy of coal coal litho lithotyp types, es, &chopf  &chopf  sedimentary rock terms may be used. 'nother system, $()/?% established a descriptive system for use by the used extensively extensively to describe describe 'ustrali 'ustralian an bituminou bituminouss U.&. Eeological &urvey, and this has been subsequently coals in outcrops, mine exposures and drill cores, is employed by many others for field use. &chopf s terms disc discus usse sedd in &ect &ectio ionn /.7. /.7./. /. 2isc 2iscus ussi sion onss of the the are out in able 1.(. n summary, three constituents are  preparation and use of megascopic coal seam logs in described, namely ‘vitrain’, ‘fusain’ and ‘attrital coal’. field field studi studies es are are also also given given by &chopf &chopf $()/?% $()/?% and he he firs firstt two two are are rega regard rded ed as larg larger er clas clasti ticc un unit itss 2utcher $()=0%. occurring within a matrix of finely divided attrital coal. he thickn thickness ess and con concen centra tratio tionn of the vitra vitrain in and 3.2. .2. I)p(re co coals fusain fusain are described described in terms terms which are quantitat quantitatively ively  precise, and the attrital coal is described as having one 4here the coal contains a significant amount of mineral of five levels of lustre, ranging from bright to dull. matt matter er,, its its ov over eral alll dens densit ityy and and ash ash yiel yieldd incre increas asee 'nother alternative has been to describe the coal significantly. 'lthough the distinction between clean with with refe refere renc ncee only only to its its rela relati tive ve brig bright htne ness ss.. coal coal and and impu impure re coal coal is gene genera rall llyy base basedd on the the >mploying terms used in the Eerman coal industry, economic constraints of mining, marketing and use, 2iesse 2iessell $()/7% $()/7% has descr describe ibedd coal coal seams seams as being being most materials regarded in the latter category have an compos composed ed of megasc megascopi opical cally ly disti distinct nct layers layers of the ash yield greater greater than 67F, and sink when placed placed in a following types of materials+ liquid with a relative density of (./?. Gowever, where $a% bbrright coal 3 the coal has more than about 5?#7?F ash, it is usually $b% ban banded ded bbrig right ht coal coal 3 more more corr correc ectl tlyy desc descri ribe bedd in no non# n#co coal al term terms, s, for  for  $c% $c% band banded ed coal coal example as a carbonaceous shale or a coaly shale. $d% ban banded ded dull dull coal coal he mineral impurity in the coal may be in the form $e% $e% dull dull coal coal.. of discrete bands, streaks or layers interbedded with the here are many similar descriptive systems in use organic constituents, or it may occur as nodules or as around the world, and all can be used on as broad or as fracture infillings. t may be made up of clay or shaley material, or of pyrite, siderite or calcite. Coal with a narrow narrow a scale scale as necess necessary ary or con conven venien ientt for the the  particular purpose for which the description is being significant amount of fine clay disseminated throughout  prepared. ' seam may be described on a centimetre the organic matter, rather than in discrete layers, is often described as stony coal or, in the U.&.'., as one scale $e.g. for research purposes%, or on a metre scale, coal’ $"ig. 1.6b%. &uch material is characteri*ed by a as desired. '9> 1.( U.&. Eeological &urvey terms for megascopic description dull appearance and, commonly, a grey, rather than a of banded bituminous coal $&chopf ()/?%.  black, coloured streak. 'n indication of the terms that may be used to describe impure coals is given in able -itrain "usain 1.1. 'ttrital coal  J 

 f  bright   bright moderately bright midlustrous

moderately dull ( dull hi hickn ckness ess cl classes $m $mm% thin bands ?.7#6 medium bands 6#7 thic k bands 7#7? very thick bands I7?

Concent entrati ation cl classes $F $F% sparse B(7 moderate (7#1? abundant 1?#/? dominant I/?

' combination of the bright#dull system and the &chopf system has been devised by the Coal Desearch &ectio &ectionn of he 8ennsy 8ennsylva lvania nia &tate &tate Univer Universit sityy for  effective seam descriptions at a megascopic level, and

3.2./ 3.2./

Megasc Megascopic opic +eat(r +eat(res es o+ lo0ra lo0ran n coals coals

"or most practical purposes, the distinction between ‘hard’ coals, of bituminous rank or higher, and ‘soft’ or  low rank coals is based on the specific energy and other  chemical properties of the materials concerned $&ection 6.(? 6.(?.6 .6%. %. n >uro >urope pean an term termin inol olog ogyy, the the low low rank  rank  H mate materi rial alss are are gene genera rall llyy desc descri ribe bedd as  brown coaf, whereas in the U.&.'. and elsewhere they are classed either as ‘lignite’ or ‘sub#bituminous coal’, depending mainly on their chemical characteristics. 'lthough the term ‘brown coal’ is, strictly speaking, applicable to a wider range of material than ‘lignite’, it is also used as a synonym for ‘lignite’ in many contexts. ignite is a dull, soft, earthy material, ranging from

'9> 1.6 2escriptive terms for coals $he 8ennsylvania &tate University%. Coarsely banded coals

'ttrital coals

 bright $banded%< $banded%< coal $I )? -%f -%f bright interbanded interbanded with dull coal $/7#)? -% interbanded dull and bright coal+%+ $17#/7 -%

 bright $attrital% $attrital% coal< $I )? )? -%

midlustrous coal J

fusain

dull interbanded with bright coal $(?#17 -% dull coal $B (? -% sapropelic $non#banded% coal K

< n practice, only one category of bright coal has been employed.   Aumbers in parentheses refer to approximate percentages of vitrain bands and streaks. 3 Contains approximately equal proportion of bright coal and dull attrital coal. J @idlustrous attrital coal may contain many fine vitrain streaks, but obvious vitrain bands should be less than (?F.

 brown to black in colour. t may occur in a massive sapropelic form or, more commonly, as a humic material  banding, but some are almost massive in hand specimen. 9oth lignites and subbituminous coals also tend to crack  with recogni*able wood, leaf and other plant and fall fall apart apart on drying drying#ou #outt with with exp exposu osure, re, a proces processs known generally as ‘slacking’.

"ig. "ig. 1. 1.66 $a% &aprop &apropeli elicc coal. coal. Aote Aote the the massiv massivee struc structur ture, e, faint bedding and conchoidal fracture $