Coalification trends in Indian coals

International Journal of Coal Geology, 13 (1989) 413-435

413

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

C o a l i f i c a t i o n t r e n d s in I n d i a n coals D. CHANDRA 1and N.C. CHAKRABARTI2

1Department o[ Applied Geology, Indian School of Mines, Dhanbad, India 2Department of Applied Geology, Geological Survey of India, 27, Jawaharlal Nehru Road, Calcutta 700 016, India (Received March 1, 1988; revised and accepted November 23, 1988)

ABSTRACT Chandra, D. and Chakrabarti, N.C., 1989. Coalification trends in Indian coals. In: P.C. Lyons and B. Alpern (Editors), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol., 13: 413-435. A study of the coalification trends of Indian coals, covering a wide range of geological age (Permian, Eocene to Pleistocene ), shows a progressive transition in physico-chemical and optical properties from peat to semianthracite. The relation between the maximum reflectance in oil of huminite/vitrinite and volatile matter (or fixed carbon) has been found to be similar to that of the normally coalified British Carboniferous coals. Also the coalification trend of Indian coals, measured in terms of elemental carbon and hydrogen, appears to follow Seyler's band, and is similar to the normally coalified British Carboniferous coals. Although coals of the two hemispheres follow the same normal coalification course, their physical, chemical and petrographical characters differ depending on the petrographic composition, i.e., the macerals, microlithotype and mineral-matter contents. The petrographic composition of a coal appears to be essentially controlled by the nature of the peat, paleodepth, paleotemperature and paleoheating in the coal basins.

INTRODUCTION

Coalification denotes an orderly increase in rank from peat to meta-anthracite. Diagenesis and catagenesis, in that order of intensity of coalification, are the two major processes that bring about the changes during coalification of bituminous and lower-rank coals. There is a wide variation in the processes responsible for coalification. A number of factors, like paleodepositional environment, lithostatic pressure due to depth of burial, temperature, duration of heating, and post-tectonic transformation, mainly by intrusives, have played important roles in influencing the properties of coal. These changes left imprints that are now indicated by the chemical properties, reflectance, maceral and microlithotype composition, and fluorescence characters of the coal. These parameters reveal clues to the genesis of coal. 0166-5162/89/$03.50

© 1989 Elsevier Science Publishers B.V.

414

To establish the coalification trend, these parameters have to be measured and interpreted. The sequential events are mainly controlled by pressure, time, and temperature in a basin and result in a progressive increase in rank, but it is very difficult to know the interplay of these factors during coalification. However, the systematic changes occurring during different stages of coalification are established (Teichmfiller et al., 1975). Coal is the end product of chemical processes whereby peat underwent changes in terms of the loss or degradation of cellulose, lignin and protein, and other organic compounds. In the initial stages of coalification, biochemical processes played a vital role. Later, geochemical processes transformed the biochemical products into the present-day coal. The purpose of the present investigation is to examine the factors controlling the coalification trends in Indian coals. In the process, the variations in chemical composition, paleodepositional environments, rank and petrographic make-up of the different coal seams of India are critically examined. GEOLOGIC SETTING

India was a part of Gondwanaland. Ninety-nine percent of the commercial coals in India are Gondwana coals of Permian age. The remainder is Tertiary coals and lignites. The geok gical distribution of coal in India is shown in Table 1. The occurrences of different coalfields in India are shown in Figure 1.

Physico-chemical characters of Indian coals Peat occurs in many places in India, including the outskirts of the city of Calcutta. Here the peat is amorphous (Pleistocene to Holocene), moist to the touch, sooty black, sometimes brownish and easily splittable. Humified, dark, soft unconsolidated layers, 5-10 mm thick and freely admixed with clastic detritus, are easily available in the buried channel system. Low moisture content (12.3%, air dried), high ash (51.5%) and volatile matter (74.9%, d.m.f.) contents characterize the Calcutta peat. As the ash content is greater than 50%, technically Calcutta peat is peaty clay (see Cameron et al., 1989). Lignites of Palana, Barmer and Neyveli (Fig. 1 ) contain moisture 6.9-15.8% (air dried) and volatile matter 52.2-64.3% (d.m.f.). These lignites are brown to dark brown in colour. In Assam, the Tertiary coals are intermediate in rank between lignite and bituminous coal. They differ from those occurring in the other parts of the country because of their greater compactness with conspicuous banding, black colour, lower moisture (0.9-1.2%, air dried), and variable volatile-matter (42.947.9%, d.m.f.) contents. These are lignitous coal according to Seyler's classification (Seyler, 1938). Indian bituminous coals of Permian age are brightly banded. The vitrain

415 TABLE 1 Geological distribution of coal in India Coalfields

Tertiary coalfields

Upper Gondwana coalfields

Lower Gondwana coalfields

Geological horizons

Localities

Early Pleistocene to Upper Pliocene

Lignites in the Karewa formations of Kashmir Valley

Miocene

Lignites in the Cuddalore Series of South Arcot, Tamil Nadu, and Varkala and Quilon in Kerala.

Oligocene to Upper Eocene

Lignites in the Barail Series, in Jaipur, Nazira, and Makum coalfields of Upper Assam; Namchik and Namphuk coalfields of Arunachal Pradesh

Middle Eocene

Lignites of Palana, Rajasthan; lignites of Kutch.

Lower Eocene

Coals in the Jaintia Series of Cherrapunji, Mawlong and Shillong in the Khasi and Jaintia hills; Garo and Mikir hills, Assam; coalfields of Western Assam Daranggiri, Rongrenggiri in the Garo hills; Jammu coalfields - Kalakot, Metka, Mahogala, Chakar, Dhanswal Sawalkot.

Upper Jurassic

Chikiala and Kota in the Kota Stage in Maharashtra; Satpura region, in Jabalpur Stage, Madhya Pradesh; and Ghuneri in Kutch below the Umia Stage.

Upper Permian

Raniganj, Jharia, Bokaro and Karanpura coalfields of the Damodar Valley in West Bengal and Bihar.

Lower Permian

All Lower Gondwana coalfields of the Indian Peninsula, including Damodar Valley, Mahanadi Valley, Brahmani Valley, Sone Valley, Pench-Kanhan Valley, Pranhita-Godavari Valley and Wardha Valley; coalfields of the Eastern Himalayas; Darjeeling district of West Bengal; Ranjit Valley in Sikkim. Abor, Daphla and Aka hills of Assam.

bands are of variable thickness (generally 1-5 mm). Coal seams are usually thick and interbanded with distinct partings that contribute to the high ash contents of Indian coals. The physico-chemical alteration in the initial stage of coalification (i.e., diagenesis) involves reduction in moisture and volatile-matter contents with increasing depth. This is mainly caused by compaction and decomposition and alteration of plant matter during burial. The different stages of coalification directly relate to the hydrogen, carbon and volatile-matter contents of the coals. The relation between the maximum reflectance in oil of vitrinite (Ro max) and volatile-matter contents of Indian coals (Fig. 2 ) indicates a gradual change in volatile-matter contents from the lowest-rank peat (Calcutta "peat", Fig.

416

SCALE

~-~

ii

R

52"

I I . AUmU,IGA 12 HUTAR 13. OALTON6$~J 14. R&JMAHAL tS. DEOGARH ~E. GIRIDIH 17 TATAPANI 18. RAbIKOLA 19. SINGRAULI 20 BiSRAMP~UR 21 .JHILIMILI 22. SONHAT

MAORJcS

""

--

53

~3. 34. 35. 36. ~,7. 38. 39. 40, 41. 42 43 44.

TERTIARY

U~RIA EORAR PENCH" K~qHAN TAWA VALLEY IB RIVER TALCHER W&RDHA VALLEY KAMPTEE UMRER BANOER 600AVARY VALLEY HIMALAYAN FIELDS

COALFIELDS

...........

,o . . . . . . . .

4(I. 47. 4e. 49.

. 52. 5:5. S4.

MAKUM OILLI-JEYPORE NAZIRA LAKHUNt

OARRANGIRI $1JU CHERRAPUNJI KALAKOT

TERTIARY LIGNITE FIELDS 55. NEYVELI 57. PALANA 56, UMARSAR]LAKHPAT 58. NICHAHOM

;,"

~

.'o"

~"

I

I

Fig. 1. Permian and Tertiary coalfields of India.

2) to the semianthracite of Metka and Kalakot coalfields of J a m m u (Fig. 2), under normal conditions of coalification. The mean curve is obtained by plotting Ro max against volatile-matter contents of the normal British Carboniferous coals by Chandra (1965a) as shown in Figure 2, which is a normal coalification curve. It will be observed that Indian coals of different geological ages (Table 1) follow the same curve (Fig. 2). The relationship between the Ro m~ and fixed-carbon contents of Indian coals is shown in Figure 3. For comparison, the mean curve for normal British

417 80

I

/~ •

A

75-

Calcuttaj West Bengal

Peat Lignite

Palana, Ra]asthon Neyveli~ Tamil Nodu Sarmer ~ Rajasthan

®

70-

65-

Coal

Makum~ Assam

EE~ []

-

Kofhagudam~ Andhra Pradeeh Srahmani, Orissa

[]

-

Ib-RiverjOrissa

~

,,

TaLcher~ Orissa

~AX

Rajmahat ~ Bihar

L

Upper Balkudra seamjSouth Karanpura~Sihar Rajna(Jar'~Sohogpur coalfield j Madhya Pradesh

60-

Bijurij SohocJpurcoalfieidj Madhyo Prodesh

(I)

Sohorjuri~Deo0arh coalf eld~B bar

~J

55-

Rakhikol seam T ~Pench ~ Kanhan coalfieldj M. P. m

~X tD

~

50-

~

45-

Kargoli seamj Bokaro ¢oalfield~ Bihar J Domagorio seam~ Roniganj coal field,West Bengal Jharia ~X'ffand T seamsjJhar[a coolfield~Sihar Seam I ~Sohag pur ( Baheraband ), Modhyo Prodesh Rajaroh~ Daitonganj c¢alfield,Bihar Metka~ Kalakot coalfields~Jammu

"

01~"

.2

•..

40-

\

35 0

\

30

®~\\0 \

25

\

\

\ \

~20

\

",¢

15-

\ I0-

\

\

\

5-

O

0

i

i

I

'0!51

I

t

i

II

1"5

i

2"0

2 !5

3!0

3!5

4!0

4!5

5'0

(Romax) ~,~ Fig. 2. Relationship between volatile matter (wt.% d.m.f.) and Ro m~x of huminite/vitrinite at different stages of coalification in India.

418

Carboniferous coals as obtained by Chandra (1962, 1965b) is drawn in Figure 3. It is to be noticed that the relationship between R . . . . and fixed-carbon contents of Indian coals of various geological ages (Table 1 ) follows the same normal curve for the British Carboniferous coals. The relationship between the elemental carbon and hydrogen contents of Indian coals is shown in Seyler's diagram (Fig. 4). It will be observed that the plots of elemental carbon and hydrogen contents of Indian coals follow Seyler's band, which was originally constructed by plotting carbon and hydrogen of normal regionally coalified, brightly banded British Carboniferous coals (Seyler, 1938). The Seyler's band is, therefore, indicative of the path of normal coalification. It may be observed from Figure 4 that some of the plots of Indian coals, belonging to the lignitous group of Seyler, are scattered above and below Seyler's band. The scattered plots are linearly disposed parallel to Seyler's band (Fig. 4). The maximum deviation is +0.3% hydrogen above the upper limit and -0.3% hydrogen below the lower limit of Seyler's band. It is significant that the scatter of the plots (0.3% hydrogen) is symmetrically distributed above and below Seyler's band. It is to be noticed that beyond 84% carbon, i.e., with the onset of Bituminous coal rank of Seyler's classification, the plots of elemental carbon and hydrogen of Indian coals tend to follow Seyler's band almost exactly (Fig. 4). The best type of Carboniferous coking coals of Great Britain are of orthoand meta-bituminous coal rank of Seyler (Fig. 4) (Seyler, 1948). It is remarkable that Indian prime coking coals of Permian age also fall exactly within the 40 i 3"5

E

~

E

3'0

\

25

x

~

2"0

"~ 0 .E

15

~

ro

~

0'5

x \

--[~" . . . . . I

95

I

t

I

I , 90

,

,

~

I 85

,

t

i

,

I 80

S

Carbon wt. Z

I

,

,

I 75

I

"~-~I . . . . . . . . . . . I

I

i

I 70

I

I

I

I

~ .... I , 65

I

I

I

I 60

'

I

I

I

55

(d.m.f.)

Fig. 3. Relationship between elemental carbon (d.m.f.) and R .... of vitrinite/huminite macerals in Indian coals.

419 7"0

-

6'0

A

1

- ~,=,~, ~6t~-~- - - ' ~ -

^ para mete C)~,c~;~ r;lortho

E

..o ~,~

I

""50

- -

-

_

l__

or bono ¢eou$ -~.~,

t,0 4"0

P

-Semianthraci~ nthrocit e / ~ / / 7 ~

g r ° u P ~ / ~ S e y l e r ' s coGlificationband I

3o

IIi 2"0 I00

I

I

I

I

I

I

95

90

85

80

75

70

Carbon

65

wt. 7. (d.m.f.)

Fig. 4. Relationship between elemental carbon (wt.% d.m.f.) and hydrogen (wt.% d.m.f.) contents of Indian coals.

ortho- and meta-bituminous coal range of Seyler's classification (Mukherjee et al., 1982; Kumar, 1984; Chatterjee, 1985). In Great Britain, hard foundry cokes are obtained from the meta-bituminous coals of South Wales and Durham (Seyler, 1948). Similarly, the meta-bituminous Permian coals of the Jharia coalfield in India also are hard foundry cokes. The Tertiary coals of J a m m u (Metka, Kalakot and Mahogala coalfields) vary from semi-bituminous to carbonaceous rank of Seyler (Fig. 4). Petrographically, they show semi-anthracitic characters. The absence of anthracite in Indian coals indicates that the coalification conditions (particularly temperature and pressure) were not high enough to produce anthracite in India. DEPOSITIONAL CHARACTERISTICSOF INDIAN COALS The biochemical and geochemical changes during diagenesis and metamorphism control the entire process of coalification. The influence of temperature, pressure, and duration of heating during burial gradually transforms peat to coal. The burial history of peat differs in temperature and in tropical zones. The accumulation rate of tropical peat is 3-4 mm per year (see also Cameron et al., 1989 ) in contrast to lower values of 1-1.3 m m per year in the subtropical areas, and 0.5-1 m m per year in the temperate regions (Anderson, 1964). The important process during the peatification is the formation of humic substances that are mainly controlled by the oxygen supply, temperature, and alkalinity

420

of the peat swamp. Peatification embraces microbial and chemical changes during peat formation (Teichmiiller et al., 1975). Climate, ecology, and Eh value (redox potential) have a pronounced influence on the bacterial activity during peatification. Humic acid is released from the lignite due to oxidation, and huminite becomes a typical coalification product. The degree of humification depends not only on the depth, but also on the facies. The peat and soft brown coal (lignite) boundary is placed at a depth of 200-400 m (Teichmiiller et al., 1975). At this depth, gradual degradational diagenetic changes bring about a progressive increase in carbon and a decrease in moisture content of the peat. In fact, moisture and elemental carbon contents are the two main parameters for the distinction between peat and soft brown coal (lignite). The optimum temperatures needed for cellulose-destroying peat bacteria in tropical areas are between 35 and 40 ° C; in the temperate zone they fall to 24-28 ° C. Calcutta "peat", which occurs in a tropical climate, is supposed to have experienced higher temperatures, greater depth of burial and more acidic conditions. The resultant end-product has lower moisture (12.3 %, air dried) and a high volatile-matter content (74.9%, d.m.f.). The high ash content (51.5%) is from stream-deposited detritus that makes it a peaty clay. Differences in physico-chemical characters between Indian lignite and those of the Ruhr Basin, Federal Republic of Germany, suggest an entirely different paleodepositional history. The compositional differences in moisture (6.926.6% ), ash (3.8-40.7%), volatile matter (39.4-64.3%), hydrogen (4.5-5.4%) and total sulphur (0.9-3.7%) observed in Indian lignites of Eocene to Pleistocene age (Table 1 ) are due to different processes of biochemical coalification, perhaps, due to variable depth of peat burial which restricted the access of free oxygen and, thus, retarded the decaying action of aerobic bacteria. The following are the several requisites necessary for the biochemical changes to produce lignite from peat: (1) periodic wetness and dryness of the peat; (2) a moderate degree of aeration; (3) preferably an alkaline (pH 8-8.5) and low acidic medium (pH 5-6); (4) presence of microorganisms during accumulation and degradation. Lignite usually becomes darker if it contains a high proportion of humic substances that are severely biochemically gelified and shows increase in reflectance values of huminite macerals. Abundance of pyrite of both framboidal and microcrystalline forms in lignite from Palana (Rajasthan), Barmer (Rajasthan) and Lakhpat (Gujarat), and in Tertiary lignitous coals of Assam (Makum coals) have generated a number of opinions on the origin of pyrite and related paleodepositional environments. The earliest formed pyrite is usually preserved as framboids and associated microcrystalline forms that are followed by non-framboidal granular to massive aggregates (Chaudhuri et al., 1982). These forms are all pre-compactional forms, whereas massive pyrite occurs as small irregular epigenetic veins within other textures and is consid-

421 ered to be of postcompactional origin (Love et al., 1983). This pre-compactional pyrite is related to the influx of marine aqueous sulphate after deposition of the peat (Lyons et al., 1989). Primary siderite (FeCO3) formed before marine transgression can also be transformed to pyrite by ascending or descending H2S in solution (Smyth, 1966). The impregnation of pyrite in coal constituents (in huminite and fusinite) may occur during coalification or after coalification. Recent studies on the distribution of sulphur in peat-forming environments of southern Florida by Cohen et al. (1984) have established that marine to brackish peat contains more pyrite (and total sulphur) than the fresh-water type. The foregoing observations are in agreement with the marine-influenced lignite, especially Tertiary lignite from Rajastha11, Gujarat and lignitous coals of Assam. Lignite from Neyveli (Fig. 1), perhaps, has undergone a lower degree of decomposition which resulted in the preservation of woody tissues (Chandra, 1958). The moisture content up to 14.7 (air dried), lower sulphur (less than 1.0), and lower rank of huminite (Ro max 0.30% ), of this lignite indicates their shallower depth of burial in a deltaic to near-shore backswamp paleoenvironment (Sidhanta, 1986). It shows more affinity to peat than woody lignite noted elsewhere. Mackowsky (1953) suggested that the mode of formation of soft brown coal (lignite) is different from that of hard brown coal (lignite), and that there is no transition from one to the other. It is also noticed that soft brown coal resembles mature peat in composition and most properties. In fact, peat can be converted to hard coals by the application of temperature and pressure (Francis, 1961 ). In India, Fox (1931) confirmed that Eocene lignite of Palana (Fig. 1 ) had formed from the peat initially deposited under marine conditions. Later the peat was exposed to the air due to the regression of sea. According to Fox (1931), as a result of this exposure of the peat to the air due to the regression of sea, the process of coal formation was totally arrested. As such, further enhancement of rank could not take place. There was partial conversion of cellulose, lignin and protein to humus or humic acid under low-temperature oxidation conditions. Paucity of resins, tannin, pigments, and wax in cell walls in Indian lignites suggests very slow processes of biochemical coalification, which inhibited the development of fluorescent characters. The Oligocene coals of the Makum coalfield, Assam, were also marine-influenced during deposition. Dynamic folding and heating during the Himalayan orogeny enhanced their rank from lignite to lignitous coal, according to Seyler's classification. The physico-chemical characters of the bituminous ~ a l s of India vary laterally and vertically in coal seams in the major Gondwana Permian basins. The degree of coalification as indicated by volatile matter and R . . . . shows a normal increase with depth (Table 2). The coal seams from depths of 900 m and above in the Jharia and East Bokaro coalfields indicate changes in rank

422 TABLE2 Variation in volatile matter and vitrinite Ro maxwith depth in a few borehole sections of Gondwana coalfields (Permian) of India Coalfields and borehole

Depth range (m)

V.M. range (wt. % d.m.f. )

Reflectance range (R . . . . )

Saharjuri (SJ-2) Jharia (JKP-5) East Bokaro (EB-38) South Karanpura (SKP-2/1) Sohagpur (SBJ-31)

270.7- 395.7 213.8- 991.9 954.5-1212.3 366.8- 739.5 209.5- 426.2

41.2-32.9 29.5-18.7 29.8-22.0 39.1-36.5 34.5-26.2

0.62-0.66 1.40-1.80 0.79-1.70 0.66-0.84 0.89-1.01

TERRESTRIAL

B

lO0~

C too~,/

_

\too% A+D

\FOREST MOOR

\O E OOR/ / ~ ~

/ / V

A = Duroclorite B = Vitri nertite I + Fusite C = Vitrite + Clorife+Vitrinertite V O = Durite + Clorodunte

tO0

D LIMNIC

Fig. 5. Microlithotype composition of a few major Gondwana coal seams of India plotted on a facies diagram (after Hacquebard and Donaldson, 1969).

423

and morpho-physical characters and petrographic composition mainly due to variation of temperature and pressure with depth (Fig. 1; Chakrabarti and Bardhan, 1986). But it is yet to be ascertained how the paleodepositional environments differed during coal sedimentation. A study of certain coal seams of Gondwana basins of India suggests contrasting paleodepositional environmental patterns that resulted in compositional (both maceral and microlithotype ) variation in the seams. Hacquebard and Donaldson (1969) have shown that different paleoenvironments of coal deposition are related to the depth of water in the peat swamps which affected both the types of vegetation developed and mode of preservation of petrographic entities. The characteristics of different microlithotypes of various moor environments are accordingly identified. These are Forest Terrestrial Moor (FTR), Reed Moor (RM) and Forest Moor (FM) and Open Moor (OM). A four-component peat facies diagram is developed following the work of Hacquebard and Donaldson (1969). It is based on microlithotype composition of the Indian coal seams as represented in a diagram (Fig. 5 ) with four apices showing duroclarite (A); vitrinertite I (where inertinite > vitrinite ) and fusite (B); vitrite + c larite + vitrinertite V (where vitrinite > inertinite) (C); and durite + clarodurite (D). These are the dominant microlithotypes that occur in Indian coals. Figure 5 suggests that most of the deposition of Indian coals was in Forest Terrestrial Moor to Forest Moor. These have resulted from deposition under dry conditions above the water table and also from periodic submergence of the peat swamp due to rise in water level, thus inundating the entire swamp area in some parts of the basin (Chakrabarti, 1985). This accounts for fusain-rich or vitrain-poor seams in the basins. The paleoecological pattern suggests deposition in telmatic to limnotelmatic water-table conditions which explain the compositional variation in the coal seams in the Indian coalfields from east to west (Fig. 7 ). Paleodepths of 1800-4000 m involving paleotemperatures of 80-220°C (Fig. 7) probably controlled the formation of the major coal seams in the Indian basins. It is apparent that low-aquatic to subaquatic depositional conditions evolved to dry terrestrial deposition conditions along with the progressive shallowing of the basins, which was accompanied by intensive oxidation from east to west. Vitrite + clarite in the seams of the Raniganj and Jharia coalfields in the east, gives way to "intermediate" and durite + fusite composition in the coalfields of the west (Fig. 7). The changing diagenetic conditions due to variations of depth, temperature and heating were instrumental in affecting the coalification trends. RANK STUDY, PALEOTEMPERATURE AND PALEODEPTH OF COAL BASINS IN INDIA

The determination of rank in assessing the degree of maturity of organic matter and establishment of coalification trends are of immense importance.

424

Several parameters are taken into consideration of rank determinations, such as, volatile matter, elemental carbon, hydrogen and reflectance in oil of huminite/vitrinite. The reflectance values of vitrinite and reflectance anisotropy have been successfully applied for estimating temperature and pressure conditions in coal basins (Chandra, 1964, 1965c; Bostick, 1974; Hower and Davis, 1981). The rank enhancement in a coal seam is pressure-temperature and time oriented (Chandra, 1965d). Several factors are responsible for determining rank of a coal in a basin. These are: (1) exposure of the strata to high temperature for brief duration due to igneous intrusions; (2) high heat flow from the basement; (3) depth of burial and depth of basement and thermal conductivity of rock enclosing the coal seams; and (4) tectonic shearing of coal-bearing strata with or without high heat generation (Chandra, 1965c; Strauss et al., 1976; Bustin, 1984). It is now imperative to know the intensity of heat flow that prevailed during coalification in the basins that preserve the coal sequences in India. A close perusal of reflectance-temperature-time nomograms by Middleton (1982, fig. 2 ) for the Sydney basin, Australia, shows how at a given time, the reflectance of vitrinite varies in different basins. The paleogeothermal gradient reconstructed following the work of Huck and Karweil (1953) shows two distinct trends of paleogeothermal gradient in the coal basins of I o cia. The first is 4 °C per 100 m and another above 5 ° C per 100 m which, in fact, demarcate the field of high and low rank coal regimes (Fig. 6). The level of coalification also depends on the thermal history of the strata. The slow and gradual rise of paleogeothermal gradient over a prolonged period of time causes a gradual increase in reflectance. The sudden rise in temperature due to basic intrusives within coal seams also enhances the rank, but much more rapidly. This leads to thermally metamorphosed coals. In this case, the chemical composition of the thermally metamorphosed coals follows a line joining the original composition (elemental carbon and hydrogen) with 100% carbon plotted on a Seyler's diagram (Chandra, 1963, 1965c ). The relationship between R . . . . and volatile matter of thermally metamorphosed coals deviates from that of normal coal. For the same carbon content, thermally metamorphosed coal shows a higher reflectance (Ro max) than normal coal (Chatterjee et al., 1964). The paleotemperatures determined from a nomogram after Middleton (1982) indicate that the temperatures between 80 and 120 °C control the coalification for attainment of rank between Ro max0.40 and 0.60% in the Indian basins. The higher-rank coals of the Raniganj, Jharia, East Bokaro, Baheraband (Sohagpur) and Rakhikol-Seam I (Pench-Kanhan valley coalfield) (Fig. 1) are indicated by higher R . . . . values greater than 0.90% due to temperatures attained between 160 and 220 ° C. The corresponding paleodepth is estimated to be from 1600 m to 4000 m (Bostick, 1974). The progressive decrease in the

425 •

Q

O

500

High ronk coat

IOOC

E

1500

J~ G E3

INDEX

0 dharia (JKP)

2000

Bokaro ( E B ) C) Sohagpur (SBJ) Sahorjuri ( S J ) IR SouthKoronpuro (SKP)

2500

0

I0

20

30

40

Volatile matter wt. ,~ (d.m.f.)

Fig. 6. Estimatedcourseof coalificationand paleogeothermalgradientsfor a fewboreholesections in India (after Huckand Karweil, 1953). rank, vitrite and clarite constituents, paleotemperature, paleodepth and increase in durite + fusite constituents from east to west in the coal seams is shown in Figure 7. It is concluded that shallowing of the basin in general influenced the maturity of coal in the basins of India. The rank can also be evaluated by another parameter, namely, anisotropic character of the vitrinite. The normal increase in rank in the borehole profiles due to increase in temperature is quite evident, but the effect of pressure on coal metamorphism is least understood. A distinction has been made between chemical and physico-structural changes in coalification (Dulhunty, 1954; Teichmilller et al., 1975) and is reflected in optical anisotropy. Vitrinite reflectance anisotropy is indicative of a pressure effect, as pressure promotes the optical anisotropy which is apparent from the bireflectance (Chandra, 1965c; Goodarzi, 1985). Vitrinite in bituminous coal is known to be optically anisotropic (Chandra, 1965c, d). As the rank increases, volatile matter is decreased or fixed carbon increased with a concomitant rise in anisotropy through progressive adjustment of predominantly aromatic lamellae in the bedding plane due to increasing load or other pressure. This anisotropy appears in vitrinite in Indian coal at Ro max 0.45% and is more pronounced above 0.90%. A progressive increase in rank and the rise in overburden pressure have the effect of producing a degree of orientation in the aromatic lamellae which results in a

JHAR~,~ t • • "~ .,'~"

2000 m-

1500 m-'-

/

c

[

)Z

1SANYAL AND SUBRAMANIAN, 1977

WEST

VOLUME %

SCALE)

"BOSTICK ~ 1 9 7 4

( N O T TO

VOLUME %

)

JHARIA SEAMS

( NOT TO SCALE)

:~ENCH AND KANHAN SEAMS

}ENCH AND KANHAN Ro max 0 " 8 - - r 0 2

WARDHA VALLEY SEAMS

20-

40--

IWARD HA VALLEY R 0 mox 0'5 ~ 0 " 7 ~

U

I,-

e, ;>-

MJ

60--

30-

soX

,ooX

t

O

Ro

~o7,

:AST

~ooZ

0"7--1'5 %

VOLUME Z

max

IINTERMEDIATES ~'

_220°C

.160=C

,oo°c

80°C

oILl

Q.

>

f

E

X O E

FEMPERATURE(°C)

pURWE+ FUSITE

VITRINERT,TE-I AND V~

I RANIGANJ

m

~

- - VITRITE - • + CLARITE

3 MIDDLETON, 1982

7.

1'1~1"6~

VOLUME

R o max

JHARIA

RANIGANJ SEAMS

\.~~..~c~--~ ~ ) 1

?ig. 7. E s t i m a t e d course of coalification assuming various t e m p e r a t u r e paleogradients in a few borehole sections.

q.OOOm

3500m-

< ~ 3ooom"E"

o

I--

~z

~"

rr

E o

"~

%WAe0MA

~KANHAN VALLEY

,t.f '/ ~ j , ,/"%. '~,PI£NC.HANO"~"~, f" %' /2

.,,.,.~ ")% "'2 ;' <

:T)

427 gradual increase in the bireflectance of the coal [(Ro m~-Ro mean)/Romax] (Hower et al., 1981; Levine and Davis, 1989). It has been shown that by plotting Ro m~ (maximum reflectance in oil) against Ro r, in (minimum reflectance in oil) in a diagram (Fig. 2, Chandra, 1963) normal coals can be distinguished from thermally metamorphosed coals. Also a distinction can be made between coals thermally metamorphosed under normal atmospheric pressure (referred to as without excess pressure) and under pressure (mentioned as with pressure). Indian coals have been thermally metamorphosed mostly by mica peridotite dykes and sills and rarely by dolerite dykes. By studying anisotropy, all the Indian coals have so far been found to be thermally metamorphosed without excess pressure. It has been observed that the volatile displacement of thermally metamorphosed coals, like that of the carbonized coals, exceeds + 2.5% (Chandra, 1988; Chandra and Srivastava, 1980). Volatile displacement is the difference between experimentally determined V.M. (volatile matter) and calculated V.M., i.e., V.M. (experimental) - V.M. (calculated). The calculated V.M. can be obtained from the following equation: V.M. (calculated) = 10.61 H - 1.24 C +84.15 where H and C are elemental hydrogen and carbon contents. Seyler (1938, 1948) introduced the concept of volatile displacement. According to him, bright coals (the plots of carbon and hydrogen of which follow Seyler's band, i.e., follow the normal coalification trend) show volatile displacement values between +2.5%. Volatile displacement, therefore, can be taken as a measure of normality of coal. If volatile displacement of any coal exceeds + 2.5%, then the coal may be said to be "abnormal" (Chandra, 1985 ). In other words, the interrelationship among volatile matter and elemental carbon and hydrogen holds up for normally metamorphosed coals within limits of volatile displacement of + 2.5%, whereas the same relationship does not hold good for thermally metamorphosed coals (Chandra, unpub, data), weathered/ oxidized coals (Chandra, 1962), burnt coals (Chandra et al., 1984), or dull coals (Seyler, 1938, 1948). In this context, it may be mentioned that although, so far, the interrelationship between volatile matter, carbon and hydrogen is known, the relationships among temperature, pressure, and degree of metamorphism of normally metamorphosed coals are still an unresolved problem. PETROGRAPHIC MAKE-UP Calcutta peat, showing partly decomposed plant materials, has not undergone extensive lignification, suberinitization (alteration of cell walls of cortex) or cutinization (alteration of cutinite). Amorphous humic particles and un-

428 decomposed detritus, such as t h a t comprising rootlets and tracheids (Fig. 8 A, B ) suggest the initiation of biochemical processes in a "peatigenic" layer. Resultant humic detritus of these cell walls do not show fluorescence of waxy components. M a k u m coals of Assam cannot be considered as lignite in view of their abnormal chemical characters (Chandra et al., 1984 ). These coals may include a substantial percentage of inertinite components, up to 11% by volume (Sanyal and Chakrabarti, 1987 ) which are almost absent in lignite of Palana, Barmer, Lakhpat and Neyveli (Fig. 8 C, D). The M a k u m coals are entirely composed of vitrinite with varying proportions of resinite, bituminite, cutinite and sporinite (fungal spores). Pyrite is quite abundant in the Tertiary coals of India, except for Neyveli lignite, which contains a low percentage of pyrite. This casts doubt about its marine origin. There is, however, very little dispute on the origin and petrographic composition of Lower Gondwana coals of India. The petrographic composition, both maceral and microlithotype composition, reveals regional variations among the seams in the different coalfields examined in the present study (Fig. 7). The total reactive or fusible constituents i.e. vitrinite, liptinite and semifusinite transitory to vitrinite (Chakrabarti, 1986) or semivitrinite (Chaudhuri and Ghosh, 1978) show a gradual decrease from the Raniganj coalfield to the Wardha valley coalfield and a concomitant rise in inertinite and mineral-matter contents. Similarly, the total vitrinite ÷ clarite gradually diminishes with corresponding increase in durite ÷ fusite (Fig. 7). This compositional variation has a distinct link with the paleodepositional and paleoecological environments that prevailed during the biochemical coalification within these coalfields. E n h a n c e m e n t in carbon content in a coal can be effected both by biochemical and geochemical processes, but the maceral types and their relative abundance are fixed mainly at the biochemical stage only and are very little influenced by later geochemical action (Sanyal and Subramanian, 1977). The inertinite constituents show the high carbon content compared to other maceral constituents because they are the result of Fig. 8. A, B. Deep brown huminitized tissues in a peat with partly decomposedplant fibers appearing opaque. The circular body represents a rootlet in which a cell cavity is filledwith mineral matter; Calcutta peat (transmitted light). C, D. Pair of photomicrographsunder plane incident light (C) and fluorescence(D). Bright pyrite framboids impregnated in light grey huminite appearing nonfluorescingunder reflected light, whereaselongatecutinite (yellowish)and bright resinite droplets(yellow)are recognizableunder fluorescence.Tertiary lignite, Lakhpat, Gujarat; E. Resin-filledyellowishtracheid preserved in vitrinite. Tertiary coal, Makum coalfield,Assam (transmitted light), F. Sporinite and oval-shapedresin bodies embeddedin vitrinite. Tertiary coal, Makum coalfield, Assam (transmitted light).

e.D

430

intensive oxidation under aerobic conditions. The profuse fungal activity and their subsequent preservation in coals are quite evident in some inertinite-rich seams (Fig. 9 A, B, C, D ) in the Pench-Kanhan and Jharia coalfields (Fig. 1 ). The variation in rank and petrographic composition may also be correlated with the evolutionary history of the coal basins, particularly those of intracratonic graben types that are characterized by tensional faults, which perhaps caused intermittent oscillation of the water table that exposed the seams to varied basement heat (Laskar, 1977). The regional variation in environment and coalification conditions in the coal seams of India suggest faster sinking rate of the easterly basins than their westerly lying counterparts. This had tremendous impact on coalification in these basins. In fact, besides Jharia, Raniganj and East Bokaro, a widespread lateral continuity of non-coking coals are found in other basins, except for a few sporadic occurrences of coking coal pockets (e.g., Baheraband in Sohagpur, Rakhikol Seam I in Kanhan valley coalfields. Madhya Pradesh, Fig. 1 ). The basic difference between coking and non-coking coals, perhaps lies in the fact that the type and proportion of vitrinite preserved in a seam vary from one seam to another. Vitrinite (A or telocollinite type) with Ro m a x greater than 0.80% is considered to have developed coking properties in Indian Gondwana coals. Vitrinite at this reflectance level develops fluorescent properties that are due to absorption by vitrinite of bituminous substances formed during coalification (Wolf et al., 1983 ). Whether this enhances coking potential of vitrinite is to be ascertained. Telinite is known to possess greater coking power than collinite. The optimum quantity of inertinite required in a coking coal is yet to be precisely determined. However, it is noticed that "intermediates" that comprise vitrinertite V (vitrinite > inertinite ) or vitrinertite I (inertinite > vitrinite ) contribute a significant percentage in the coking coals of India (in Jharia, Raniganj, Bokaro, Sohagpur and Pench-Kanhan coalfields (Fig. 10 A, B ). Petrographically, an Indian coking coal contains > 50% vitrinite with Ro maxbetween 0.80% and 1.40%. Chemically, it is found to contain less than 2% moisture, 20-30% volatile matter and elemental carbon between 85 and 91%. In view of the recent development of fluorescence microscopy, it has been easier to diagnose and quantify liptinite constituents present in a coal. Bituminite, fluorinite, alginite, resinite in high volatile bituminous coals, and secondary resin in cell lumens of fusinite and semifusinite are frequently noticed in medium to low volatile bituminous coals in India. Fluorescent inertinite (or fusible/reactive inertinite) corresponds closely to "reactive" fractions determined by coking experiments in Australian coals (Diessel, 1985). As such, their presence is likely to increase the coking character of a coal. The fluorescent colour of sporinite diminishes from bright yellow to faint reddish brown colour with increasing rank. The reduction in fluorescence with increasing rank has been related to the destruction of cellulose and other primary vegetal tissues during coalification (Wolf et al., 1983). As observed in Indian coals, this

431

Fig. 9. A, B. Aggregates of sclerotinites, partly deformed showing vacuoles, slits and without internal structures. Permian coal, Rakhikol, Seam I, Pench-Kanhan coalfield, Madhya Pradesh (reflected light). C. Sclerotinites show the collapse of cell lumens, Permian coal, Jharia Seam XII, Bihar (reflected light). D. Radial cracks developed in vitrinite around sclerotinite. Permian coal, Ekhehra, Seam I PenchKanhan coalfield, Madhya Pradesh.

432

Fig. 10. A. High-rank vitrinite includes microfragmental bits of fusinite. Permian coal, Kargali seam, Kathara Mines, Bokaro coalfield, Bihar {reflectedlight). B. Duroclarite grading into vitrinertite I, Permian coal, Kargali seam, Kathara Mines, Bokaro coalfield, Bihar (reflected light). reduction in fluorescence corresponds to a change from 70% to 89% C of elemental carbon during coaliflcation. CONCLUSIONS The synthesis of the observations made by chemical, petrographic, reflectance and fluorescence studies reveals several aspects of the coalification trends in Indian coals. The petrographic characters of the peat to bituminous coal to semianthracite examined, covering the entire Gondwana and Tertiary basins in India, show a gradual change in morpho-physical character of macerals, reflectance values (Ro max) of huminite/vitrinite, and fluorescent colour and intensity with increasing rank. The trends of coaliflcation of Indian coals have been found to follow the same trend as those of normally coalifled British Carboniferous coals. "In this connection, although the coals of two countries may have the same normal coalification trend, the physical and chemical properties differ widely. This is because the properties of a coal depend, not only on rank, but also on the type of coal. Type of a coal depends on the maceral or microlithotype composition, along with the mineral-matter composition. The amount of maceral constituents or microlithotypes and mineral matter

433 of a coal depends on the nature and origin of coal. The original composition of the peat, the nature of its accumulation, and consequent alteration, by biochemical and geochemical agencies, and a host of other factors play an import a n t role in the composition of different macerals, microlithotypes and mineral matter of coal. As such, there is always a difference, say for example, between an Indian Permian coal and a British Carboniferous coal of the same rank. Essentially there is a difference in the properties, both physical and chemical, due to high inertinite and high ash contents of Indian coal. Characteristically, Indian Permian coals contain a high percentage of inertinite, as compared to the British Carboniferous coals, due mainly to seasonal variations, i.e., alternation of dry and wet conditions, during the formation of Indian coals. Also the high ash contents in Indian Permian coals are due to the allochthonous origin of t h e i n d i a n peats (coals), in contrast to the in-situ origin of the British Carboniferous coals. ACKNOWLEDGEMENTS The contents of this paper are partly drawn from the findings of a research programme being pursued by the junior author for a doctoral thesis for which permission has been granted by the Director General, Geological Survey of India. Borehole data referred to in the paper are taken from the unpublished reports of the Coal Wing, Geological Survey of India, Calcutta.

REFERENCES Anderson, J.A.R., 1964.The structure and developmentof the peat swamp of Sarwak and Brunei. J. Trop. Geogr. Singapur., 14: 7-16. Bostick, N.H., 1974. Phytoclasts as indicators of thermal metamorphism,Franciscan assemblage and Great Valleysequence (Upper Mesozoic),California.In: R.R. Dutcher, P.A. Hacquebard, J.M. Schopfand J.A. Simon (Editors), Geol. Soc. Am., Spec. Pap., 153: 1-17. Bustin, R.M., 1984. Coalificationlevels and their significancein the GroundhogCoalfield,North Central British Columbia.Int. J. Coal Geol., 4(1 ): 21-49. Cameron, C.C.,Esterle,J.S. and Palmer, C.A., 1989.The geology,botany and chemistryof selected peat-forming environments from temperate and tropical latitudes. In: P.C. Lyonsand B. A1pern (Editors), Peat and Coal: Origin, Facies, and DepositionalModels. Int. J. CoalGeol., 12: 105-156. Chakrabarti, N.C., 1985. Petrography rank and depositional environment of certain coal seams, Saharjuri Coalfield,Santhal Pargana District, Bihar. Indian Miner., 39 (1): 12-27. Chakrabarti, N.C., 1986. Petrography and rank in relation to coking characters of certain coal seams in Sohagpur Coalfield,Madhya Pradesh, Proc., National Seminar on Coal Resourcesof India, Varanasi, India, pp. 244-263. Chakrabarti, N.C. and Bardhan, B., 1986. Vitrinite reflectanceand reflectanceanisotropyin evaluation of coal metamorphism - A study of deep borehole core samples from Jharia and East Bokaro Coalfields,Bihar. Indian Miner., 40 (3): 52-60.

434 Chandra, D., 1958. Microfossils in lignite of India and Pakistan. J. Paleontol. Soc. India, 3: 211213. Chandra, D., 1962. Reflectance and microstructure of weathered coals. Fuel, 41: 155-163. Chandra, D., 1963. Reflectance of thermally metamorphosed coals. Fuel, 42 {1 ) :69-74. Chandra, D., 1964. Use of reflectance in evaluating the pressure effect on thermally metamorphosed coals. 22nd Session of the International Geological Congress, Report, Pt.XVI, pp. 387392. Chandra, D., 1965a. Reflectance of Indian coals. Q. J. Geol. Min. Metals. Soc. India, 37: 1. Chandra, D., 1965b. Reflectance of New Zealand Coals. Econ. Geol., 60 {3): 621-629. Chandra, D., 1965c. Use of reflectance in evaluating temperature of carbonized or thermally metamorphosed coals. Fuel, 44:171-176. Chandra, D., 1965d. Reflectance of coal carbonized under pressure. Econ. Geol., 60 (3): 621-629. Chandra, D., 1985. Evaluation of coal characteristics. Proc. 1985 Int. Conf. on Coal Sciences, Sydney, Aus., Oct. 28-Nov. 1, 1985. Pergamon Press, pp. 600-603. Chandra, D. and Srivastava, G.P., 1980. Volatile displacement of burnt coals. J. Geol. Soc. India, 21 (6): 306-310. Chandra, D., Ghosh, S. and Chaudhuri, S.G., 1984. On certain abnormalities in the chemical properties of Tertiary Coals of Upper Assam and Arunachal Pradesh. Fuel, 63 {9): 1318-1323. Chatterjee, C., 1985. The mapping and appraisal of the physico-chemical characteristics of important coals seams above No. VII Seam of the Barakar Formation of the Jharia Coalfield. Ph.D. Thesis, Indian School of Mines, Dhanbad, Vol. I, 136 pp. Chatterjee, N.N., Chandra, D. and Ghosh, T.K., 1964. Reflectance of Poniati seam affected by a mica peridotite dyke. J. Mines, Metals Fuels, 12 (11 ): 346-348, 360. Chaudhuri, S.G. and Ghosh, S., 1978. A preliminary study of reactive semifusinites of Indian coking coals. J. Mines, Metals Fuels, 26(3): 137-141. Chaudhuri, S.G., Ghosh, S. and Chandra, D., 1982. Origin and mode of occurrence of pyrite in Assam coals. Fuel Sci. Technol., 1 (1): 41-46. Cohen, A.D., Spackman, W. and Dolsen, P., 1984. Occurrences and distribution of sulphur in peat forming environments of Southern Florida. Int. J. Coal Geol., 4 ( 1 ): 73-96. Dulhurty, J.A., 1954. Geologic factors in the metamorphic developments of coal. Fuel, 33: 145152. Diessel, C.F., 1985. Fluorometric analysis of inertinite. Fuel, 64: 1542-1546. Fox, C.S., 1931. The Natural History of Indian Coals. Mem. Geol. Surv. India, 57, 283 pp. Francis, W., 1961. Coal, its Formation and Composition. Edward Arnold, London, 806 pp. Goodarzi, F., 1985. Optical properties of vitrinite carbonized at different pressures. Fuel, 64: 156162. Hacquebard, P.A. and Donaldson, J.R., 1969. Carboniferous Coal deposition associated with flood plain and limnic environments in Nova Scotia. In: E.C. Dapples and H.E. Hopkins (Editors), Environments of Coal Deposition. Geol. Soc. Am., Spec. Pap., 114: 143-191. Hower, J.C. and Davis, A., 1981. Application of vitrinite reflectance anisotropy in the evaluation of coal metamorphism. Geol. Soc. Am. Bull., 92: 350-366. Huck, G. and Karweil, J., 1953. Versuch einer Modellvorstellung Vom Feinbau der Kohle. Brennst.Chem., 34(9/10): 129-135. Kumar, V., 1984. Study of coals from the Parej area, West Bokaro Coalfield, District Hazaribagh, B ihar (India), with special reference to the zero seam. D. Phil. Thesis, University of Ranchi, Ranchi, 214 pp. Levine, J.R. and Davis, A., 1989. Reflectance anisotropy of Upper Carboniferous coals in the Appalachian foreland basin, Pennsylvania, U.S.A. In: P.C. Lyons and B. Alpern {Editors), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal, Geol., 13: 341-374. Lyons, P.C., Whelan, J.F. and Dulong, F.T., 1989. Marine origin of pyritic sulfur in the Lower

435 Bakerstown coal bed, Maryland (U.S.A.). In: P.C. Lyons and B. Alpern (Editors), Peat and Coal: Origin, Facies, and Depositional Models. Int. J. Coal Geol., 12: 329-348. Laskar, B., 1977. Evolution of Gondwana coal basins. 4th Int. Gondwana Symp., Calcutta, India, Proc., I, pp. 223-237. Love, L.G., Coleman, H.L. and Curtis, C.D., 1983. Diagenetic pyrite formation and sulphur isotope fractionation associated with a Westphalian marine incursion, northern England. Trans. R. Soc. Edinburgh, 74: 165-182. Mackowsky, M.Th., 1953. Probleme der Inkohlung. Brennst.-Chem., 34 (11/12 ): 182-185. Middleton, M.F., 1982. Tectonic history from vitrinite reflectance. Geophys. J. Astr. Soc., 68: 121-132. Mukherjee, A.K., Chatterjee, C.N. and Ghose, S., 1982. Coal resources of India - its formation, distribution and utilization. Fuel Sci. Technol., 1: 19-34. Sanyal, S.P. and Chakrabarti, N.C., 1987. Petrographic and reflectance measurement studies of Gondwana and Tertiary Coals in India to evaluate the applicability of Coal Petrography in their Coking properties. (Studies of Tertiary Coals from Makum Coalfield, Assam ). Geological Survey of India, unpubl. Rep. for 1984-85, 13 pp. Sanyal, S.P. and Subramanian, C.S., 1977. Petrology of Gondwana coals of India - A comparative study. 4th Int. Gondwana Symp., Calcutta, India, 1977, Proc. I, pp. 305-319. Seyler, C.A., 1938. Petrology and classification of coal. Proc. S. Wales Inst. Eng., 53: 254-407. Seyler, C.A., 1948. The past and future of coal - the contribution of petrology. Proc. S. Wales Inst. Eng., 63 (3): 213-243. Sidhanta, B.K., 1986. The age of Neyveli lignite with reference to stratigraphy and palynology. Indian Miner., 40(3): 61-82. Smyth, M., 1966. A siderite-pyrite association in Australian Coals, Fuel, 45: 221-231. Strauss, P.G., Russel, N.J., Bennet, A.J.R. and Atkenson, C.M., 1976. Coal petrography as an exploration in the Circum-Pacific, In: W.I.G. Muir (Editor), Coal Exploration. Proc. First Int. Symp. London, England, May 18-21, 1976, pp. 401-447. Teichmtiller, M. and Teichmtiller, R., 1975. The geological basis of coal formation, In: E. Stach, M.-Th. Mackowsky, M. Teichmtiller, G.H. Taylor, D. Chandra and R. Teichmtiller (Editors), Stach's Textbook of Coal Petrology. Gebruder Borntraeger, Stuttgart, pp. 5-54. Wolf, M., Wolff-Fischer, E. and Ottenjahn, K., 1983. Fluorescence properties of vitrinites, 36th I.C.C.P. Meeting, Oviedo, Spain, Report Commission 3.