ter 7PETROGRAPHY AND

RANK OF COAL

7.1 INTRODUCTION

Coal is not a homogeneous substance but consists of various

constituents and occurs in rhythmic succession with sedimentary

rocks like sandstone, shale etc. and hence it is considered as a rock.

But unlike the inorganic rocks, it is composed of decayed and

metamorphosed vegetal materials which were deposited along with

other sedimentary rocks. Like the minerals in rock, coal is also

Chapter 7 97

composed of some constituents of organic origin which have some

distinguishing physical properties.

These two fundamental properties of coals viz., the rock

character and the vegetal composition led to the study of coal by the

geologists in two different methods in two sides of the Atlantic. One

in America and the other in the West European countries. The

American concept is based on studying and describing coal in terms

of its initial organic components. On the other hand, in England, the

study of coal was as a rock, composed of petrologic units, irrespective

of its initial botanical composition (Stopes, 1919). The coal is

regarded to be composed of four kinds of constituents or rock types or

lithotypes. The lithotypes are the megascopically distinguishable

bands in coal, viz., vitrain (brilliant bands), clarain (silky lustrous

bands), durain (hard compact material) and fusain (charcoal like

material).

Contribution made by Stopes in megascopic petrology of coal

gained general acceptance in Europe and has been a steady

development in technique and terminology. The microscopic study

was employed to the study of lithotypes and has resulted in

recognition of macerals (Stopes, 1935) or optically homogeneous

aggregates of organic substances possessing more or less distinctive

physical and chemical properties analogous to the minerals as

constituents of rocks. Individual macerals can be grouped according

to their physical characters and thus three maceral groups are found.

Chapter 7 98

The three maceral groups are vitrinite, exinite and inertinite.

Coal petrology is a rapidly expanding subject of great

technological importance and with a highly specialised nomenclature

and complicated by the several system of classification (Francis, 1961;

Krevelen, 1961).

The more discriminating observations and improved methods in

coal petrography coupled with active contribution and co-operation of

the members of the International Commission for Coal Petrology

(I.C.C.P - composed of representatives from all parts of the world) has

resulted in a co-ordinated petrographic classification of bituminous

coal material published in International Handbook of Coal Petrology

(1963, Table 7.1). Though this system is almost purely based on

polished section technique, it has also recognised the presence of

botanical entities like spores, cuticles, fungal bodies etc. described by

thin section microscopy in America.

7.2 MACERAL GROUPS

The macerals are classified into three groups - vitrinite, exinite

and inertinite. This subdivision is conventional and represents a

simplification which is useful in practice. Each group includes a

series of macerals which can be regarded as belonging together,

either because of similar origin (such as exinite) or because of the

mode of conservation (vitrinite and inertinite).

The constituent macerals of the three groups as proposed in

Chapter 7 99

Stach’s Textbook of Coal Petrology (1975) are given in Table 7.2 and

the nomenclature of this classification is used in the study of Bapung

coal.

7.3 MEGASCOPIC CHARACTERS OF BAPUNG COAL

Megascopically the coals of the Bapung coalfield are black to

dark black in colour with dull to glossy lustre. In these coals the

characteristic dull and bright bands of humic coals are absent. The

coal is seen as a homogeneous mass of vitrain. Megascopically the

coal indicates high sulphur content which occurs as minute yellow

particles or granules. On weathering, the coal breaks parallel to the

bedding plane.

7.4 MICROSCOPIC CONSTITUENTS OF COAL (MACERAL)

The microscopic analysis of the coal is carried out in terms of

group maceral composition. Polished blocks of the coal are studied

under reflected light with oil-immersion lens using a Leitz Microscope.

7.4.1 Vlfrlnlte group

The dominant maceral group in the Bapung coal is vitrinite.

Collinite is the dominant maceral of vitrinite group and bears no plant

structure (Fig. 7.1). Collinite is light grey in colour and shows low to

moderate reflectance and occurs as groundmass and sometime in

definite bands.

Chapter 7 100

-7.4.2 Exinitegroup -

The macerals of this group are derived from spores, cuticles,

resins and algae. The exinite group of maceral in Bapung coal is

represented by sporinite and resinite.

The sporinite is the most important maceral of exinite group.

Sporinite occurs as thread like bodies within vitrinite. It has a low

reflectivity than vitrinite and almost opaque (Fig. 7.2).

Resinite is next to sporinite in occurrence in the exinite group.

Resinites are found as rounded and oval shaped bodies mostly as

inclusions in vitrinite and are almost opaque in reflected light. The

resinite includes the plant resin and wax which are also found as

rodlets in vitrinite (Fig. 7.3).

7.4.3 Inerftnlte group

Inertinite macerals show highest reflectance and exhibit well

preserved cell structures. This group of macerals in Bapung coal is

represented by fusinite, semifusinite and sclerotinite.

Fusinite is characterised by presence of cell structure and

higher reflectance (Fig. 7.4 and 7.7). Occasionally fusinite bands

without cell structure are also observed (Fig. 7.5). In some cases cell

structure is crushed producing ‘Bogen structure’ (Fig. 7.6).

Semifusinite shows reflectance between vitrinite and fusinite

and cell structures are less preserved than fusinite (Fig. 7.8). This

maceral shows slightly higher reflectivity than the vitrinite (Fig. 7.9)

Chapter 7 101

and occurs as irregular bodies or as small droplets.

Sclerotinite occurs as circular or irregular body of varying sizes

having high ref!ectance(Fig. 7.10). The maceral represents the

remains of fungal sclerotia. Sclerotinites are found with single and

multicell structure (Fig. 7.11 & 7.12).

7.4.4 Mineral matter

The mineral matter observed in reflected light is dominantly

represented by pyrite. It occurs as disseminated grains and specks

inside vitrinite and sometimes as framboids in definite bands replacing

vitrinite (Fig. 7.13 & 7.14).

7.5 PETROGRAPHIC MODAL COMPOSITION OF BAPUNG COAL

Petrographic modal analysis is carried out with an electric point

counter attached to a mechanical stage in the microscope to

determine the maceral composition. Individual macerals are counted

and finally computed to corresponding groups.

The vitrinite group of maceral is dominant in Bapung coal and it

is mainly represented by collinite. The content of vitrinite group

ranges from 74.43 to 82.62%. The average percentage of the vitrinite

at the top of the seam is 80.03, at the middle of the seam is 79.28

and at the base of the seam is 77.76 (Table 7.3).

In visible mineral matter free basis, vitrinite percentage ranges

from 76.9 to 87.26. The average percentage of vitrinite at the top of

Chapter 7 102

the seam is 83.65, at the middle of the seam is 82.44 and

at the bottom of the seam is 80.74 in visible mineral matter free basis

(Table 7.4).

The second important group of maceral is the inertinite. The

percentage of this group of maceral ranges from 8.69 to 18.83 (Table

7.3). The average percentage of inertinite at the top of the seam is

13.56, at the middle of the seam is 15.29 and at the bottom seam is

15.86.

In the visible mineral matter free basis the percentage of the

inertinite group of maceral ranges from 9.27 to 19.09 (Table 7.4). On

this basis the percentage of inertinite group at the top of the seam is

14.14, at the middle of the seam is 15.89 and at the bottom seam is

16.43.

The percentage of exinite group is less. Its percentage ranges

from 1.17 to 4.32 (Table 7.3). The average percentage of exinite

group of maceral at the top seam is 2.10, at middle seam is 1.61 and

at the bottom seam is 2.74.

In visible mineral matter free basis the average percentage of

the exinite group at the top seam is 2.20, at the middle seam is 1.68

and at the bottom seam is 2.83 (Table 7.4).

The visible mineral matter under microscope in the Bapung coal

is represented mainly by pyrites. The percentage of mineral matter

varies from 1.36 to 6.60. The average mineral matter per cent at the

top seam is 4.31, at the middle seam is 3.83 and at the bottom seam

Chapter 7 103

is 3.65 (Table 7.3).

The petrographic composition of macerals are shown in stacked

bar diagrams in Fig. 7.15 & 7.16.

7.6 MICROLITHOTYPES IN BAPUNG COAL

The Bapung coal never shows sufficient banding character to

determine microiithotypes visually under microscope. The dominant

microlithotype of the coal was determined using ternary classification

scheme of Francis (1961) and the dominant microlithotype is

determined as Vitrinertite - V (Fig. 7.17).

7.7 VITRINITE REFLECTANCE

The reflectance of vitrinite is measured on polished blocks of

the coal using Leitz MPV-2 microscope under oil immersion lens. The

vitrinite reflectance (maximum reflectance with polariser) of Bapung

coal ranges from 0.50 to 0.67% with an average of 0.60% (Table 7.5).

The average vitrinite reflectance of the top seam is 0.64%, that of

middle seam is 0.58% and of the bottom seam is 0.59%.

7.8 RANK OF BAPUNG COAL

The nomenclature subcommittee of ICCP (1963) agreed to

accept the term "rank” as an international scientific term to designate

the degree of coalification attained by a given coal.

The rank of a coal is not directly measurable quantity. It is

Chapter 7 104

necessary to prefer to a specific physical or chemical property which

exhibits adequate change in the course of coalification. Such

properties are numerous but all of them do not change conformably

with carbonification and therefore many are not reliable indices of

rank.

Moreover, the properties of the different constituents do not

change at the same pattern. It is necessary, therefore, in strict

comparison to determine rank on one particular maceral. In

determining the rank of hard brown coal, bituminous and anthracite,

vitrinite is the best reference maceral for the following reasons.

(a) It is the most abundant and therefore, the most

representative maceral.

(b) Its characteristics change fairly continuously and

commensurately with the course of coalification.

The rank of hard coals can be determined by the reflectance of

vitrinite.

According to the ASTM classification the rank of the Bapung

coal ranges from Sub-bituminous B to Sub-bituminous A on the basis

of vitrinite reflectance (Table 7.6).

7.9 MICROHARDNESS OF VITRINITE

Microhardness of vitrinite in Bapung coal is given in Table 7.7.

Identical relation of the microhardness of vitrinite of the seams exists

in the area i.e., top seam shows hardness from 20.2 to 23.6, middle

Chapter 7 105

seam shows hardness from 20,8 to 25.1 and bottom seam shows from

23.1 to 27.6.

The values of microhardness of vitrinite are plotted in Fig. 7.18

with carbon (Table 6.8) as the abscissa and Vicker’s Microhardness

(MH)v (Table 7.7) as the ordinate, it is observed that there is

concentration of the points from 20-27 (MH)v- This indicates that the

vitrinite of the Bapung coal has attained maturity. It was due to the

tectonic influence of the region after the formation of the coal (Nath

and Ahmed, 2002).

Chapter 7 106

Table 7.1 Generalised petrographic classification of coal (after ICCP, 1963)

Megascopicaily identified : components

Lithotypes or banded ingredient• Vitrain• Clarain• Durain• Fusain and• Mineral matter

Microscopically identified : components of the lithotypes

Microlithotypes (natural associations of macerais)

• Vitrinite• Ciarite• Vitrunertinite• Ciarodurite• Durite and• Fusite

Macerais (the fundamental or ultimate units)

• Group macerais, consisting of macerais of general similarity in composition

- vitrinite- exinite (or liptinite) and- inertinite

• Individual macerais, consisting of coal entities derived from or formed by the individual botanical entities of the coal (not to be considered only as plant fossils but are also coal substance)

- Collinite- Telinite- Sporinite- Cutinite- Alginite- Resinite- Micrinite- Sclerotinite- Semifusinite and- Fusinite

Chapter 7 107

Table 7.2 Summary of the macerals of hard coals (In part, after Stach et al. 1975)

Group macerals Maceral

Vitrinite TeliniteColliniteVitrodetrinite

Exinite SporiniteCutiniteResiniteAlginiteLiptodetrinite

inertinite FusiniteSemifusiniteMicriniteMacriniteSclerotiniteInertodetrinite

Chapter 7 108

Table 7.3 Petrographic composition of Bapung coal (in volume per cent)

SampleNo. Vitrinite Exinite Inertinite Mineral

matter

1 81.76 3.25 8.69 6.30

2 78.25 1.52 14.73 5.50

3 80.05 2.95 11.40 5.60

4 77.35 1.27 18.08 3.30

5 78.05 1.28 16.42 4.25

6 82.62 2.16 11.17 4.05

7 ------ ____ 76.50_____ 1.98 18.05 3.47

8 82.02 1.70 13.08 3.20

9 81.86 1.25 13.73 3.16

10 79.24 2.16 12.00 6.60

11 77.92 2.18 13.60 6.30

12 78.06 1.17 14.72 6.05

13 81.54 1.25 14.16 3.05

14 78.54 1.35 15.76 4.35

15 80.04 2.10 13.91 3.95

16 76.25 3.22 18.46 2.07

17 78.06 1.57 17.07 3.30

18 81.03 1.82 13.30 3.85

19 74.43 4.32 18.04 3.21

20 79.32 2.52 15.52 2.64

21 78.39 1.65 17.08 2.88

22 76.09 3.72 18.83 1.36

23 79.23 2.12 16.74 1.91

24 77.65 3.74 15.36 3.25Average 78.93 2.18 15.00 3.90

Chapter 7 109

Table 7.4 Maceral composition of coal in visible mineral matter free basis (in volume per cent)

Sample No. Vitrinite Exinite Inertinite

1 87.26 3.47 9.27

2 82.80 1.61 15.59

3 84.79 3.13 12.08

4 79.99 1.31 18.69

5 81.51 1.34 17.15

6 76.11 2.25 11.64

7 79.25 2.05 18.69

8 84.73 1.76 13.51

9 84.53 1.29 14.78

10 84.84 2.31 12.85

11 83.16 . 2.33 14.51

12 83.09 1.25 15.67

13 84.11 1.29 14.61

14 82.11 1.41 16.48

15 83.33 2.19 14.48

16 77.86 3.29 18.85

17 80.72 1.62 17.65

18 84.27 1.89 13.83

19 76.90 4.46 18.64

20 81.47 2.59 15.94

21 80.71 1.70 17.59

22 77.14 3.77 19.09

23 80.77 2.16 17.07

24 80.26 3.87 15.88

Average 82.15 2.26 15.58

Chapter 7 110

T a b le 7 .5 R e f le c ta n c e o f v it r in ite o f B a p u n g c o a l

Sample No. Vitrinite reflectance (max)%

1 0.66

2 0.62

3 0.58

8 0.67

9 0.59

10 0.61

11 0.65

12 0.63

13 0.57

14 0.64

15 0.56

16 0.66

20 0.58

21 0.50

22 0.55

Average 0.60

Chapter 7 111

Table 7.6: Rank of coal based on vitrinite reflectance according to A.S.T.IW. classification

Chapter 7 112

Table 7.7 Microhardness of Vitrinite of Bapung coal

SampleNo.

Firstdiagonal

Seconddiagonal

Mean H V Vickers Hardness

Average Identification time {in sec.)

96 94 95 20.21 92 90 91 22.4 22.1 10

90 88 89 23.492 90 91 22.4

2 88 92 90 22.9 22.7 1090 90 90 22.988 88 88 23.9

3 86 87 86.5 24.8 23.1 1097 92 94.5 20.897 96 96.5 19.9

4 96 95 95.5 20.3 20.5 1294 92 93 21.494 96 95 20.5

5 92 90 91 22.4 21.6 1290 93 91.5 22.180 82 81 28.3

6 84 84 84 26.3 27.6 1282 80 81 28.397 86 91.5 22.1

7 92 90 91 22.4 21.9 1095 92 93.5 21.388 76 79 29.7

8 89 90 89.5 23.1 24.6 1092 96 94 2180 82 81 28.3

9 90 88 89 23.4 26.5 1080 83 81.5 27.990 92 91 22.4

10 88 86 87 24.5 23.6 1087 88 87.7 24.188 88 88 23.9

11 84 85 84.5 26 25.1 1085 86 85.5 25.482 82 82 27.6

12 88 88 88 23.9 25.6 1085 86 85.5 25.497 96 96.5 19.9

13 95 94 94,5 20.8 21.0 1292 90 91 22.4

- ....................- -------90 --------- ------- 90 — 90 22.914 89 88 88.5 23.7 23.0 12

92 89 90.5 22.690 92 91 22.4

15 88 87 87.5 24.2 23.5 1288 88 88 23.997 96 96.5 19.9

ie 96 94 95 20.5 20.2 1095 96 95.5 20.392 94 93 21.4

17 95 96 95.5 20 3 20.8 1096 93 94.5 20.792 90 91 22.4

18 86 87 86.5 24.8 24 1088 85 86.5 24.8

Figures 7.1 -7.14

PHOTOMICROGRAPHS OF MACERALS (All photomicrographs are taken in reflected light with

oil immersion lens, X200)

Fig. 7.1 Collinite type of vitrinite

Fig. 7.2 Sparcely distributed sporinite (sp) in vitrinite

Fig. 7.3 Resinite (Re) in vitrinite

114

Fig. 7.4 Fusinite with typical cell structure (Fu)

Fig. 7.6 Fusinite bands showing partially developed bogen structure (Fu)

115

Fig. 7.7 Fusinite (Fu) and vitrinite (V) bands

Fig. 7.8 Semifusinite along with vitrinite and fusinite

Fig. 7.9 Semifusinite along with fusinite

1 1 6

F u s i n i t e

U n i c e l l e d

s c l e r o t i n i t e

I n e r t o d e t r i n i t e

F i g . 7 . 1 0 S c l e r o t i n i t e ( S ) w i t h i n e r t o d e t r i n i t e a n d f u s i n i t e

F i g . 7 . 1 1 T h r e e c e l l e d s c l e r o t i n i t e ( S )

F i g . 7 . 1 2 M u l t i c e l l e d s c l e r o t i n i t e ( S )

117

Fig. 7.13 Framboidal pyrites (Py)

Fig. 7.14 Pyrite rich bands (Py)

C h a p te r 7 118

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Chapter 7 119

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Chapter 7 120

VITRINiTE (V) 100%

EXINITE (E) 100%

INERTINiTE (!)50 100%

Fig. 7.17 Triangular representation of macerals in microlithotypes

Chapter 7 121

1570

-p—

80 % of carbon 90 100Fig. 7.18 Plots of Bapung coal In Vlcker's microhardness

curve