International Journal of Coal Geology, 2 (1982) 181--194 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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D O P P L E R I T I Z A T I O N O F X Y L I T I C C O A L I N T H E L I G H T O F P E T R O G R A P H I C A N D C H E M I C A L I N V E S T I G A T I O N S

MARIAN WAGNER

Institute of Fuel Materials, University of Mining and Metallurgy in Cracow, al. Mickie- wicza 30, 30-059 KrakOw (Poland)

(Received July 17, 1981; revised and accepted February 26, 1982)

ABSTRACT

Wagner, M., 1982. Doppleri t izat ion of xylit ic coal in the light of petrographic and chem- ical investigations. Int. J. Coal Geol., 2:181--194.

The paper presents the results of petrographic and chemical investigations of xyli t ic coal, xyli te and their ash. As is known, a xylit ic coal has the function of a l i thotype whereas xylites are inclusions brown coals only. All these xylites were arranged according to the increasing degree of doppleri t izat ion of xylem in the sequence: common xylite, poorly, moderately and intensely doppleri t ized varieties, dopplerite coal. The distinctive criteria were their differentiated physical properties, which are also reflected in the variable petrographic and chemical compositions. The petrographic differences result from the replacement of textini te by ulminite and gelinite, while the differentiation of the chemical consti tut ion is due to the increasing carbon content and the increasing number of functional groups that determine the aromatic nature of the internal struc- ture of the coal. Infrared absort ion spectroscopy, Raman spectroscopy and chemical analysis of ash have shown that the aromatizat ion of coal and the resulting increase in the degree of order of the structural unit are due to the formation of organomineral compounds in the course of doppleri t izat ion. I t appears from the investigations that the doppleri t izat ion of vegetable mat ter gives rise to gels that are a mixture of organo- mineral humic compounds. The process of humification leads to the formation of gel made up of humins. I t follows then that there are two ways of initial coalification of plant material, both referred to as biochemical gelification.

INTRODUCTION

X y l i t i c c o a l is t h e foss i l w o o d o f T e r t i a r y c o n i f e r s . I t o c c u r s in b r o w n c o a l s e a m s in t h e f o r m o f p r e s s e d s t e m s l y i n g h o r i z o n t a l l y , u p r i g h t t r u n k s , s t u b s a n d o t h e r t r e e f r a g m e n t s o f d i f f e r e n t sizes. X y l i t i c c o a l is c h a r a c - t e r i z e d b y d i v e r s e p h y s i c a l f e a t u r e s in t h a t i t m a y r e s e m b l e t h e p r e s e n t - d a y w o o d , s h o w l o n g i t u d i n a l o r b o t h l o n g i t u d i n a l a n d t r a n s v e r s a l s p l i t t i n g , o r i t m a y o c c u r in t h e f o r m o f gel t h a t r e s e m b l e s a w o o d f r a g m e n t o n l y in s h a p e .

T h e t r a n s f o r m a t i o n o f s o f t b r o w n c o a l t o h a r d b r o w n c o a l v a r i e t i e s

0166-5162[82/0000--0000[$02.75 © 1982 Elsevier Scientific Publishing Company

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involves intense gelification of fossil wood. At the bright sub-bituminous coal stage it is no t possible to distinguish the xylite varieties.

The gelification of fossil wood can be considered as physical, chem- ical and biochemical changes of lignin and hydrocarbons. It is generally held that it is the first stage of transformation (decomposit ion) of plant matter, which results in the formation of humic or organomineral gel. At the subsequent stages of coalification this gel is converted into vitrain.

The transformation of plant material at the peat and brown coal stages is referred to as humification or biochemical gelification (Stach et al., 1978, pp. 247--250). It is the first stage of enrichment of the decomposing plant mat ter in carbon, involving moderate oxidation and the formation of humic acids which in the subsequent stages of coalification are con- verted into humins, i.e. compounds of low chemical activity.

In an aqueous medium, humic acids can react with inorganic bases or salts. The products of these reactions are organomineral compounds called dopplerite. The complex of physical and chemical phenomena leading to the formation of such compounds will be referred to in this paper as doppleritization.

There are very few papers published to-date that deal with the process of doppleritization. More attention has been given to dopplerite originating from peat and brown coal deposits. Dopplerite has been defined as a Ca humic compound or complex humic compound of Ca, Fe, A1, K and Na (Potoni~ and Stockfish, 1932; Rammler and Jacob, 1951).

It appears that the biochemical gelification of plant mat ter in a coal sedimentation environment proceeds in two different ways. One leads to the formation of humins, i.e. purely organic compounds, whilst the other gives rise to dopplerite. Therefore the term "humif icat ion" is not synonymous with "biochemical gelification", as it defines only one of the possible ways of transformation of humic material.

This paper aims to determine the geochemical condit ions of doppleritiza- tion of xylitic coal. Petrographic and chemical investigations were carried ou t on xylitic coal samples showing varying degrees of doppleritization, derived from the same coal seam.

MATERIAL AND METHODS

The macroscopic features alone do not allow to distinguish xylites that have been subject to humification from those that have undergone dopp- leritization. It is, however, relatively easy to distinguish one variety from the other if the conten t of internal ash, i.e. that combined with the or- ganic mat ter of coal, is determined, but such a procedure requires a strict control of the puri ty of the investigated material in thin sections. As ap- pears from the author 's experiments the intensely doppleritized xylites have an ash content of more than 10 wt.%, while the xylites that have undergone humification contain only about 6 wt.% of ash.

1 8 3

The present investigations were carried out on doppleritized xylites derived from the so-called Glogbw bed (Upper Oligocene) in the area of GlogSw (Frankiewicz, 1975). The experimental material was represented by samples of common cylite (L-O), poorly (L-l), moderately (L-2, L-3) and intensely doppleritized (L-4) xylites, and doppleritic coal (L-5). The internal ash content in this series of xylites varied from 1.2 to 11.1 wt.% (Table I). The detailed investigations comprised:

(a) Macro- and microscopic petrographic observations in polarized trans- mit ted light, carried out with a Zeiss "Laboval 2 pol" microscope

(b) Chemical analysis of C and H contents (by Sheffield's method), the O content in the carboxyl groups of coal, and the determination of CaO, MgO, A1203, Fe203, Na20 and K20 contents in the xylite ash ob- tained at 450°C

TABLE I

Quan t i t a t ive analys is of IR absorp t ion spec t ra of xy l i t i c coal

rain rain fmin CC~ Car nar

Sample A a C a H a C/H HCH 3 HC~ Har CCH 3 max fmax CCH 2 C max nat

40.52 --7.94 --0.02 L-0 1.3 47.2 5.9 8.0 2.48 3.19 0.23 21.01 0.07

81.04 32.58 0.38

42.63 --9.15 --0.02 L-1 1.1 49.0 5.8 8.4 2.05 3.48 0.29 16.72 0.06

85.26 33.49 0.41

40.00 2.15 0.07 L-2 4.1 53.7 5.5 9.7 1.74 3.58 0.22 12.96 0.21

80.00 42.15 0.47

23.83 25.14 0.34 L-3 10.8 53.7 5.0 10.8 2.46 2.14 0.39 18.32 0.07

47.73 49.000 0.58

21.79 26.25 0.37 L 4 10.9 53.4 4.7 11.7 2.45 1.83 0.45 18.34 0.06

43.58 48.04 0.59

10.41 39.74 0.55 L-5 11.1 55.1 4.6 11.9 3.27 0.93 0.67 23.74 0.11

20.82 50.15 0.65

A a = ash content calculated in the analytical state; C a and H a ffi C and H contents calculated in the analytical state; C/H = atomic ratio of C to HCH~ ; Cc~ = percentage of C and H in CH~-type groups relative to their content in coal; Car ffi percentage of C in aromatic bonds relative to their total content in coal; fmax, fmln = maximum and minimum coefficient of aromatization; nar/nal ffi quantitative ratio of vibrating aromatic to aliphatic bonds systems.

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(c) Infrared absorption spectroscopy in the basic infrared region, using a UNICAM SP 1200 grid spectrometer

(d} Raman spectroscopy carried out with a Zeiss GDM-1000 spectrometer equipped with a monochromator and an ILA-120(2W)Ar laser. The laser beam power was 40 and 70 mW, depending on the kind of xylite, at a wavelength of 514.5 nm. The resolution of the Raman spectrometer with monochromato r was about 10 cm -1 .

Coal samples for spectroscopic investigations were prepared with the u tmos t care. Each sample was dried at 105°C in a nitrogen atmosphere for 2 hours. Then the samples were ground in a Fritsch planetary mill for 16 hours in the damp-free atmosphere, and prepared in KBr discs in proport ions by weight of 1:300 coal to potassium bromide by pressing at a pressure of 10 MPa.

PETROGRAPHIC FEATURES OF DOPPLERITIZED XYLITES

The change from common xylite through doppleritized xylites to doppleritic coal is progressive. It manifests itself in the gradual disappearance of the macroscopic features of wood, the change of colour from brown to black, and the change in lustre from dull to vitreous. In contrast to common xylites, the doppleritized varieties do not show fibrous parting or elasticity. The latter have a conchoidal fracture and exhibit great friability due to the dense ne twork of endogenous fissures. These features are particularly conspicuous in partly dried coal, being less pronounced in freshly exposed seains.

The degree of doppleritization of xylites is also clearly visible under the microscope. On the basis of microscopic features, three degrees of doppleri t ization can be distinguished, corresponding to the macroscopic division (Jacob, 1958; Suss 1959; Brzyski and Majewski, 1974).

Common and poorly doppleritized xylites are made up of textinite. It has been noticed that in the doppleritized varieties the zones of spring growth of xylem are generally disturbed, showing locally the features pe- culiar to texto-ulminite. The textinite is partly impregnated with corpo- huminite.

Xylites showing a medium degree of doppleritization are made up of textinite and texto-ulminite. The structure of spring growth zones of xylem is frequently obliterated due to the crushing, tearing and random displace- ment of the cell walls. The cell walls of the summer growth of xylem are well preserved. The cell lumina are filled with gelinite.

Intensely doppleritized xylites are made up almost entirely of eu-ulminite. The tracheids are poorly marked off, being embedded in the granular gel mass. Doppleritic coal has a similar structure. In the gel ground mass, made up of uliminite, fragments of corroded tracheids are visible only locally.

From the above observations it is evident that intense dopplerit ization

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of xylitic coal involves not only the impregnation of xylem with gel but also the chemical destruction of the cell walls (Roselt, 1969, 1976). These processes result in the progressive replacement of textinite by ulminite and eventually by gelinite (levi-gelinite). The humic material owing its origin to the destruction of wood tissue partly or completely fills the fis- sures. From the above considerations it follows that the doppleritization of xylitic coal is a process involving several stages.

I N F R A R E D A B S O R P T I O N S P E C T R A

IR spectra of the investigated xylite varieties are differentiated, the differences consisting in the disappearance or development of absorption bands at the specified frequencies (Fig. 1). The IR spectrum of common xylite has been found to be similar to that of lignin, while the absorption displayed by doppleritic coal has several features in common with the spectrum of sodium acetate (CH3COONa), an organomineral compound used for spectroscopic analysis.

L-O

L4

L~2

L-3

L-4

[_-5

CH,COONa

35 30 25 20 40 15 12 10 B 4X100 cn~ 1

Fig. 1. I n f r a r e d absorption spectra of xyl i tes . L-O = c o m m o n xyl i tes ; L-1 = p o o r l y dopp- ler i t ized xylite; L-2, L-3 = m o d e r a t e l y dopp le r i t i zed xyl i tes ; L-4 = in tense ly dopp le r i t i zed xylite; L-5 = dopp le r i t i c coal.

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At present much at tent ion is given to the absorption displayed by coal at 1500--1650 cm -1 . The bands produced by xylitic coal varieties in this range are distinct, although they are characterized by the variable mor- phology of lines.

The spectrum of common xylite shows an intense and sharp peak at 1510 cm -1 , and a broad but less intense band at 1610 cm -1 . Its left part shows a weak inflexion at 1700 cm-' (Fig. 1). In xylites showing a higher degree of doppleritization the 1500 cm-' band disappears, but simulta- neously the 1600 cm-' band increases in intensity and becomes markedly broadened.

If we assume that the 1600 cm -' absorption band reflects solely the aro- matic structure of coal, then the dependence between the coefficient of aro- matizat ion f and the optical density of this band should be linear. The appropriate calculations have shown (Table I), however, that this assump- tion is incorrect (Fig. 2). It appears therefore that the growth of the 1600 cm -1 band is associated not only with aromatic --C=C - vibrations, but also with vibrations of functional groups containing oxygen. The relationship between the optical densities of the 1600 and 1700 cm -1 bands (Fig. 2) indicates that the intensity of their vibrations depends on the bonds con- taining oxygen.

A ~ B D17 0 ~ ar I frnax

i

0,4 ~ ~ . 0,7 o~- . . . ~ " o, o1 I --------:~.

i I i 0,2. 0,5 , ~ . I

i / 0,1 } 0,4 + .

0,0 0,5 1,0 D160~ Q2 0,4 {3,6 O,B 1,0 D1600

Fig. 2. A. Relationship between the optical densities of the 1700 and 1600 cm -1 ab- sorption bands. B. Relationship between the max imum coefficient of aromatization and the optical density o f the 1600 cm -1 band.

The 1700 cm -1 band in the spectrum of lignin is to be attr ibuted to the C=O groups of ketone radical. In common xylite it is displaced to a frequency characteristic of quinones. In the spectra of doppleritized xylites this band is caused by C=O stretching vibrations of aromatic acids. This is evidenced by sharp peaks at 1220 (vc_o) and 880 cm -1 (VoH) that disappear, however, in doppleritic coal. The absorption close to 1700 cm-' dis- played by this coal may be due to the carbonyl group of ketones.

The 1600 cm-' band produced by common xylite and poorly or mod- erately doppleritized varieties is accompanied by absorption with peaks at 1500 and 1260 cm -1 , which markedly decrease in intensity, and have

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been at tr ibuted to skeleton vibrations of heteroaromatic rings (Czuchajowski and Sliwiok, 1974, p. 68). The intensity of these bands indicates that in the process of doppleritization of xylites, the aromatic bonds of lignin are substi tuted by heteroaromatic linkages.

Xylites showing a high degree of doppleritization display an absorption centered close to 1630 cm -1 , which in doppleritic coal exceeds the vibra- tions of heteroaromatic systems in intensity. Its growth is associated with vibrations of chelate bonds between the carboxyl groups of ketoadipic acids, or, more precisely, with vibrations of the =C--C=O . . . . . . HO--type bonds. The inflexions on the right part of this band are presumably due to the incorporation of a metal ion in the carboxyl group and to the for- mation of a linkage intermediate between single and double bonds in CO0- , which also points to the rise of an organomineral compound. The bands appearing near to 470 and 2360 cm -1 lend additional support to the hy- pothesis of incorporation of metal in the organic structure of coal.

The 1600 cm -1 band is certainly also caused by aromatic --C=C - vi- brations, ye t the effect of such vibrations on its intensity is insignificant. This is implied by the relationship between the optical densities of the 1600 and 1700 cm -1 bands, as well as by the comparison of this band for brown coal and bituminous coal in which aromatic skeleton vibrations alone are responsible for its development (Friedel, 1970, p. 146; Hacura et al., 1977, pp. 1--17).

The aromatic skeleton vibration of the structural unit of coal lies in the range from 650 to 900 cm -~ . As doppleritization progresses, the nature of aromatic substitution changes, which testifies to the aromatic conden- sation of this unit. At the initial stages of doppleritization, single benzene- type rings are substituted in the positions 1, 3, 4. Intensely doppleritized coal varieties display absorption between 730 and 790 cm -1 , arising from rings substituted in the positions 1 and 2. Simultaneously a peak indicative of substitution disappears in the position 3. The inference that aromatic condensation takes place in the structural unit is also borne out by the 2830 cm -~ band caused by aromatic C--H vibrations. In the spectrum of common xylite this band is barely distinguishable whereas in doppler- itized coal it is pronounced. The sharpness of the 2830 cm -1 band is also due to the elimination of CH2 vibrations that occur close to 2870 cm -~ . The band at 1450 cm -1 has the same character. It is at tr ibuted to CH2 groups and disappears with the progressing doppleritization of xylites.

The infrared spectrum of xylite coal contains a band that is absent in the spectrum of bituminous coal. It is a strong absorption centered close to 1440 cm -~ , arising from methoxyl (OCH3) groups. In intensely dopp- leritized xylites this band disappears.

The absorption appearing near to 2040 cm -~ and caused by hydroar- omatic skeleton vibrations has a different character (Friedel, 1970, p. 146). It is absent in the spectrum of common xylite and distinct in dopp- leritic coal. Hydroaromatic compounds are also responsible for the band

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at 1000 cm -1 . These two bands testify to the presence of five-membered rings in the structural unit of coal, which may form by condensation of ketoadipic acids (van Krevelen and Schuyer, 1959, pp. 98--103).

A strong and diffuse absorption band appearing close to 3400 cm -I is difficult to interpret. It generally testifies to the presence of hydroxyl (OH) groups, presumably of phenolic nature.

QUANTITATIVE DETERMINATION OF SOME FUNCTIONAL GROUPS

A qualitative description of IR spectra fails to provide all the informa- tion necessary to account for the structure of the compounds studied. Therefore, various approximate methods are used to calculate the content of certain groups in their structure. In this paper Oelert 's me thod (1970) was used to determine the percentage of hydrogen atoms in CH3, CH2 and aromatic CH compounds, and the content of carbon in CH2 and aromatic CH groups; Brown's method (1955) to determine the quantitative ratio of vibrating aromatic to aliphatic bonds; and Bloom's method (fide Roga and Wnekowska, 1966, pp. 206--207) to estimate the percentage of ox- ygen in COOH groups.

Most hydrogen atoms occur in aliphatic compounds. Their amount is greatest in the CH3-type compounds (Table I) because in doppleritic coal hydrogen makes up about 67.1% of the total content of this element. The lowest content of hydrogen in CH3 groups has been noted in mod- erately doppleritized xylite, being only about half of that in doppleritic coal.

The H content on CH2 groups is greatest in moderately doppleritized xylite. As the degree of doppleritization increases, hydrogen in these groups decreases from 64.6% down to 19% of the total H content in these coal varieties. From the data listed in Table I it appears that in the process of dopplerit ization of xylites, the number of CH3 groups decreases and then substantially increases while the reverse relation applies to CH2 groups.

The percentage of hydrogen in aromatic groupings is no t high, varying from 4 to 14% of the total content. At the initial stage of doppleritiza- tion it decreases, causing the opening of a part of the aromatic rings of lignin, and then increases nearly three-fold. This behaviour may be due to the aromatic condensation of the structural unit of coal.

The total hydrogen content in the doppleritization series decreases by about 20% (Table I).

The carbon content in CH2 and CH3 groups is subject to changes similar to those observed for hydrogen. Moderately doppleritized xylite has the lowest C content in CH3 groups (about 13%) while in doppleritic coal it is nearly twice as high. On the other hand, the carbon content in CH2 is highest in common xylite and poorly doppleritized varieties, showing a nearly fourfold decrease in the direction of doppleritic coal.

The carbon content in aromatic compounds is greatest in doppleritic

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coal, running up to 40% at the least and 50% at the most (Table I). In xylites showing a lower degree of doppleritization it is considerably lower. The increasing degree of aromatization of coal is best reflected by the co- efficient of aromatization f, which in common xylite is 0 at the least and 0.38 at most, amounting respectively 0.55 and 0.65 in doppleritic coal.

The ratio of vibrating aromatic to aliphatic bonds is highest in mod- erately doppleritized cylite. It is also high in doppleritic coal but low in the other xylites. This suggests that the medium degree of doppleritiza- tion involves the condensation of aromatic rings into one system which breaks up again and is subject to recondensation at the subsequent stages of this process. Such systems would be likely to form if one of the con- densed rings were a quinone ring that would then be subject to opening due to the formation of heteroaromatic bonds and then ketoadipic acids and organomineral compounds (Fig. 4, stages II--IV).

The total carbon content in the doppleritization series, recalculated to the analytical state, increases by about 6%.

The content of oxygen in COOH is a measure of the amount of these groups in the xylite varieties under study. Their amount is greatest in in- tensely doppleritized xylite and less in doppleritic coal. The simultaneous increase in the content of metallic elements in ash {Table II) shows ex- plicitly that dopplerit ization involves the replacement of carboxyl by an or- ganomineral system, possibly according to the reaction:

I I I I

=C--C=O ' ' ' H O - ~ =C--C=O . . . M e - - O - .

Infrared absorption spectra yielded by xylites from which bitumens have been extracted (with the Soxhlet method) are nearly identical with the spectra presented above. A slight difference has only been noted in the intensity of bands corresponding to certain aliphatic groupings.

RAMAN ROTARY SPECTRA

The xylites under s tudy show marked differences in the shape of Raman spectra (Fig. 3). Poorly doppleritized xylite does not display any vibra- tion. In xylites showing a medium degree of doppleritization (L-2, L-3) the 1600 cm I band increases in intensity and a band begins to develop close to 1380 cm -1 . In doppleritic coal these bands are pronounced, and the shape of spectral lines is similar to that of low-rank bi tuminous coal ( type 31 according to Polish standards or type 800 according to the in- ternational classification).

The occurrence of bands in Raman spectra of intensely doppleritized xylites testifies to the formation of locally ordered graphite-like domains, because the 1600 and 1380 cm -' bands are associated with vibrations of E2g and A~ classes of the crystal symmetry group D~h of graphite (Tsu et al., 1977). The average size L of these areas is probably about 5 nm, similar to that in sub-bituminous coal and somewhat less than in bitumi-

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L-3

20 1B 16 14 12 10 B F, xlO0 cm 1

Fig. 3. Raman rotary spectra of xylites. L - 1 = poorly doppleritized xylite; L - 2 , L - 3 =

moderately doppleritized xylites; L - 4 -- intensely doppleritized xylite; L - 5 = doppleritic coal; B C = low-rank bituminous coal (type 31 according to Polish standards or type 800 according to international classification).

nous coal (Zerda et al., 1981}. The structurally ordered areas are to be located in the core of the statistical structural unit of coal. From the above considerations it seems feasible that such a unit, presumably made up of one or two aromatic rings, is formed in the process of doppleritization of xylites.

CHEMICAL COMPOSITION OF XYLITE ASH

The ash content recalculated to the analytical state increases from 1.3 wt.% in common xylite up to 11.2 wt.% in doppleritic coal (Table II). It is internal ash, structurally connected with the organic matter of coal.

The contents of oxides and sulphur determined in the ash are differen- tiated. The common xylite ash has the highest content of CaO, A1203 and K20. Most oxides, except MgO and Fe203, decrease in the ash of mod- erately doppleritized xylite. The ash of intensely doppleritized xylite has the lowest content of such oxides as MgO, Fe203, Na20 and K:O, where- as in the doppleritic coal ash nearly all oxides and sulphur increase.

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TABLE II

Chemical analysis of xylite ash and oxygen content in COOH grouping (in percentage)

Sample A a CaO MgO A1203 Fe203 N a 2 0 K:O OcOOH Sash

L-0 1.38 9.15 2 .52 10.99 10.06 2.77 0.20 3.35 0.62 L-2 4.11 3 . 1 9 3.02 6.54 11 .13 2.40 0.13 4.96 0.97 L-4 10.89 3 . 8 4 0.76 6.86 5.65 2.19 0.06 5.29 2.14 L-4 11.14 4.86 1.26 5.72 5.66 3.38 0.08 4.52 3.11

The increased contents of oxides in common xylite and doppleritic coal are associated with the chemical mechanism of transformation of lignin, and with the final stage of the process referred to as doppleritiza+ tion. The transformation of lignin to soluble compounds involves its re- action with alkalis in a weak basic medium, and thence the fairly high content of CaO, Na~O, K:O, as well as A1203 and Fe203, in common xylite ash. The resulting salts, called alkalilignin, are not stable and readily change into acids and other products (Prosiflski, 1969, p. 334).

The second stage of formation or organomineral compounds is asso- ciated with the last stage of doppleritization. The reaction of humic acids with inorganic bases or dissociated salts gives rise to dopplerite, i.e. CaO, MgO and Na20 humic compounds. The increased contents of oxides of these metals compared with their amount in the ash of intensely dopp- leritized xylite lend support to this statement.

Taking into consideration Dragunov's formula for humic acid, given by Roga and Tomkow (1971), it can be stated that the replacement of carboxyl groups of this acid by COMe" groups (Me = 1/2 Ca) results in a 4.5% Ca content in the molecular mass of a humic compound. In the case of Na, it will be considerably lower.

The occurrence of Ca, Mg and Na humic compounds in brown coal has been reported by Kuhl (1960), Kruszewski (1968), and other authors.

GENESIS OF DOPPLERITIC COAL

The doppleritization of fossil wood is a chemical process that occurs in the environment of a peat-bog or a brown coal deposit (Welte, 1952). This process operates in wood that has not ye t undergone profound de-- struction during its decay, which is evidenced by the substantial content of lignin. The environment in question shows a weak alkaline reaction. Under continental conditions, an environment of this kind is provided by a low peat-bog under a thin layer of hard water. The effect of OH- ion on lignin involves the disruption of etheric bonds between its radicals and demethoxylat ion, while the formation of free phenol groups may be re- sponsible for the recondensation of radicals.

Furthermore, the alkaline medium causes changes in the side chains

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of lignin, and particularly the enolization of ketone systems (Fig. 4) as a result of the formation of soluble salts of alkaline metals. The further evolution of these salts involves the formation of quinones and reactive ketoadipic acids due to the opening of a part of rings. It seems feasible that the mixture of the decomposit ion products of cellulose and lignin in a weakly oxidizing environment gives rise to ketoadipic acids, commonly referred to as humic acids. The presence of these compounds is responsible for the strong acidity of the environment, typical of peat-bogs and brown coal deposits.

CH~ ~H2 OH C-O

i~ CH~ H-C II

_OCHs .,~C-ON° HO', CHz / * 2 NI:OH I,, ~ ~ Jr" H-C - - 0 - H# V'nr~v~"

I -CH:. H-C-OH OH

~'OCH~ 0 I

I II IIIQ lllb

CH~ ICH~ H-C CH II R~

&-ONo OH CH OH R' OH A H,CO ~x.~ HBCO.~ H~CO~f ~-H xl~,o~o , ~ IL~O ~ l l̀ II j ' 'HZ-'7~'U'C CZOH

014 H-C 0 R R

CH~

0H V IV ""°2'2"°'2:

OH

temp CO /C,.~ ,c *neH(O3 H2 CI H R o %1,-c ~OH

O-,Me.O

Fig. 4. The hypothetical course of doppleritization of lignin radicals. I = the stage of dissolution of lignin (the formation of alkalilignin); H = the stage of formation of qui- none; I I I = the stage of formation of ketoadipic acid; I V = the stage of formation of dopplerite (organomineral compound). V = the stage of formation of vitrain in sub- bituminous coal.

The formation of dopplerite is p romoted by brackish sea waters or al- kaline fresh waters. This process involves the bonding of cations of some metals to humic acids (~-ketoadipic acids). In reactions of this type a water molecule is presumably expelled. This causes the condensation of organo- mineral sol, which may then be subject to in situ diagenesis, or may be displaced to a different geochemical environment and undergo coagula- tion (Breger, 1955, p. 132).

During diagenesis the organomineral, presumably lyophobic, sol co- agulates. The coagulation may be caused by dehydrat ion due to an increasing pressure of the overburden layer and the resulting increase in temperature, or by a change in the electric charge in response to structural changes, e.g. of clay minerals, in the changing environmental conditions (Wagner, 1980). The coagulation is accompanied by the process of ageing of col-

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loids, as well as by recrystallization and chemical changes. One of the changes induced by elevated temperature is the decomposit ion of organomineral compounds (Fig. 4). Hence the vitrain of sub-bituminous and bituminous coal does no t contain the same amount of ash combined organically with coal as dopplerite, although there are exceptions to this rule.

In conclusion, it can be stated that the doppleritization and humifica- tion of wood in a coal-forming environment ultimately give rise to chem- ically different products, even though the physical nature of these pro- cesses is similar as they involve colloidal transformation. When the coag- ulation of organomineral sol is not induced by the increasing temperature, the dopplerite retains its chemical character, which fact has been noted in some peat beds and brown coal deposits.

ACKNOWLEDGEMENTS

The author wishes to express his thanks to Dr W. Carius from the Ped- agogical University of Erfurt /GDR / for taking Raman spectra, to Dr T. Zerda from the Silesian University for taking IR spectra, and to Dr K. Matl from the University of Mining and Metallurgy Cracow for the critical reading of the manuscript and valuable discussion.

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