Physical Variations in Highly Metamorphosed Antarctic Coals

12 Physical Variations in Highly Metamorphosed Antarctic Coals

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 5, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch012

N. SCHAPIRO and R. J. GRAY Applied

Research Center, U.S. Steel Corp., Monroeville,

Pa.

The presence of coal in Antarctica has been known since early 1900. The recent discovery of numerous coal beds, one over 13 feet thick, has stimulated interest in these coals. A set of 36 samples, for which chemical analyses were available, were examined

microscopically;

optical

reflectance,

Knoop indention hardness, and electrical resistivity were determined. These coals range in rank from semianthracite to natural coke and are generally high in ash, fixed carbon, and oxygen. The coals have been altered by igneous intrusives; the closer the coal to the source of heat, the higher the rank reflectance and hardness and the lower the electrical resistivity. Changes in physical properties with degree of coal alteration may be useful in estimating the temperature of alteration and rank of coal prior to alteration and in geologic correlation problems.

|n the early 1900s petrified trees, imprints of fossil leaves, a n d thin coal seams were discovered i n Antarctica. This discovery was important since it was evidence of forests that once grew i n what is now virtually an ice-covered continent. However, between 1957 and 1961 the discovery of numerous coal beds (2, 5, 7 ) , some of which are over 13 feet thick, stimulated the interest i n Antarctic coals. C o a l is the only economically important mineral resource that has been discovered to date i n Antarctica. Most of the coal now known to be present occurs i n a block-faulted mountainous area that extends almost completely across the continent. Since this exceptional area of rock exposed i n an otherwise ice-covered continent contains coal, the exposure of coal-bearing strata may be only an indication of a vast new and previously unsuspected coal field. T h e amount of coal present is very important, but the rank a n d grade of 196 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


12. SCHAPIRO AND GRAY

Variation» in Antarctic Coals

197

the coal w i l l largely determine its potential use a n d , therefore, the extent to w h i c h the coal i n Antarctica is exploited. T h e stratigraphie sequence i n w h i c h the coals occur is or has been overlain b y diabase sills that were formed b y molten material. T h e degree of thermal alteration of the coal depends on the distance of the coal from the heat source as w e l l as the amount of heat avail able. I n addition to generally describing the microscopic characteristics of Antarctic coal, the present investigation was conducted to determine changes that occur i n the chemical composition, reflectance, microhardness, a n d elec trical resistivity of the coals caused b y thermal alteration. T h e Antarctic coal samples are reportedly of Permian age a n d were col lected i n 1957 as part of the Antarctic exploration program of the O h i o State University Institute of Polar Studies. T h e bulk of the samples are from two localities shown i n Figure 1 : ( 1 ) the Terrace Ridge i n the O h i o Range of the Horlick Mountains, a n d ( 2 ) M o u n t G r a n i n the Granite Harbour area of South Victoria L a n d . I n each instance the coals were sampled from each of the accessible coal seams i n the stratigraphie sequence exposed. Thus, these sam-

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BELLINGSHAUSEN SEA

EAST

ANTARCTICA

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COAL LOCALITIES IN ANTARCTICA A LOCALITY MTS (STCPMCMMN, ΙΜ·) t MT GCOMOPTCftlS (UONC.t**) » MT WtAtfCft (M.ACK**M.IMT) MT M N M N (COUIO.IMS)

4 rmcrjor % MT auCKUT (se«w«o.iW4-o€iCNN»«.i»iOJ * τ

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Figure 1.

1*4)

Map of Antarctica

Q l*W STATIONS JtjOjJM

showing the general area of coal sample

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

localities


198

COAL SCIENCE

pies represent a n oriented sequence ranging from youngest to oldest coal through a thickness of about 1000 feet. F o r comparison purposes, samples from the Schultheiss escarpment i n the Horlick Mountains and also samples from Plunkett Point near the Beardmore Glacier were included i n this study. A l l of these coals occur i n association w i t h diabase sills formed b y molten material that invaded the coal-bearing sediments i n the area. A fairly complete discussion of the geologic sequence that accompanies the coal-bearing strata, together w i t h a more complete description of the physiography of the exploration area is given b y D o u m a n i and L o n g ( 2 ) . Materials

and Experimental

Work

T h e 36 Antarctic coal samples were analyzed chemically (proximate a n d ultimate analyses) at the U . S. Bureau of Mines i n Pittsburgh. T h e results of these analyses accompanied the samples when received from J . M . Schopf. T h e samples were collected by W . E . L o n g i n the O h i o Range on N S F grant Table 1. Chemical, Optical, and Physical

Sample Number

Description

77» 78* 80 81 82» 83* 85» 86» 87» 88» 89» 91» 92» 94 < 95 98» 100»

Coaly Shale Coal Coaly Shale or Impure Coal Shale, Black Coaly Coal Coal Coaly Shale Shale, Black, Rough, Blocky Coal Coal Coal Coal, Blocky and Platy Coal Coal—Diabase Relation Coal Coal, Impure Coal

6

6

e

12-21-1 12-21-2* 12-21-4* 12-21-5 12-21-7* d

d

12-21-9 12-21-10* 12-21-11* 12-23-1 12-23-2' 12-23-3" 4

e

12-24-4' 1-10-1 e

Dirty Diamond Adit ( Fresh Coal ) * No. 1 Coal No. 2 Coal No. 3 Coal No. 4 Coal No. 5 Coal No. 6 Coal No. 7 Coal No. 9 Coal No. 1 Coal, Mt. Grans Seam No. 3 Coal Specimen of No. 1 Coal in Contact with Igneous Rock No. 5 Coal Coal Float

Seam Thickness, feet 3 2 3 4 8 3 2 2 3 8 2 8 3

2.3 0.5 2.8 2.0 8.0 2.5 1.5 7.5 4.1 2.0 0.7


Distance Below Sill, feet 52 132 500 530 648 652 850 878 889 959 962 1023 1117 1167 1021 962 1000


12.

SCHAPIRO AND

GRAY

Variation» in Antarctic Coals

199

G - 1 7 2 1 6 , and i n Victoria L a n d b y John M u l l i g a n , also on an N S F grant to the Bureau of Mines. E a c h sample was crushed b y hand w i t h a mortar and pestle to approximately 0.625 by 0.25 inch. A split of each crushed sample was embedded i n epoxy resin, polished, examined microscopically, photo graphed, and then its reflectance was determined. Knoop indention hardnesses were also determined by using a 20-gram load on the diamond indenter. T h e indenter was kept i n contact w i t h the sample for 15 seconds. Additional splits of each sample were crushed to minus 60 mesh, dried at 100°C. for 24 hours, and their electrical resistivity was determined at 20,000 p.s.i. T h e sample locations, chemistry, petrography, reflectance, electrical resistivity, and hard ness of the 36 Antarctic coal samples studied are included i n Table I. Results and

Discussion

Chemistry. T h e moisture- and ash-free fixed carbon figures for this coal are relatively high (Table I ) . Most of the samples exceeded 8 0 % fixed carbon, and all the samples exceeded 8 0 % ultimate carbon. About one-third of the

Properties of Antarctic Anthracite Chemical Analysis* (moisture-free) Proximate

Ultimate

VM

FC

A

H

C

Ν

Ο

S

Moisture

1.3 3.3 4.7 5.5 13.7 15.5

12.2 32.4 16.3 19.1 54.9 73.4

0.3 0.4 0.4

13.0 33.3 18.2

0.5 0.7 0.7

1.1 1.6

0.1 0.2 0.1

0.6 1.7 2.9

1.3 1.9

58.2 76.6

1.4 1.5

7.4 8.5

0.3 0.4

7.0 8.0

13.5 11.9 14.4 13.7 15.4 6.7 6.5 11.9 14.0

46.3 52.5 63.6 44.7 65.1 29.1 45.6 52.6 61.9

86.5 64.3 79.0 75.4 31.4 11.1 72.3 80.1 40.2 35.6 22.0 41.6 19.5 64.2 47.9 35.5 24.1

1.4 1.7 2.0 1.6 2.0 1.3 1.7

49.3 54.2 65.6 48.7 68.1 30.9 46.4

1.4 1.4 1.6 1.3 1.5 1.1 1.2

7.2 6.6* 8.3 6.5 8.4 2.2 2.4

0.5 0.5 0.5 0.3 0.5 0.3 0.4

1.8

64.0

1.7

7.9

0.5

5.7 4.9 5.8 5.3 6.1 2.3 2.7 5.4 5.5

9.5 9.2 23.2 11.5 12.7

78.8 81.6 62.3 75.9 69.0

11.7 9.2 14.5 12.6 18.3

1.3 1.6 1.4 2.2 2.3

79.9 83.1 77.1 78.5 71.7

0.8 0.9 0.8 1.2 1.3

6.0 4.8 5.9 4.9 5.8

0.3 0.4 0.3 0.6 0.6

6.1 6.8 7.7 3.7 3.6

12.8 7.9 11.9 6.3 5.3

64.2 34.1 77.3 78.3 83.3

23.0 58.0 10.8 15.4 11.4

2.1 1.2 2.0 1.2 1.2

66.9 35.6 79.3 78.9 84.5

1.2 0.6 1.2 0.5 0.7

6.0 4.1 5.8 3.7 1.9

0.8 0.5 0.9 0.3 0.3

3.8 2.8 5.6 14.9 14.1

4.5 11.8 6.6

77.2 72.5 80.0

18.3 15.7 13.4

0.4 1.8 1.9

77.8 74.7 79.4

0.0 0.9 0.9

3.5 5.7 4.0

0.0 1.2 0.4

1.5 7.8 6.2


COAL SCIENCE

200

Table I. Chemical Analysis* Sample Number 77» 78» 80 81 82» 83» 85» 86» 87» 88» 89» 91» 92» 94 95 98» 100» 6


6

c

e

12-21-1* 12-21-2* 12-21-4* 12-21-5* 12-21-7* 12-21-9* 12-21-10* 12-21-11* 12-23-1· 12-23-2' 12-23-3' 12-24-4' 1-10-1'

Proximate Description Coaly Shale Coal Coaly Shale or Impure Coal Shale, Black Coaly Coal Coal Coaly Shale Shale, Black, Rough, Blocky Coal Coal Coal Coal, Blocky and Platy Coal Coal—Diabase Relation Coal Coal, Impure Coal Dirty Diamond Adit ( Fresh Coal ) » No. 1 Coal No. 2 Coal No. 3 Coal No. 4 Coal No. 5 Coal No. 6 Coal No. 7 Coal No. 9 Coal No. 1 Coal, Mt. Gran Seam No. 3 Coal Specimen of No. 1 Coal in Contact with Igneous Rock No. 5 Coal Coal Float

VM

FC

9.6 9.2 22.4 22.4 20.0 17.5

90.4 90.8 77.6 77.6 80.0 82.5

22.6 18.4 18.5 23.4 19.1 18.7 12.5 18.4 18.4

77.4 81.6 81.5 76.6 80.9 81.3 87.5 81.6 81.6

10.8 10.1 27.1 13.1 15.5

89.2 89.9 72.9 86.9 84.5

16.7 18.8 13.4 7.5 6.0

83.3 81.2 86.6 92.5 94.0

5.5 14.0 7.6

94.5 86.0 92.4

Plunkett Pt. Graphite • Abbreviations: V M = Volatile Matter; F C = Fixed Carbon; A = H = Hydrogen; C = Carbon; Ν = Nitrogen; S = Sulfur. » Terrace Ridge, Mt. Schopf, Ohio Range of Horlick Mts.

Ash;

samples exceeded 9 0 % ultimate carbon. T h e hydrogen values ranged from 0 . 5 - 3 . 3 % . Although these carbon values are high and the hydrogen values are low, the carbon values are not nearly as h i g h as the reflectance data indicate. Nearly all the coals examined are relatively high i n ash. O n l y two of the samples from the Terrace Ridge area contain less than 2 0 % ash, on a moisturefree basis. Most of the remaining samples from this area range from impure coal to coaly shale. Most of the mineral matter is finely divided (dehydrated) clay, silica, and pyrrhotite. This is an important point since the proximate and ultimate analyses for material that deviates so widely from what is normally considered coal cannot be considered particularly reliable for comparison pur-


12.

SCHAPIRO AND

GRAY

201

Variation* in Antarctic Coals

Continued (moisture- and ash-free)


Ultimate H

C

Ν

Ο

S

2.2 1.1 1.9

93.5 93.3 86.7

3.6 1.9 3.3

3.1 7.6

0.7 0.6 0.5

1.9 2.1

84.9 86.2

2.0 1.7

10.8 7.5

0.4 0.5

2.3 2.7 2.5 2.7 2.4 3.6 3.3

82.4 84.1 84.1 83.4 84.6 86.3 89.1

2.3 2.2 2.0 2.2 1.9 3.1 2.4

12.1 10.3 10.7 11.1 10.5 6.2 4.5

0.9 0.7 0.7 0.6 0.6 0.8 0.7

2.3

84.3

2.2

10.6

0.6

1.5 1.8 1.6 2.5 2.8

90.6 91.6 90.1 89.8 87.8

0.9 1.0 0.9 1.4 1.5

6.6 5.1 7.0 5.6 7.1

0.4 0.5 0.4 0.7 0.8

2.7 2.8 2.3 1.4 1.4

86.8 84.8 88.9 93.3 95.4

1.5 1.5 1.4 0.5 0.8

8.0 9.7 6.4 4.5 2.1

1.0 1.2 1.0 0.3 0.3

0.5 2.1 2.1

95.2 88.5 91.8

0.0 1.1 1.0

4.3 6.9 4.6

0.0 1.4 0.5

Average MicroReflectance, Hardness % (in oil) kg./sq. cm.

Electrical Resistivity, ohm-cm. (20,000 p.s.i.g.)

179.8 188.6 77.8 85.8 69.7 56.7 40.5 39.1 42.9 39.5 40.4 33.4 34.8 23.8 35.7 37.8 42.7

2.45 2.67 2.70 3.09 7.73 2.29 6.62 6.67 4.55 2.40 1.22 6.02 1.32 2.88 1.85 2.41 1.28

3.1 5.9 5.9 5.4 4.2 3.5 6.0 3.8 3.8 4.5 6.0 5.4 5.5 5.1 5.5 5.9 5.0 6.0

11.5 10.4 6.2 7.0 6.0 6.8 5.5 4.2 4.8 5.4 4.7 2.8 3.1 2.6 2.9 3.5 5.0

X Χ X Χ Χ X X Χ X X χ X Χ X Χ X

10-' 10 10' 10 10 10° 10 10 10* 10* iœ 10* 10 10 10 10

34.3 116.6 124.7 91.0 65.0 49.3 96.6 60.0 55.7 53.2 69.5 74.0

1.98 3.98 2.69 3.34 1.08 5.06 3.24 1.34 8.74 3.62 1.20 1.12

X X Χ X X X

10 10 10 10 10 10*

X X Χ X X

10 10 10* 10 10

35.1 52.6 59.8 55.2 34.5 27.0

3.80 2.56 1.84 3.93 1.84 4.00

Χ ΙΟ" Χ 10 X 10

4

4

6

T

8

8

s

8 s

e

2

5

T

e

2

4

s

X 10 Χ 10"

5

Four miles northwest of Mt. Schopf, Ohio Range of Horlick Mts. * Second mountain south of Mackay Glacier. • Mt. Gran, Central Victoria Land. c

poses with coal. A l l except two samples from the Granite Harbour area con tain less than 2 0 % ash; however, only one sample contained less than 1 0 % ash. In general, all the coal samples were l o w i n sulfur. Most of the samples contained 0.5% or less sulfur. T h e oxygen content of these coals, w i t h few exceptions, was unusually high. T h e reason for considering the dry, ash-free oxygen values exceptionally h i g h is based on comparing the oxygen values to carbon content. A l l the samples except one exceeded 4 . 3 % oxygen, on an ash- and moisture-free basis. Approximately one-third of a l l the samples analyzed exceeded 1 0 % oxygen. B r o w n and Taylor ( I ) indicate that the oxygen occurs i n nonacidic functional groups i n the Antarctic coal. T h e y also



202

COAL SCIENCE

demineralized (acid-washed) coal and found that the coal analyzed the same on the ultimate basis, indicating the high oxygen content of the actual coal substance. Chars prepared from brown or subbituminous coals are k n o w n to contain a high proportion of nonacidic oxygen. It has been suggested that the unusually high oxygen content may be caused by the weathering conditions i n Antarctica ( 6 ) . However, i n a recent study of coals that h a d been altered to natural coke i n the Somerset area i n Colorado, similar high oxygen contents were reported (4). C o a l Petrography. O f the 36 samples examined microscopically, approxi mately one-half contained gas vacuoles produced by thermal distillation of the coal. Thus, these coals that might otherwise have been referred to the rank of anthracite can be regarded as natural coke. A l l the samples, including those w i t h gas vesicles, appeared to be fairly dense, with no macroscopic indication of a coke-structure development. The samples w i t h the most advanced vacuole development appeared to be almost metallic i n luster. A l l the coal entities normally encountered—vitrinoids, micrinoids, semifusinoids, and f u s i n o i d s — were present, whereas the exinoids and resinoids, w h i c h are difficult to identify w i t h certainty i n semianthracite or anthracite, were not positively identified i n

Figure 2. Photomicrographs showing some of the common enti ties found in Antarctic coals; reflected light, X 150 A—Vitrinoia with cracks. Β—Bright inerts in a mixture of vitrinoids (gray) and chys (bhck). C—Semifusinoids preserving cellular structure. D—Clay minerals (hhck) containing thin vitrinoid bands (gray)


12.

SCHAPIRO AND GRAY

Variations in Antarctic Coals


Figure 3. Examples of the Microstructural variations that accompany the transition from natural coke to coal. The photomicrographs are arranged in sequence of increasing distance below the intrusive (sill) that was the source of heat; reflected light X 255 Large vacuole (bfock) surrounded by coke wall (gray). This natural coke appears 132 feet below a 600-F diabase sill

Small vacuoles (black) surrounded by material transitional between coke and anthracite. This coal occurs 648 feet below the sill

Transitional material between anthracite and coke. The variations in the appearance of the banding are caused by variations in composition and anisotropy. No vacuoles developed. Banding was retained in this unit, which occurs 652 feet below the sill

Semianthracite or anthracite containing recognizable coal entities that do not appear to be altered in this unit, which occurs 1117 feet below the sill


203

COAL SCIENCE


204

Figure 4.

The appearance of natural coke that occurs in the Antarctic coal measures; reflected light X 150

these coals. Representative macérais a n d maceral associations are illustrated in Figure 2. T h e natural coke-to-semianthracite sequence is illustrated i n Figure 3. A l l the photomicrographs are of coals from the Terrace Ridge area and are arranged i n the order of increasing distance below the sill. T h e photomicrographs i n Figure 4 illustrate the nature of the anisotropic domains developed i n the coke. T h e size and intensity of anisotropy suggests that the cokes are similar to those produced i n by-product ovens from marginal h i g h - to medium-volatile rank coals. However, exceptionally high hydrogen coals are known to produce cokes w i t h similar anisotropy, even though they are from high volatile coals. Additional pétrographie features are presented i n Figure 5 , showing pyrolytic graphite, compression-crack patterns i n organic inert particles, a n d two typical areas of a highly mineralized coal. T o a petrographer familiar w i t h North American coals the most striking difference i n the Antarctic coals, apart from rank, is the abundance of semifusinoids, w h i c h is not uncommon for Permian coal deposits i n such Southern Hemisphere countries as South A f r i c a and Australia. T h e coal from each of these areas, including Antarctica, is dominated b y a fossil flora known as the Glossopteris flora, w h i c h is considered characteristic of a postulation of an ancient land mass i n the southern hemisphere, known as Gondwanaland. Some of the semifusinoid forms observed are illustrated i n Figure 6. T h e presence of an abundant semifusinoid, fusinoid, or micrinoid content for a coal is usually considered indicative of a poorer coking coal since these macérais either w i l l not fuse or w i l l undergo transient



12.

SCHAPIRO AND GRAY


205

fusion during carbonization. This last observation is superfluous because all the entities i n the Antarctic coals included i n this study are noncoking, being too high i n rank to soften during carbonization. However, it is important to consider entity composition because if lower rank coals are found i n Antarctica, they are likely to contain an abundance of the poorer fusing macérais. Reflectance. T h e optical properties (reflectance) are not i n accord w i t h the chemical properties for these coal samples, and the maximum reflectance of the coals indicates that they are higher i n rank than w o u l d be concluded from the chemical data alone. These discrepancies are not surprising since these coals are thermally metamorphosed and may not follow the normal coalification curve (8). F o r the subject samples, it was decided that chemical data d i d not suitably indicate rank or the degree of thermal metamorphism, particularly i n those instances where the samples contained so m u c h ash that they were not suitable for routine chemical tests. T h e maximum reflectance i n o i l of these coals ranges from 2.6% to 11.5% (Table I ) . T h e lower reflectance is similar to that encountered i n some semianthracites and anthracites, whereas the upper reflectance is more nearly that of graphite or long term, h i g h tem-

Figure 5.

Some of the unusual material or structural features in Antarctic coals; reflected light, X 150

A—Pyrolytic carbon showing ribbon-like structure in vitrinoid bands. Β—Faint gray lines define compression cracks in a bright micrinoid par ticle. C—Pyrrhotite (white) formed by the thermal decomposition of pyrite impregnating semifusinoids (gray). D—Bright coke particles in a baked-bone coal layer



206

COAL SCIENCE

Figure 6. Variations in semifusinoids in Antarctic coals. These semifusinoids are a nonfusing entity present in abundance in these coals; reflected light, X 150 perature coke. In this study the reflectance measurements were used to differentiate the degree of coal alteration. F o r instance, when the reflectance is plotted against the distance of the coal or coke below the igneous sill i n the Terrace Ridge area (Figure 7 ) , the reflectance decreases from 11.5% at a distance of 52 feet below the sill to 2.8% at a distance of 1167 feet below the sill. Thus, the reflectance changes regularly w i t h the distance of the coal below the sill. W h e n the normal rank indicators, namely volatile matter or ultimate carbon, are plotted against distance below the sill, there is no general trend (Figure 8) even when the chemical analyses that are expected to be most erratic (over 5 0 % ash) are eliminated. However, the hydrogen, w h i c h is normally considered the best indicator of temperature changes i n the coal, does tend to increase as the distance below the sill increases (Figure 9 ) . T h e hydrogen values i n the demonstrations are calculated to a dry, ash-free basis. H o w ever, analysis of low ash coal concentrates from which the mineral matter has been removed w o u l d be desirable. E l e c t r i c a l Resistivity. There is a close relationship between the reflectance and electrical resistivity of coal. As the reflectance increases, the resistivity decreases and higher reflecting, higher temperature cokes are less resistant than the lower reflecting, lower temperature cokes. The resistivity of the A n tare-


12.

SCHAPIRO AND GRAY


-

•


\

i

\

X

•χ

-

V

:X

1

1

400 600 000 1000 DISTANCE BELOW SK.L. ! · · !

Figure 7.

1200

·

Relation between distance of coal below sill and reflectance

• •

?! It

• •

) COALS WITH MOftE THAN SO PERCENT ASH

® ®

i

400

600 tOO 1000 DISTANCE BELOW SILL. f t « »

1200

Figure 8. Relation of distance of coal below sill to ultimate carbon and volatile matter


207

COAL SCIENCE

208

• COALS WITH MORE THAI1 SO PERCENT ASH


•

•

·/ y

I 0

ι 200

1

600 tOO 1000 4 00 DISTANCE BELOW SILL. f M l

Figure 9.

IS 00

Relation between distance of coal below sill and hydrogen content

•

-

y

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Figure 10.

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4 0 0 6 0 0 B O O ÎÔÔÔ DISTANCE BELOW SILL. (Mt

1200

I I

Refotion between distance of coal below sill and electrical resistivity


12.

SCHAPIRO AND GRAY


209

tic coals ranges from 1.40 Χ 10" ohm-cm. for one of the most thermally altered samples to 6.35 Χ 10 ohm-cm. for one of the least altered samples (Table I ) . In the resistivity determinations, the entire sample was used, therefore, the effect of the kind and amount of mineral matter enters into the results. H o w ever, the resistivity changes are much more sensitive to coal rank differences than to changes i n ash (mineral) content. T h e resistivity increases as the distance below the sill increases ( F i g u r e 1 0 ) . Hardness. T h e Antarctic coals have Knoop hardnesses that range from 27 kg./sq. m m . to 188.6 kg./sq. c m . (Table I ) . Other natural cokes are generally the hardest, a n d the semianthracites are generally the softest. Hardness i n creases as the distance from the sill decreases for the coals from the Terrace Ridge area (Figure 1 1 ) . However, when a l l the samples are considered, the hardness does not necessarily correlate i n a completely regular manner w i t h another single set of test data. 2


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Figure 11. Relation between distance of coal below sill and indention microhardness T h e Knoop hardness of coal i n the rank range of semianthracite to anthra cite increases regularly w i t h rank. However, the hardness of carbonized coal increases w i t h temperature u p to about 1 2 0 0 ° C . and then decreases at higher temperatures ( 3 ) . It is possible that coal brought to similar stages of chemical rank m a y have different hardnesses, these depending upon the agency or method responsible for the rank change. H o n d a and Sanada (3) indicate that the Knoop hardness numbers of coals carbonized at 1 2 0 0 ° C . are i n the range of 2 0 0 - 3 0 0 kg./sq. c m . , whereas those carbonized at 2 0 0 0 ° C . are between 100 and 200 kg./sq. c m . Several of the Antarctic coals have Knoop hardness n u m -


210

COAL SCIENCE

bers between 100 and 200 kg./sq. c m . , but this has been interpreted to mean that these coals reached temperatures between 7 0 0 ° and 1 0 0 0 ° C . Cokes from coals w i t h lower volatile matter have higher hardness numbers than cokes similarly prepared but from higher volatile coal. Therefore, it is necessary to know the rank of the coal from w h i c h the coke was produced before hardness data can be used to postulate accurate temperatures.


Summary A set of 36 Antarctic coal samples were analyzed chemically, examined microscopically and photographed, and their optical reflectance and electrical resistivity were determined. T h e coals were associated w i t h intrusives, a n d i n degree of metamorphism they range from semianthracite through anthracite to natural coke. T h e coals are generally high i n ash, fixed carbon, and oxygen and low i n hydrogen and sulfur. T h e oxygen values are unusually high com pared to equivalent rank commercial anthracites used i n the U n i t e d States. A l l of the coals or cokes are too high i n rank to be considered for coke produc tion. T h e entity composition of the coals is similar to that of Pennsylvania anthracites, but the Antarctic coals contain an abundance of semifusinoids. These semifusinoids undergo only transient fusion even i n lower rank, higher volatile coals. In general, the reflectance and K n o o p indention hardness of the coals increase and the electrical resistivity decreases as the rank of the coal increases. T h e coals are altered by contact w i t h an igneous sill; the closer the coal to the sill, the greater the alteration, the higher the reflectance and hard ness, and the lower the electrical resistivity. Therefore, these physical proper ties are useful i n postulating temperature equivalents and should also be useful i n planned exploration if the degree of metamorphism of the coal is to be related to the extensive sill complex i n the area. Literature

Cited

(1) Brown, H. R., Taylor, G . H., Fuel 40, 211 (1961). (2) Doumani, G . Α., Long, W . E . , Sci. Am. 207, 168 (1962). (3) Honda, H., Sanada, Y., Fuel 36, 403 (1957). (4) Johnson, V . H., Gray, R. J., Schapiro, N., "Abstracts of Papers," 145th Meeting, A C S , September 1963, p. 3K. (5) Mulligan, J. J. et al., U.S. Bur. Mines Rept. Invest. No. 6218 (1900). (6) Schopf, J. M., personal communication. (7) Schopf, J. M., U.S. Geological Survey Rept. No. 2, (1962). (8) Van Krevelen, D . W . , Schuyer, J., "Coal Science," D . Van Nostrand Co., Inc., New York, 1957. R E C E I V E D January 25,

1965.


211


General Discussion George K a p o : A recent development (3) i n the solubilization i n water of organic compound^ normally insoluble i n water such as vitamin A acetate, alizarin, actinomycin C and F , etc., may be useful i n humic acid chemistry. The technique of solubilization consists of dissolving the water insoluble compound i n a water miscible solvent, adding an aqueous solution of the sodium salt of hippuric or cyclohexylsalicylic acid, and evaporating the solvent. This results i n an aqueous solution of the material; removing water by thin film evaporation or freeze drying leaves a water soluble powder. Assuming the effect of the additive can be accounted for, this technique could provide a means of direct titration i n aqueous solutions of the functional groups i n humic acids. Here the water miscible solvent could be an acetone-water mixture. Peter H . G i v e n : W h i l e listening to the papers of D r . F l a i g and D r . Spademan (page 695), it occurred to me that perhaps they are discussing essentially the same process from different viewpoints. D r . F l a i g has elucidated m u c h of the chemistry of the microbiological decay of lignin i n straw. D r . S packman, by petrological methods, has observed the effects of the differential decay of different types of cell w a l l i n fossilized woody tissue. Is D r . F l a i g i n fact studying the chemistry of what D r . Spackman sees under the microscope? O n the other hand, if the decay observed by D r . Spackman is not caused by microorganisms, by what k i n d of process is it caused? It seems clear that some land of mechanical breakdown of major plant tissue occurred before this became thoroughly incorporated into the growing sediment. N o doubt further biochemical attack took place during this latter stage and before burial. A colleague ( D r . C . Clagett, Department of Biochemistry, Pennsylvania State University) has recently drawn my attention to some organisms k n o w n as the slime molds. These are large multi-nucleate but noncellular saprophytic organisms w h i c h can live on particles of organic matter, including decayed and macerated plant tissue, and also on bacteria, using some of the latter's protein for their o w n metabolism. Although nonphotosynthetic and able to grow i n the dark, they develop a pigment when growing i n light, and light appears to be necessary for them to produce spores and reproduce by a sexual mode. It occurred to me that these organisms may have played a part, w i t h others, i n bringing about the decay of woody tissue i n coalification. One interesting point is that they do not bring about the complete breakdown of cellulose and lignin as fungi and bacteria may do if provided w i t h enough oxygen and nutrients. A third point that may merit some discussion is the origin of nitrogen i n coals. Coals commonly contain 1 - 2 % nitrogen, and this represents an appre^ ciably higher N / C ratio than found i n woody tissue. Mason (2) has suggested that the extra nitrogen is derived from the proteins i n the bacteria that brought about the original decay; bacteria have a relatively high total nitrogen content. Aureal Cross: I really cannot believe that the type of cell-wall alteration seen by D r . Spackman i n the Brandon lignite resulted from microbial attack.


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Such attack can always be detected afterwards by a mottled or pitted appearance of the cell walls. Also, I do not think that Myxomycètes, the slime molds, were important primary agents i n the biochemical stages of coal formation because their preferred habitat is, for example, rotting tree stumps rather than a peaty mass. Norbert Berkowitz: Surely it is possible that the primary alteration of the cell walls is caused by hydrolysis and oxidation by water vapor and air, or they may undergo these reactions inorganically after actual immersion i n water. It is of course possible that breakdown products resulting from these reactions were used or altered by microorganisms. Cornelius Steelink: Perhaps plant cellulosic material, after hydrolysis, could be used by soil microorganisms to produce phenolic substances, w h i c h could then be incorporated into humic acids. I think it is significant that nonligninderived phenols occur i n podzol humic acids. D r . G i v e n : O f course, the sort of decay studied by D r . F l a i g , if it affected a major part of the lignin and if the cellulose and hemicelluloses were similarly largely broken d o w n , w o u l d cause all cellular organization and even individual cell structure to be destroyed. However, I am not sure how far these two conditions are likely to be met i n practice. M y original thought was that D r . Flaig's type of biochemical breakdown had occurred to a small extent or i n a small proportion of the cell w a l l material i n the Brandon lignite not sufficient to destroy the cellular organization, but perhaps this is not a very reasonable concept. However, the discussion suggests a further thought—that i n a pillar section of a bituminous coal seam the fraction of material retaining fairly massive portions of organized cellular structure is small. Surely, therefore, extensive maceration of cellular tissue must have occurred; this could well have been caused by microbial agencies, and i n any case there is not enough dimensionally intact fossilized cell structure left to show the mottling referred to by Cross and others. In other words, where extensive microbiological breakdown to relatively simple molecules has occurred followed by alteration and repolymerization as described by D r . F l a i g , no physical traces of the activities of the organisms can be discerned afterwards from the nature of things. W i l l i a m Spackman: O n l y about 2 0 % of the cells of woody tissue contain protoplasm and hence proteinaceous material. It is true therefore that woody tissue is a poor source of nitrogen. Irving A . Breger: There is little doubt that nitrogen was introduced at a very early stage of coal formation. R. Stephan: One way i n w h i c h nitrogen could be fixed i n the coal-forming materials is by a reaction between quinones (formed by breakdown of lignin) and amino acids (from plant protoplasm or microorganisms). A n alternative possibility is the interaction between reducing sugars (from cellulosic material) and amino acids. These sugars can be either glucose or its oligomers, a l l derived by chemical and/or bacterial degradation from cellulose. Condensation of amino acids and sugars occurs both i n weakly acidic and alkaline media ( p H range 5 . 5 - 8 . 5 ) . T h e resulting complex, once formed, is fairly unstable and rapidly decomposes to dark brown polymers. These polymers contain nitrogen, very likely i n the form of ring structures because the α-amino nitrogen as



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found i n the starting materials (proteins, peptides or amino acids) can no longer be detected. D r . Spademan's point that only about 2 0 % of the cells of woody tissue contain protoplasm a n d therefore amino acids, does not preclude the above hypothesis. In fact, this amount of nitrogenous material w o u l d be more than ample to account for the small percentage of nitrogen found i n coal. D r . G i v e n : A s I understand it, there is not enough protein i n woody tissue to account for the 1 - 2 % nitrogen commonly found i n coals unless one supposes that a great deal of preferential loss of C a n d H occurred, w i t h enrichment of N , a n d this supposition seems most improbable. However, amino acids from enzymes a n d bacterial cells most probably w o u l d be released into the coalforming mass, a n d D r . Stephan's suggestions on h o w these acids could be built into stable structures retaining the nitrogen are most interesting. O f course, this view of the source of some of the nitrogen i n coals implies that a significant amount of carbon a n d hydrogen i n coals originated i n microorganisms rather than the plant material; the C / N weight ratio i n amino acids is i n the range from 2:1 to 8:1 or so. Marlies Teichmuller: D u r i n g coalification gelling takes place i n the early stage of a Hartbraunkohle (lignite). F r o m a pétrographie viewpoint it is the most striking change i n the whole rank range (starting from peat a n d ending w i t h anthracites). T h e coal at this point of the rank range changes from a brown, d u l l , earthy a n d soft substance to a black (almost glass-like) substance of lustrous, tough, a n d hard appearance. T h e bedding becomes much more pronounced. U n d e r the microscope one observes that cavities of cell tissues and pores i n the detrital matter disappear a n d that the humic substances have been i n a somewhat fluid or at least a plastic state. I w o u l d like to draw the attention of organic a n d physical chemists to this very interesting phenomenon. Perhaps it can be clarified by colloid chemical studies. A t least it seems to me that this process is mainly a colloidal, a n d perhaps purely physical one since the chemical alterations at the boundary between soft a n d hard brown coals are small (at least as measured b y C , H , O , a n d volatile matter content). Stach (4) obtained the same effect b y an artificially peptizing humic substances i n centimeter pieces of brown coal by means of alkali treatment. After drying the gelled coal pieces at room temperature, he found hard, lustrous, black pieces of "Glanzbraunkohle" (subbituminous coal). T h e carbon content had not changed. D r . Berkowitz: I must question the validity of D r . Teichmuller's rather definite conclusions about the relative roles of time, temperature, and pressure i n the coalification process. F r o m an examination of Ruhr coals, D r . T e i c h muller said that only temperature plays a significant role. I suggest that conclusions drawn from data for coals i n other areas (e.g., Alberta a n d Pennsylvania) w o u l d lead to the conclusion that pressure rather than temperature was the determining variable; therefore, I doubt whether D r . Teichmuller s quite unqualified statements could have general validity. Indeed, from first principles one w o u l d deduce a rather complex a n d variable situation. Thermodynamically, one could perhaps rule out time as an important parameter since, unless one accepted the concept of a "tunnelling factor," time alone w i l l not



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allow a reaction to proceed w h e n such reaction is thermodynamically impossible. If it is thermodynamically possible, temperature w o u l d play a much more important role. O n the other hand, pressure effects could (depending on the k i n d of pressure) both accelerate and retard reaction. F o r example, if pressure operates to retain gaseous reaction products (e.g., methane) i n the coal, it w i l l cause retardation; if it operates to produce greater permeability of the strata (and thereby to allow release of gaseous reaction products), pressure w o u l d accelerate the reaction, f question the wisdom of needlessly attempting to force nature into a sort of intellectual straightjacket w h i c h w o u l d obscure rather than elucidate reality. D r . Teichmuller: I a m sorry that the geological observations are not consistent w i t h D r . Berkowitz s ideas concerning the influence of pressure on coalification. A greater permeability of the sediments cannot be caused by the overburden pressure since permeability decreases w i t h increasing depth. As to the folding pressure, it commonly leads also to compression. It is true that tectonic pressure may also lead locally to fracturing and mylonitJzation if shearing occurs. Crushing of the coal and the associated sediments may therefore be observed occasionally at planes of movement, as overthrusts or faults, but according to our observations i n different coal fields of Europe, this does not lead to an increase i n coal rank. A n exception to this rule has been described i n our paper from the Sutan overthrust near Bochum where we assume the local increase i n rank to be an effect of friction heat caused by exceptionally rapid movement. T h e geological observation that even i n a folded formation coal rank increases with depth (measured from one point of the earth's surface) cannot be explained as an effect of folding pressure because this has a more or less horizontal direction. In the U . S. the influence of the theory of D . W h i t e and his students (e.g., Stadnichenko) is very strong. W h i t e assumed that the tectonic pressure is the main cause of the coalification process. T h e same theory has been favored in Europe for a long time, but since systematic studies have been made of the vertical and regional alterations of rank i n different coal fields of Europe and since also the chemists—on the basis of experiments and reaction kinetic considerations—concluded that temperature is the decisive factor for rank increase and that pressure even retards the chemical reactions during coalification, this theory has been abandoned more and more. It is true that the most intensely folded areas often bear the highest rank coals, but these areas commonly had subsided to the greatest depth prior to folding. It is a geological rule that the very deep sinking areas (foredeeps, geosynclines) w i l l be folded later most intensely. D r . Breger: It seems significant that coal like materials with coking ability have never been produced i n artificial coalification experiments using heat as well as pressure. D r . Teichmuller: C o k i n g properties seem to depend largely on the physical microstructure of coals (porosity), as Berkowitz and Schein have shown ( I ) . T h e swelling is possible only when the gases are kept from escaping b y a toonarrow pore structure. I imagine that for getting such a pore structure, pressure must be applied while heating the coal. This is the case i n nature



6 · Π · Γ Ο Ι . Discutsion

215

where normally coalified coals, during subsidence, are exposed to elevated temperatures as w e l l as to (overburden) pressure. Perhaps the experiments on artificial coalification d i d not pay attention to the " r i g h t " relations (i.e., as i n natural conditions) between the temperatures and pressures applied. M o r e over, no experiment can imitate the long geological time w h i c h was available for the coalification processes i n nature. Therefore, i n experiments higher temperatures must be used than normally were available during natural coalifi cation. This also may lead to different reactions and different coalification products. E l s o Barghoorn: It is worth noting that i n coal balls the organic matter, despite exposure to metamorphosing conditions, is still i n the peat stage because it has been protected against pressure by its position i n petrified concretions. D r . Teichmuller: T h e excellent structural preservation of plant remains i n coal balls ( w h i c h is the same as i n recent peats) of course is caused by the very early petrification i n the peat stage. It can be seen easily i n thin sections that only the structure of the plant remains is preserved whereas the organic substance has been replaced almost totally by the petrifying mineral (calcite, dolomite). Its former location may be recognized only by a darker, brownish color of the mineral ( w h i c h may be caused by submicroscopic inclusions of humic matter adsorbed by the formerly colloidal inorganic matter). A t least i n coal balls from the German Carboniferous the amount of microscopically visible organic matter (mostly fusinite, but sometimes also exinite and vitrinite) is very small. It can be seen and identified best i n polished sections. A c c o r d i n g to my observations on polished sections the organic matter of coal balls has the same rank as the coal i n w h i c h the concretions occur; at least the reflec tance of the vitrinite is the same. James M . Schopf: D r . Teichmuller probably has underestimated the sig nificance of the small amount of organic matter i n peat-like state of preserva tion that is present i n coal balls. It is, i n fact, impossible for excellent structure to be preserved without the presence of the substance. Excellent structural preservation, so common i n calcareous coal balls, is not compatible w i t h any theory of replacement. This conclusion is supported most decisively by using the peel technique for preparing microscopical sections of the petrified plant tissues. B y this method, the supporting and protecting mineral is removed by etching. T h e organic substance of the tissues remains intact on the etched surface and is removed as a sheet of microscopic thickness for study by apply i n g a dissolved plastic strong enough (when dry) to be removed i n a coherent sheet. T h e peel technique has virtually replaced thin section methods for paleobotanical studies of petrified plant tissues i n recent years. This technique is significant for detailed interpretation of the organic structures preserved by "intracrustation" within the rock. T h e peel can only remove solid substances. A n y vestiges of actual mineral replacement w o u l d show only as surface replicas on the peel, and these are generally unsatisfactory for botanical examination. General success of the peel technique for use i n European as well as American coal ball studies shows that a substantial part of the original plant substance is still there. F r o m my o w n observations, I feel that comparing the coal ball tissues w i t h those i n peat or i n peaty litter is fully justified. It seems evident



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that the tissues were protected from alteration by the mineral medium w i t h w h i c h they were permeated. T h e coal ball plant fragments can, i n fact, be regarded as an excellent sample of the vegetable material present i n coal measures peat before any overburden was em placed. T h e petrifying medium may be regarded principally as a replacement of the water w h i c h saturated tissues and filled the interstices. In such uncompressed peat the water content generally is i n the neighborhood of 9 0 % , and this corresponds to about the present proportion of mineral i n coal balls to organic residue. None of the lignocellulosic plant tissues i n coal balls has been altered to vitrinite in any of the various European and American coal balls I have examined. It w o u l d be very interesting to know the source of D r . Teichmiiller's coal ball vitrinite reflectances. I believe they must refer to streaks of unmineralized peat w h i c h can be found associated between centers of coal ball mineralization w h i c h had been compressed and coalified rather than actually petrified. None of the plant material within coal balls I have examined can be taken as having any bearing on metamorphism or on the rank of the coal. Jack Simon: In Illinois the systematic increase i n rank from the northwest to southeast can hardly be explained by the increase of subsidence; this, to be sure, does exist i n the same general direction, but the increase i n depth does not seem to be great enough for this gradual loss of moisture and for the gradual increase i n calorific value across the Illinois basin from northwest to southeast. D r . Teichmuller: Besides the deeper subsidence perhaps additional heat, caused by intrusive bodies at depth, might be the cause for the higher rank of coals i n southeastern Illinois. The igneous activity seems to be greatest i n that area according to geological-mineralogical observations. I was informed at least that igneous dikes occur i n that area. D r . Schopf: A study has been completed recently by L . P. T a n , formerly of the Geological Survey of T a i w a n , on the metamorphism of T a i w a n Miocene coals. The results of this study w i l l be published soon by the T a i w a n Geological Survey. They are very similar to those previously developed for N e w Zealand Cretaceous coals by Suggate (5) except that the T a i w a n "coal b a n d " is i n effect slightly displaced above the N e w Zealand coal band, according to the customary plotting. Differences i n age of the coal—i.e., the period of incoalation, may explain w h y N e w Zealand coal attains about 4 0 % volatile matter at about 3000 ft. maximum overburden depth whereas i n T a i w a n , the same volatile content is not attained until maximum overburden exceeded 7000 ft. Recent mapping and coal analyses i n T a i w a n and a favorable geologic setting have provided good control. It is, of course, very difficult to separate the effects of geothermal gradient from overburden pressure. In a given region the two are intercorrelated. D r . Teichmuller: V e r y often it is impossible to separate the influence of overburden pressure and the influence of rock temperature on rank of coal i n a subsidized formation. Nevertheless sometimes it can be done. In the Saar Basin, where thick sandstone layers occur between claystones, i n the coal-bearing Carboniferous



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it can be shown that the degree of rank increase w i t h depth has been influenced by the different thermal conductivity of the rocks. Because of the higher thermal conductivity of sandstones, the increase is slower i n the sandy series than i n the clayey series. T h e question of whether rank increase w i t h depth is caused by overburden pressure or by rock temperature could be clarified best by studying the relations between the course of "isobaths"—i.e., rock isotherms and isorank lines i n young sediments. If the overburden pressure is decisive there must be a strict relation between depth and rank of coal whereas if temperature is the main factor there should be a relationship between rock temperature and rank. T h e rock temperature is determined not only by the depth of subsidence but also by the depth of the basement ( w h i c h has a high thermal conductivity) a n d the different thermal conductivities of the overlying rocks. F r o m Russian investigations we know that the depth of the basement ( w h i c h may vary laterally) greatly influences the course of the isotherms i n the overlying rocks. A n ideal area for the proposed investigations w o u l d be the southern U . S . where measurements of rock temperature as well as basement depth have been made i n numerous o i l wells. C o a l rank could be studied by means of reflectance measurements of vitrinites from coal seams and humic inclusions i n claystones out of deep wells. Literature

Cited

(1) Berkowitz, N., Schein, H. G . , Fuel 31, 19 (1952). (2) Mason, Brian, "Principles of Geochemistry," 2nd ed., p. 226, Wiley and Sons, New York, 1958. (3) Rothbaum, H. T., J. Appl. Chem. 14, 382 (1964). (4) Stach, H., Brennstoff-Chem. 14, 201 (1933). (5) Suggate, R. P., New Zealand Dep. Sci. Ind. Res. Bull. 134 (1959).


Physical Variations in Highly Metamorphosed Antarctic Coals

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