Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.)

International Journal of Coal Geology, 13 ( 1989 ) 455-479 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

455

Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.) COLIN R. WARD

Department o[ Applied Geology, University of New South Wales, P.O. Box 1, Kensington, N.S. W. 2033, Australia (Received February 22, 1988; revised and accepted August 31, 1988)

ABSTRACT Ward, C.R., 1989. Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.). In: P.C. Lyons and B. Alpern (Editors), Coal: Mineralogy, Classification, Coalification, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol., 13: 455-479. Although other forms of inorganic components may be significant in lower-rank coals, the mineral matter in most bituminous coals is dominated by silicates, carbonates, sulphides, phosphates and other crystalline mineral groups. These may be present in various forms, includingthin bands and laminae intimately associated with the organic matter as well as lenticles, nodules, crystal aggregates and cell infillings, all of which indicate approximately contemporaneous formation with the peat bed. Other minerals, however, occur as veins or cleat and fracture fillings, indicating precipitation after most of the compaction and, presumably, after most of the rank advance. Some of the penecontemporaneous mineral matter is of detrital origin, representing epiclastic or pyroclastic particles washed or blown into the peat swamp. Other mineral matter, however, including much of the pyrite, quartz and siderite, as well as abundant well-crystallized kaolinite, appears to have been formed by precipitation processes either within the swamp waters or in the pores of the peat deposit. Pyrite, largely formed by bacterial reduction of dissolved sulphate ions, seems to be related in abundance to marine transgressions, which allowed permeation of sea water into the swamp and through the peat bed beneath. Siderite appears to form in the absence of sulphate-rich sea water, probably from C02 released by organic decay. The clay fraction of the noncoal rocks in many coal-bearing sequences is typically dominated by illite or interstratified clay minerals, with kaolinite, chlorite and possibly montmorillonitealso present to a variable, though typically minor extent. Although perhaps modified by chemical reactions or ion-exchange mechanisms in the peat swamp, these clay minerals may also be present in the coal as well, especially near the base of top of the seam. The kaolinite in the noncoal rocks is usually poorly crystallized and typically a minor constituent, but well-crystallized kaolinite is characteristically present, and often the dominant component in the mineral fraction of bituminous coal beds, as well as in some of the lutites (including underclays) intimately associated with the seams. A progressive upward gradation to a particularly kaolinite-dominated mineral assemblage occurs in some seams of the Sydney basin, Australia. This gradation is thought to be due to either increased blockage of detrital contaminants as the swamp developed, or to in-situ chemical or biological leaching of the mineral fraction and reconstitution of the residues in the upper parts of

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456 the peat bed. Superimposition of such cycles is also noted within the seam in some instances, suggesting repetition of several phases of this type during major peat accumulations.

INTRODUCTION

In its broadest sense, the term "mineral matter" with respect to coal embraces three types of components: (a) discrete crystalline mineral particles occurring in the coal in some way; (b) inorganic elements or compounds, usually with the exception of nitrogen and sulphur, that form part of the coal's organic structure; (c) dissolved ions and other inorganic material in the pore water or surface water of the coal concerned. Only the crystalline inorganic particles, of coarse, can be classified as "minerals" in the strict geological sense. Indeed, Kiss and King (1979) have suggested that careful distinction should be made between "minerals" and other "inorganics" in low-rank coals, where the noncrystalline components, chiefly as metal ions associated with carboxyl groups or chelate complexes, make up a significant part, if not the dominant fraction of the total mineral matter (Kiss and King, 1977; Miller and Given, 1978; Given and Spackman, 1978). In higher-rank coals, however, most notably in bituminous coals, the mineral matter is almost entirely present in crystalline form. MODES OF MINERAL OCCURRENCE

Minerals in bituminous coals are often visible in hand specimen, and mineral occurrences can often be distinguished, at a megascopic scale, in X-radiographs of drill cores (Jones, 1970). Megascopic mineral components include discrete bands and laminae of clay or clay-rich material, lenticles, nodules and concretionary bodies of sulphide, carbonate or silica, and, more rarely, pellets or tube-like aggregates (Fig. 1 ) typically made up of crystalline clays such as kaolinite. Other common occurrences, usually formed at a later stage in the coal's diagenetic history, include deposits of minerals such as carbonate, pyrite, and clay in cleat fractures and in veins cutting across the coal seam. In addition to this megascopic mineral matter, coals may also contain significant mineral components visible only at a microscopic scale. These are often intimately admixed or intergrown with the macerals themselves, and are commonly referred to as the "inherent" mineral matter of the coal seam. Minerals in coal at a microscopic scale may include thin bands or laminae, often containing small subangular particles, such as quartz grains, suggesting a detrital origin. They may, however, also include infilling s of cell lumens with clay, carbonate, sulphide, apatite or silica (e.g., Balme and Brooks, 1953; Cook, 1962), polycrystalline nodules, and submicroscopic framboids (Scheihing et

457

Fig. 1. Vermicularaggregates of kaolinite (white) on a coal seam bedding plane, Surat basin, Queensland,Australia. Width of fieldapproximately3.5 ram. al., 1978). The mode of occurrence of minerals in coal, as observed under the microscope, is discussed in some detail by Kemezys and Taylor (1964), Mackowsky (1968), Finkelman (1980) and Stach et al. (1982). ANALYTICALTECHNIQUES A comprehensive review of the methods that may be used to determine the total proportion of mineral matter in coal, as well as to identify the assemblage of mineral species present, is given by Ward (1986). Methods for determining the mineral matter present in a coal sample include petrographic techniques (International Committee for Coal Petrology, 1963; Stach et al., 1982), interpretations from chemical data (King et al., 1936), procedures based on heating the coal in air at temperatures of up to 370 ° C (Hicks and Nagelschmidt, 1943 ), dissolving the minerals out of the coal with acid (Radmacher and Mohrhauer, 1955), and oxidizing the organic matter at low temperature using hydrogen peroxide (Ward, 1974) or an electronically excited oxygen plasma (Gluskoter, 1965). Adolphi and StSrr (1985) also describe a glow-discharge, low-temperature ashing method. The oxygen-plasma technique, however, is the most widely used method in modern mineral-matter studies, allowing both precise estimation of the total mineral-matter content (Frazer and Belcher, 1973; Miller

458

et al., 1979) and isolation of the minerals from the coal, without significant alteration, for subsequent geological studies (Gluskoter, 1967; Ward, 1977; Finkelman et al., 1981 ). In the oxygen-plasma method, the coal is exposed, in an evacuated chamber, to a slow stream of oxygen that has passed through a high-energy electromagnetic field. The oxygen is converted to a mixure of atomic and ionic species, and slowly destroys the organic matter, at a temperature of around 150 ° C, leaving a light-coloured, dry, essentially unaltered mineral residue. The inorganic components of the coal's organic matter, particularly with low-rank coals, may form additional crystalline residues during the course of the oxidation process, but these can be minimized by special operating techniques (Miller et al., 1979). Once isolated from the coal, the mineral residues may be analyzed by a variety of methods, including X-ray diffraction, thermal analysis, infrared spectrometry and electron microscopy (Russell and Rimmer, 1979). These provide a valuable extension to petrographic studies, allowing a more precise estimation of total mineral content and better identification of minerals, especially the clay minerals, than is possible by optical microscopic techniques. MINERALS P R E S E N T

The minerals present in bituminous coals have been summarized by a number of authors, including Kemezys and Taylor (1964), Mackowsky (1968) and Stach et al. (1982) on the basis of petrographic observations and Gluskoter ( 1967 ), O'Gorman and Walker (1971), Rao and Gluskoter (1973), Soong and Gluskoter (1977), Ward (1977, 1978, 1986) and Doolan et al. (1979) from mineralogical analyses of plasma-ashing residues. Many of the minerals noted in these studies, however, are only very minor constituents or only found in particular coal deposits. The bulk of the mineral fraction in most bituminous coals or their oxidation residues consists of a selection of components from those listed in Table 1.

Quartz Quartz is a relatively abundant constituent of some bituminous coals, but is more commonly only a minor component of the mineral fraction. Rao and Gluskoter (1973) and Ward (1977) found that quartz makes up less than 20% of the mineral matter for most coals from the Illinois basin, U.S.A., and low quartz contents are also indicated by O'Gorman and Walker (1971) in most bituminous coals from the Appalachian region. Gaigher (1980) also indicates a low quartz content in most of the economically important coals of South Africa. A wider range of quartz contents, however, though still mainly low, is indicated by Ward (1978) for a selection of Australian coal seams. Petrographic studies (Kemezys and Taylor, 1964; Mackowsky, 1968) sug-

459 TABLE 1 Major minerals identified by various authors in bituminous coals Silicates

Kaolinite A12Si205(OH) 4 Illite KA12(A1Si~)Olo(OH) 2 Montmorillonite Na (A1Mg)Si40lo(OH) 2 Chlorite (MgFeA1)6(A1Si)40x0(OH)s Interstratified (mixed-layer) clay minerals Quartz SiO2 Chalcedony SiO2 Feldspar KA1Si3Os

Carbonates Calcite CaCO3 Siderite FeCO3 Dolomite CaMg(COa)2 Ankerite (CaFeMg)CO3 Dawsonite NaA1CO3(OH) 2 Strontianite SrC03 Aragonite CaCO3

Sulphides

Sulphates

Pyrite FeS2 Marcasite FeS2 Sphalerite ZnS2 Galena PbS Millerite NiS Chalcopyrite CuFeS2

Gypsum CaSO4'2H20 Barite BaSO4 Anhydrite CaSO4 Coquimbite Fe2(SO4)3"9H20 Szomolnokite FeSO4"H20 Natrojarosite NaFea(SO4)2(OH) Thenardite Na2S04 Bassanite CaSO4' ½H20 Celestite SrSO4

Phosphates

Other minerals Anatase TiOe Rutile TiO~ Boehmite AlO (OH) Haematite Fe203 Goethite Fe (OH)3 Zircon ZrSiO4

Apatite CasF (PO4)a Goyazite SnA13(P04) 2(OH )5"5H20

gest that some of the quartz in bituminous coals occurs as silt-sized, probably detrital grains, washed or blown into the swamp in conjunction with peat accumulation. Other quartz, however, is in microcrystalline form (Kemezys and Taylor, 1964; T h o m p s o n et al., 1983), representing chalcedonic material deposited as cell infillings and possibly gelatinous masses in the pores of the peat deposit. Silica, in the form of diatom frustules, plant phytoliths and sponge spicules, is one of the most a b u n d a n t mineral components found in some present-day peat deposits ( R a y m o n d and Andrejko, 1983; Davis et al., 1984 ). This material is subject to fairly rapid corrosion from chemical leaching and the activities of microorganisms (Andrejko et al, 1983), and thus may not survive, in its orig-

460 inal form, to become part of the actual coal bed. It may, however, represent a source of silica for subsequent precipitation as some of the chalcedonic deposits.

Pyrite and related minerals Pyrite and sulphates derived from pyrite oxidation constitute up to 35% of the mineral matter (oxidation residue) in coals from areas such as the Illinois basin (Rao and Gluskoter, 1973; Ward, 1977), but lower values, usually no more than a few percent, are indicated for most Australian and South African deposits (Ward, 1978; Gaigher, 1980). Although highly pyritic coal is known where fluvial sandstone occurs in the overlying strata (Belt and Lyons, 1989 ), the occurrence of abundant pyrite in coals has long been considered to reflect the presence of marine strata a short stratigraphic distance above the seam (Williams and Keith, 1963). Its abundance in the coals of the Illinois basin is controlled mainly by the extent of penecontemporaneous freshwater channel and overbank deposits above the individual coal beds (Gluskoter and Hopkins, 1970 ), with relatively low values where the nonmarine deposits are thick. High, often excessive pyrite is typically present in areas where the coal is directly overlain by marine sediments. The pyrite appears to represent material formed from the interaction of dissolved iron and hydrogen sulphide, the latter being produced either from mobilization of sulphur-bearing compounds in the peat's organic matter (A1tschuler et al., 1983 ) or by bacterial reduction of sulphate ions in the sea water permeating through the peat bed. Its virtual absence in coals overlain by thick deposits of terrestrial strata, and its abundance in coal capped by transgressive marine successions suggest that the availability of sulphate in the pore waters of the peat, during deposition or shortly after burial, is a major controlling factor in its formation. Other pyrite, however, is formed at a later stage, possibly by a remobilization phase from the organic matter in conjunction with rank advance, precipitation from deep circulating fluids or redistribution in some way of earlier deposits, and builds up as an infilling of cleat and other fractures in the coal bed. Most of the iron involved in the pyrite formation was probably derived from the detrital sediment associated with the original peat deposit (Spears, 1987). Small amounts of other sulphides are sometimes seen in polished section, however, probably reflecting the presence of other metallic ions in sufficient quantities to become involved in the sulphide formation process. Lawrence et al. (1960), for example, report the occurrence of millerite (NiS) in an Australian coal, and Hatch et al. (1976) describe the occurrence of abundant sphalerite as a cleat infilling in coals from a particular area of Illinois. Marcasite, a polymorph of pyrite, is also noted by Harvey et al. ( 1983 ) in coals from the southeastern part of the Illinois basin.

461

A range of iron sulphate minerals, such as coquimbite (Fe2 (SO4) 3"9H20 ), szomolnockite (FeSO4"H20) and jarosite (KFe3 (SO4) 2 (OH) 6) are also found in the mineral fraction of some coal samples. These may be produced by oxidation of pyrite in the coal with exposure or during storage (Rao and Gluskoter, 1973), or by interaction of the pyrite with activated oxygen during the plasma ashing process (Frazer and Belcher, 1973). Gypsum (CaSO4"2H20), bassanite (CaSO4" 1/2H20) and anhydrite (CaSO4) are also found in some oxidation residues. This material may be produced by interaction of sulphuric acid from pyrite oxidation with calcite during storage followed by dehydration in the ashing process (Rao and Gluskoter, 1973) or by interaction of organically combined calcium and sulphur as the hydrocarbons are broken down by the oxygen plasma. Coal from arid areas, such as Leigh Creek in South Australia (Kemezys and Taylor, 1964) or Utah in the U.S.A. (Ward, 1986), may also contain gypsum, possibly due to precipitation of the material from ground water. Carbonate minerals

Siderite, calcite, dolomite and ankerite are common, and sometimes abundant constituents of the mineral fraction of bituminous coals. The siderite mostly appears to have been formed early in the coal's diagenetic history, and typically occurs as small nodules and similar bodies (Kemezys and Taylor, 1964) or as an infilling of cell lumens (Beeston, 1981 ). Concretionary masses of earlier-formed calcite a n d / o r dolomite may also be present in the form of coal balls in some seams (Moore, 1968), or as cell infillings and replacement features (Kemezys and Taylor, 1964), but calcite, dolomite and ankerite are mostly later-stage deposits, occurring in cleats and other fractures. The bulk of this material was, therefore, apparently formed after the coal had been completely compacted and undergone most of its rank advance. The siderite in coal is thought to have been formed mainly by interaction of iron with dissolved CO2, released by fermentation of the organic matter (Gould and Smith, 1979). Spears (1987) indicates that pyrite formation, if it occurs, is very effective at removing iron from solution, and hence siderite is most abundant in areas where the availability of sulphate is low and a certain amount of iron might be expected to be present after any sulphides have been precipitated. Siderite is the dominant iron-bearing mineral in most Australian coals (Ward, 1978), where the pyrite content is low, but is virtually absent from the pyrite-rich coals of the Illinois basin (Rao and Gluskoter, 1973; Ward, 1977) and the Appalachian region (O'Gorman and Walker, 1971 ). Calcite and dolomite (including the iron-rich variety referred to as ankerite ) are found in both pyrite-rich and pyrite-poor coal seams. They may be a dominant component in the mineral fraction of some samples but are completely

462 absent from others, a feature probably controlled by their occurrence mainly as cleat and fracture infillings rather than as an intimate mixture with the organic matter. Many authors (e.g., Hatch et al., 1976; Botz et al., 1986; Spears, 1987) consider calcite in fracture fillings to have been deposited during the latter stages of diagenesis, at temperatures of up to more than 100°C. Spears (1987) suggests that the calcium was derived from diagenetic modification of detrital minerals in and around the coal seam, while the necessary carbonate was derived from decarboxylation and other reactions in the organic matter. However, calcium is also a common component of the organic matter itself in a number of low-rank (e.g., subbituminous) coals (Miller and Given, 1978; Nankervis and Furlong, 1980), and its release from combination in this form during the course of coalification may represent another potential calcium source.

Other nonclay minerals Small amounts of feldspar are present in a number of coal samples from various parts of Australia (Ward, 1978), the Illinois basin (Harvey et al., 1983 ), and in bituminous coals from other areas as well. Much of this material is probably of epiclastic or pyroclastic origin, washed or blown into the swamp during peat accumulation. The phosphate mineral apatite is also found in some coal seams. Cook (1962) describes it occurring as a petrifaction in a coal from the Ipswich area of Queensland, and Ward (1978) and Corcoran (1979) have noted its presence in a number of other Australian seams. Goyazite or a similar alumino-phosphate mineral of the crandallite group is also noted in a number of Australian coals by Ward ( 1974, 1978), and has been identified by Palmer and Wandless (1985) in coals from the Appalachian region of the U.S.A. The apatite was probably formed from phosphate released by decaying organic matter, whereas the goyazite mineral is thought to represent the result of interaction between similar phosphatic material and the aluminous components that produced the kaolinite (see below) in the clay-mineral fraction.

Clay minerals The clay minerals represent the most widespread and abundant nonorganic components of many coal seams, particularly those with low carbonate or pyrite contents. A variety of different species may be present, including illite, montmorillonite, chlorite, and a range of mixed-layer or interstratified clay minerals, as well ,as kaolinite, with the kaolinite often in a distinctive, wellcrystallized form. Many of the clay minerals, apart from the well-crystallized kaolinite, resemble those found in the associated fine-grained noncoal sediments, and are thought to be essentially of detrital origin. Detailed study, how-

463 ever, as dicussed below, may reveal differences that can be attributed in some way to leaching or other degradation processes in the depositional or immediate post-depositional environment of the coal seam, or possibly to diagenetic crystallization of plant-derived inorganic components. In some cases, too, the material may be at least partly of pyroclastic rather than epiclastic origin. Most of the nonkaolinite clay material, and also much of the kaolinite as well, typically occurs in thin bands and laminae intimately associated with the different maceral components. Thicker, often quite persistent bands of claystone and similar rocks are also found within many individual coal seams. Kaolinite within the coal, however, can also occur as cell-cavity infillings in particular macerals (Balme and Brooks, 1953; Lyons et al., 1982), as rounded pellets and vermicular crystal aggregates (Kemezys and Taylor, 1964) and even in the interstices of microcrystalline pyrite framboids (Scheihing et al., 1978), which suggest in all cases kaolinite formation by an authigenic or early diagenetic process within the peat bed. Kaolinite, however, is also found as an infflling of cleat fractures, having probably formed by a different in-situ process during a later diagenetic phase. The interplay of factors that control the clay minerals in coal seams is complex, and each individual area or coal-measure sequence probably requires a separate study to establish the origin and rationale for distribution of the different components involved. Some examples, however, are provided by the following case studies with which the author of this paper has been associated. ILLINOIS BASIN, U.S.A. The two main economic coal seams in the Illinois basin, the HarrisburgSpringfield (No. 5) and the Herrin (No. 6) Coal Members, contain a clay mineral assemblage consisting of kaolinite, illite, and an irregularly interstratiffed illite/montmorillonite or degraded illite component, along with a small amount of chlorite in some areas as well {Gluskoter, 1967; Rao and Gluskoter, 1973; Ward, 1977 ). X-ray diffractograms of the clay fraction from typical samples are shown in Figure 2. Comparative studies by Ward (1977) show the coal to contain significantly more kaolinite in its clay fraction than the associated noncoal lutites, and also a lesser proportion of mixed-layer clay or degraded illite (Fig. 3). Traces of chlorite~ a common constituent of the noncoal strata, are present only in a few coal samples, with the data suggesting that it mostly occurs in the bottom parts only of the coal bed (Table 2). The contrast in mineralogy between the nonkaolinite clays in the coal and those in the adjacent strata may reflect prolonged exposure of the detrital mineral assemblage of the basin generally to organic acids within the coal swamp. Huang and Keller (1972) produced a marked sharpening of the 10A XRD peak in experiments where an illite sample from an underclay at Fithian, Illinois,

464

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Fig. 2. Typical X-ray diffractogramsof mineral matter isolated from coal of the Springfield (No. 5) Coal Member of the Illinois basin (Ward, 1977). Copper K-alpha radiation. A = anhydrite; Cq-coquimbite; Ct=calcite; D=dolomite; G=gypsum; I-illite; J=jarosite; K---kaolinite; P = pyrite; Q= quartz; S i - - siderite; S z = szomolnockite. was treated for an extended period with salicyclic acid. The change was attributed to selective solution of the mixed-layer or degraded material with a larger "d" spacing in the sample, leaving an apparently higher proportion of wellordered illite in the remaining residue. In a detailed study of the underclay beneath the Herrin (No. 6) seam in the Illinois basin, Rimmer and Eberl (1982) noted a general increase in the amount of mixed-layer material and a decrease in the proportion of discrete illite with respect to expandable clays, as well as a decrease in chlorite, from the base of

465 ILLITE

KAOLINITE

EXPANDIBLES

Fig. 3. Triangular diagram showing clay mineralogy ( < 2 #m fraction) of roof and floor strata (open circles) and low-temperature (oxygen plasma) ash of coal samples (solid circles) for the Springfield Coal Member of the Illinois basin. Partly after Ward (1977). TABLE 2 Mineral matter in subsections of the Springfield Coal Member, southwestern part of the Illinois basin (Ward, 1977) Interval thickness (m)

Rock type

Roof 0.25 0.28 0.33 0.47 0.13 Floor

marine coal coal coal coal coal underclay

Mineral matter in coal (wt %)

Quartz ( %)

Bulk mineralogy Calcite (%)

-14.8 12.7 6.7 16.7 21.2

18 11 6 16 24 --

Pyrite (%)

not determined - tr 30 11 43 36 * 10 10 41 6 8 not determined - -

< 2/~m fraction Clay Kaolinite (%) (%)

52 35 48 33 62

14 34 53 45 36 29 # 6#

Illite (%)

Expdbl. clays (%)

56 38 36 43 53 58 60

30 28 11 12 11 13 34

* includes dolomite # includes chlorite the u n d e r c l a y up to its c o n t a c t with t h e coal bed. T h i s progressive c h a n g e in m i n e r a l o g y was a t t r i b u t e d to acid l e a c h i n g o f t h e s u b s t r a t e in a s s o c i a t i o n with c o a l - s e a m d e v e l o p m e n t . A slight reversal in t h e t r e n d was noted, however, with illite i n c r e a s i n g b y a small a m o u n t at t h e top of the u n d e r c l a y in some sections i m m e d i a t e l y b e n e a t h t h e coal bed. T h e a u t h o r s suggest this m a y possibly reflect a local increase in K + ions derived f r o m vegetal m a t t e r in t h e peat, producing a m o r e stable illite-like s t r u c t u r e in some of the e x p a n d a b l e c o m p o n e n t s . A s t u d y of t h e m i n e r a l o g i c a l t r e n d s in t h e Springfield (No. 5 ) Coal M e m b e r

466

of the basin (Ward, 1977) shows that pyrite is least abundant in the mineral fraction in areas where the coal is overlain by more than 7 m of nonmarine shale close to a major penecontemporaneous paleochannel in southeastern Illinois (the Galatia Channel of Hopkins et al., 1979), and most abundant in those parts of the seam furthest away from the channel or the basin edge (Fig. 4). Similar trends with respect to another paleochannel body were noted by Rao and Gluskoter (1973) in the overlying Herrin (No. 6) seam. The restriction of low-pyrite coals to the vicinity of the paleochannels was confirmed by Harvey et al. ( 1983 ), with the note that some high-pyrite coals also occur near the paleochannels, particularly with the Springfield Coal Member. This clearly reflects the reduced extent of sea water influx where the coal is overlain by beds in each case associated with the channel body (Fig. 5). The clay minerals in the Springfield Coal Member also were found by Ward (1977) to show a certain amount of lateral variation within the seam. Kaolinite was found to be relatively abundant, as a percentage of the total clay fraction, in the pyrite-rich coal in the far south-east (Fig. 6), but to represent a somewhat minor proportion of the total clay component both in the area of the paleochannel and along the western margin of the basin. This may reflect a decrease in the abundance of detrital clay minerals (illite and expandable-

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'tr('".-,.~ Fig. 7. Lateral variation in the relative proportions of illite and expandable-lattice clay minerals in the clay ( < 2 #m) fraction for the mineral matter of the Harrisburg-Springfield Coal Member, Illinois basin (Ward, 1977).

lattice components) in the areas of the basin furthest from the sources of clastic input and an increase, in their absence, in the amount of authigenic kaolinite in the clay fraction. The increase in abundance of pyrite noted in the same area, relative to the total mineral assemblage, may also be partly explained in a similar way. The percentage of illite relative to the total nonkaolinite clay fraction (illite + expandables) was also found to be lowest in the area of the Galatia Channel and towards the western margin of the basin (Fig. 7), and the relative percentage of mixed-layer and degraded material (expandable clays) highest in these areas. The nonkaolinite fraction with the higher proportion of expandable lattice clay minerals most closely resembles the clay fraction of the associated noncoal sediments (see data in Ward, 1977), and thus its abundance was thought to reflect the greater level of clastic influx in these parts of the original peat swamp. The increase in illite away from these areas, on the other hand, may be explained by increased solution of the expandables in organic acids, or by increased opportunities for interaction of the expandable clays with K + ions to form illite in the more marine distal parts of the basin.

469 SYDNEY BASIN, AUSTRALIA The Permian coals of the Sydney basin in the eastern part of Australia (Fig. 8) form part of a thick terrestrial sequence, with associated lithofacies ranging from alluvial fan to delta-plain deposits (Hobday, 1987). Most seams have a very low pyrite content, and the mineral fraction tends to be dominated by the clay minerals, particularly well-crystallized kaolinite (Ward, 1978). With some exceptions, the coals also typically contain a high proportion of inertinite macerals, particularly semifusinite and inertodetrinite, apparently reflecting a certain amount of in situ oxidation of the peat bed during the'course of its accumulation.

Nature of the mineral matter The Bulli and K a t o o m b a seams, at the top of the coal-bearing sequence in the southern and western parts of the basin, contain a mineral assemblage made up almost entirely of well-crystallized kaolinite. This contrasts quite markedly to the mineral assemblage in the noncoal lutites of the area, including those forming the immediate floor of the coal bed, which typically consist of abundant quartz and a clay fraction made up of illite, irregular mixed-layer

Fig. 8. Outline map of the Sydney basin. Australia, showing locations of seam sections sampled for mineral-matter studies. I and 2-- BuUiseam; 3 = Katoomba seam; 4 = Lithgowseam; 5 = Dudley seam.

470

clay minerals or degraded illite and only a small amount of a much more poorly crystallized kaolinite. Detailed study shows that the coal in the lower part of these seams also contains a quartz-rich mineral assemblage (Fig. 9), with mixed-layer clay minerals and in some cases illite as minor components. Unlike the noncoal strata immediately beneath the seam, however, they also contain a significant proportion of well-crystallized kaolinite. This assemblage passes up within a short distance to a mixture of quartz and well-crystallized kaolinite alone, and in the top part of the seam to a mineral fraction almost completely dominated by well-crystallized kaolinite. Goyazite, an aluminophosphate mineral of the crandallite group, also occurs in small amounts, particularly near the top of these seams, and calcite is also present in some cases as well. At one locality, calcite forms a prominent horizon near the top of the Bulli seam, occurring as closely spaced fine subhorizonKATOOMBA SEAM Clarence Colliery m

Roo 0.0

Sample

C1 C2

C3

0.4 0.8

C3 1.2

C5

1.6 2.0 2.4

04

C5

2.8

C6

3.2

C7

3.6

Floo 4.0

C6

C7

C8

L,9 C10

C9

4.2

410

I

30

CoKo( R a d i a t i o n

I

20

I

10

D e g r e e s 2{}

Fig. 9. Seam section profile of Katoomba seam, showing X-ray diffractograms of mineral matter in representative subsections: A p = apatite; C h = chlorite; D = dolomite; F = feldspar; G = goyazitegroup mineral; I=illite; K = kaolinite; M£ = mixed-layer clay minerals and/or montmorillonite; P=pyrite; S=siderite; Black bars in column sections represent lithotype characteristics (macroscopically identified "brightness") according to Standards Association of Australia, 1986.

471 tal veins. Other veins, often with a cone-in-cone structure are also present in the associated strata (Ward et al., 1986). The Lithgow seam, near the base of the coal measures in the western part of the Sydney basin, is associated with lutites that contain quartz and a distinctive regular mixed-layer illite-montmorillonite-chlorite clay-mineral assemblage (Loughnan and Craig, 1961 ). The same assemblage also occurs, along with some well-crystallized kaolinite, in the basal part of the seam (Fig. 10). A short distance above the base, however, the mixed-layer clay largely disappears and the coal contains only quartz and well-crystallized kaolinite. Kaolinite then becomes almost the sole constituent in a locally developed claystone band 0.2 m from the seam floor. Quartz and the regular mixed-layer clay mineral occur again in the coal immediately above this band (Sample F8 in Fig. 10), but the mineral assemblage once again changes back, progressively up the seam, to one with well-crystallized kaolinite alone in another claystone band (Sample F6) and the coal immediately beneath. A third mineralogical succession of this type, though with a higher overall proportion of well-crystallized kaolinite, is again developed between the top of this noncoal layer and the ultimate roof of the seam. A different pattern of mineral distribution is found in the Dudley seam of

LITHGOW SEAM Fernbrook Colliery m

~ 4t,.["J-'-lSample

N

R o o f ~ F1 F2 0'0 " ~ F 3 0.4

F6

0"8~

F7

1.2

F8

1.6~

e,oorL,__,J q2

I

40

I

30

CoKo~ Radiation

I

20

I

10

Degrees 20-

Fig. 10. Seamsectionprofileofthe Lithgowseam,showingX-ray diffractogramsof mineralmatter in representativesubsections.Abbreviationssame as Fig. 9.

472

the Newcastle coalfield, adjacent to the faulted northern margin of the Sydney basin. The shaly rocks of the roof and floor of this particular seam, as well as a noncoal band near the roof (Fig. 11 ), consist mainly of quartz and feldspar, with illite, irregularly interstratified illite/montmorillonite, a trace of chlorite and a small amount of poorly crystallized kaolinite. A thick band of claystone and stony coal in the lower part of the seam, however (Sample P6), consists mainly of montmorillonite, with a small amount of quartz and kaolinite as well. Montmorillonite is widespread in the northern part of the Sydney basin (Holmes, 1983 ), and probably represents horizons of pyroclastic material derived from contemporaneous volcanic activity further to the north. The montmorillonite-rich band in the Dudley seam is quite different in mineralogy from either the coal (see below) or the roof and floor strata, and was probably formed from a pyroclastic or reworked pyroclastic outburst that temporarily overwhelmed the peat accumulation. Like the other seams of the Sydney basin, the coal of the Dudley seam conDUDLEY SEAM Pacific R

Colliery Sample N P1 P2 _P_3_p4

P1 P2 P3

P5

P5 P6 P6 P7 -

-

P9 - P 8

P9

P13

-p~l- P10 -P12 P13 -

F

-

P14

P14 4~0

3=0 CoKo( R a d i a t i o n

i

20

i

10

I

2

D e g r e e s 29-

Fig. 11. Seam section profile of the Dudley seam, showing X-ray diffractograms of mineral matter in representative subsections. Abbreviations same as Fig. 9.

473 tains an abundance of well-crystallized kaolinite and only a small amount of quartz. Most subsections of the seam also contain a little montmorillonite or an irregular mixed-layer clay mineral, although the montmorillonite is somewhat altered by its intimate association with the coal and does not fully collapse to an illite-like structure on heating. Although present in the roof and floor strata, discrete chlorite is not present in any part of the actual coal seam. Under the electron microscope, the c~al from the Dudley and several other seams in the Newcastle district of the Sydney basin are seen to contain small polycrystalline spheroidal aggregates, identified from their chemical composition and electron diffraction pattern as being made up of halloysite or poorly crystalline kaolinite (Ward, in press). These aggregates, up to about 0.5 zm in diameter, are similar in appearance and constitution to spherulitic halloysite or allophane occurrences described by Sudo et al. (1981), and are generally regarded as representing fine pyroclastic debris subjected to alteration under swampy or soil-forming conditions. Their presence in the coals of the Newcastle district, and their apparent absence in coals from other areas, provides additional evidence of a contribution from contemporaneous volcanic activity to the mineral matter in this more rapidly subsiding part of the basin.

Origin o[ the clay minerals It appears from studies such as these that the clay minerals in the coals of the Sydney basin were introduced to the peat by three different processes:

(a) detrital input in epiclastic sediment Although some of the quartz may be authigenic in origin, evidence for significant input of detrital sediment is found in the abundant quartz and mixedlayer clays in the coal near the base of all seams studied to date, in the basal part of the intervals immediately above the kaolinite claystone bands within the Lithgow seam, and in the coal and associated noncoal band just below the roof of the Dudley seam. Illite and chlorite, although present as detrital components in the shaly sediments elsewhere in the sequence, appear to have been removed from the detrital assemblage in the coal itself, probably by similar processes to those responsible for the progressive upward diminution of these minerals in the underclays of the Illinois basin (Rimmer and Eberl, 1982).

(b) direct or indirect input of pyroclastic material This process is best represented in the Dudley seam and some of the associated coals in the Newcastle area, where montmorillonite is present both as discrete intraseam bands and as a contaminant, in places, of the coal itself. It is also the most likely process to have formed the spherulitic clay particles that appear to be peculiar to these seams, and may be at least partly involved in the formation of some of the kaolinite bands themselves.

474

(c) authigenic precipitation within the swamp and the pores o[ the peat deposit Although other processes may also be involved in some way (see below), insitu formation within the swamp or inside the peat deposit appears to provide the best explanation for the ubiquitous occurrence of abundant well-crystallized kaolinite in the coals of the Sydney basin. The presence of kaolinite as ~ell infillings, petrifactions and vermicular crystal aggregates provides further evidence of at least some in situ mineral growth. Several different processes have been put forward over the years to explain the origin of kaolinite in coal and some of its intimately associated noncoal materials such as tonsteins or flint clays. These include the introduction of detrial sediment derived from lateritic weathering of the hinterland (Loughnan, 1975), deposition and accompanying alteration of volcanic ash (e.g., Price and Duff, 1969 ), chemical or biochemical leaching of epiclastic sediment in the peat swamp (Staub and Cohen, 1978), and precipitation as a largely biogenic colloid (Moore, 1964). It has also been suggested (Spears, 1987) that kaolinite might be formed when alumina, soluble in highly acid peat waters, becomes less soluble as pH increases after burial. Perhaps one of the most significant features of the Sydney basin coal seams is the tendency, seen particularly clearly in the western and southern parts of the basin, for well-crystallized kaolinite to increase in abundance, and quartz and other clays to decrease, progressively up the seam profile. In the Lithgow seam, moreover, such a sequence appears to be repeated several times, resulting in three stacked profiles of this type with a kaolinite claystone band at the top of each. Many of the Permian coal seams of eastern Australia display a general decrease in vitrinite, and a corresponding increase in inertinite from base to top (Smyth and Cook, 1976), a feature generally explained by atmospheric oxidation associated with drying out of the peat swamp. It is also perhaps consistent with the processes leading to development of peats in a raised-bog type of environment (McCabe, 1987 ), where the relative amount of detrital epiclastic sediment would also be expected to decrease upwards through the peat deposit. Downward percolation of water through the peat in such cases was probably responsible for the removal of mobile elements from the organic matter and any detrital epiclastic sediment that was present, along with removal of any biogenic silica, from the upper levels of the peat bed. This might be expected to leave an insoluble aluminosilicate residue that, with the remaining organic matter possibly acting as a catalyst, would ultimately have crystallized as kaolinite. Some of the soluble components may also have been re-precipitated at lower levels in the peat bed, providing another explanation for some of the quartz and mixed-layer clays immediately above permeability barriers such as claystone bands, as well as in the basal parts of the seam. Complete removal of the organic fraction from the peat by such oxidation

475 would be expected to result in a residue made up of the minerals that would otherwise have been distributed within the peat bed. For seams such as the Lithgow, Bulli or Katoomba, this would provide a layer of well-crystallized kaolinite overlying a kaolinite-dominated coal ply. A process of this type, with peat becoming re-established if and when the area is again inundated by swamp waters, provides an alternative explanation to the often-cited episodes of pyroclastic activity (e.g., Price and Duff, 1969) for the formation of thin but persistent bands of kaolinite claystone in coal seams. The coal above such a horizon, moreover, would be expected to contain a greater proportion of quartz and epiclastic detrital clays, as it does in the Lithgow seam, whereas that above a kaolinized tuff band would be expected to be dominated, at least in its basal portion, by further reworked kaolinitic debris. The more vitrinite-rich coal of the Dudley seam does not display this type of profile as well as the others studied, although similar contrasts between the clay assemblage in the coal and that in the associated noncoal rocks are still present. In part this is due to the presence of an influx of montmorillonitic material during peat accumulation, and in part perhaps to a lesser degree of leaching associated with plant decay. Clays of detrital origin are also more abundant near the top of this particular seam, which suggests that an increase in detrital influx was a factor in bringing about the cessation of peat-forming conditions. A similar profile, with abundant nonkaolinite minerals near the top and base of the seam and a kaolinite-dominated assemblage in the central part, is described by Caswell (in Spears, 1987) for a coal bed in the United Kingdom, and such studies of mineral distribution appear to be warranted on a wider geographic basis and in a wider range of paleodepositional systems. CONCLUSIONS Although transitional mineral assemblages may be developed within either coal or noncoal strata near the top or the bottom of individual seams, the inherent mineral matter in the bituminous coals described above differs in a number of ways from the mixture of minerals normally found in the fine-grained noncoal rocks elsewhere in the associated sedimentary sequences. Components such as discrete quartz and feldspar particles, illite, chlorite and mixedlayer clay minerals are found in both coal and noncoal sediments and are probably of detrital origin, whereas montmorillonite and the small kaolinite spherulites in the Sydney basin apparently represent pyroclastic input into the peat swamp. Some of the clay minerals common elsewhere in the fill of both basins have been altered, however, and others removed completely by processes associated with peat accumulation. The well-crystallized kaolinite characteristically found in the coal appears to be the result of in situ leaching and reprecipitation processes within the peat swamp. Such kaolinite is more abundant in the inertinite-rich fluvial coals of'

476 the Sydney basin t h a n in the vitrinite-rich, marine-influenced coals of the Illinois basin, suggesting either t h a t atmospheric oxidation of the peat provides more favourable conditions for kaolinite development or t h a t the influence of sea water is less favourable. The upward increase in kaolinite abundance through the seam profile in the Sydney basin is also consistent with an in situ leaching process, and would be particularly favoured by peat development in a raised-bog depositional setting. Apart from the clay minerals, pyrite or siderite may also be formed in the peat bed by interaction of iron with organically derived products in the swamp or pore waters. Apatite or goyazite-group phosphate minerals can be formed in some cases as well, while a certain a m o u n t of silica, possibly remobilized from material of biogenic origin, may also be deposited in the pores of the peat bed. W i t h the exception of any permineralized peat masses represented by coal balls, however, most of the calcite, dolomite and ankerite in bituminous coals appears to form at a later stage, possibly with release of calcium from organic combination during the course of rank advance. ACKNOWLEDGEMENTS The work described in this paper was completed in part with assistance under the Australian Research Grants Scheme of the Australian Department of Science. Studies of the coals in the Illinois basin were carried out in conjunction with the Illinois State Geological Survey, and with financial support from the U.S. Environmental Protection Agency. T h a n k s are expressed to a number of individuals both at the Illinois Survey and the University of New South Wales for assistance at various times with the analyses, as well as to the various colliery companies for provision of the coal samples. The paper was reviewed by M. S m y t h and C.A. Palmer.

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479 Karr (Editor), Analytical Methods for Coal and Coal Products, Vol. III. Academic Press, New York, pp. 133-162. Scheihing, M.H., Gluskoter, H.J. and Finkelman, R.B., 1978. Interstitial networks of kaolinite within pyrite framboids in the Meigs Creek Coal of Ohio. J. Sediment Petrol., 48 (3): 723-732. Smyth, M. and Cook, A.C., 1976. Sequence in Australian coal seams. Math. Geol., 8(5): 529-547. Soong, R. and Gluskoter, H.J., 1977. Mineral matter in some New Zealand coals. New Zealand J. Sci., 20: 273-277. Spears, D.A., 1987. Mineral matter in coals, with special reference to the Pennine coalfields. In: A.C. Scott (Editor), Coal and Coal-bearing Strata - - Recent Advances. Geol. Soc. Spec. Publ., 32: 171-185. Stach, E., Mackowsky, M.Th., Teichmtiller, M., Taylor, G.H., Chandra, D. and Teichmtiller, R., 1982. Stach's Textbook of Coal Petrology, 3rd edition, Gebrtider Borntraeger, Stuttgart, 535 pp. Standards Association of Australia, 1986. Symbols for graphical representation of coal seams and associated strata. Australian Standard 2916-1986, 12 pp. Staub, J.R. and Cohen, A.D., 1978. Kaolinite enrichment beneath coals - - a modern analog, Snuggedy Swamp, South Carolina. J. Sediment Petrol., 48 (1): 203-210. Sudo, T., Shimoda, S., Yotsumoto, H. and Aita, S., 1981. Electron Micrographs of Clay Minerals. Elsevier, Amsterdam, 203 pp. Thompson, C.L., Lyons, P.C., Finkelman, R.B., Brown, F.W. and Hatcher, P.G., 1983. Microscopy of sclerotinites in the coal beds of the central part of the Appalacian coal field, U.S.A.J. Microsc., 132 (3): 267-277. Ward, C.R., 1974. Isolation of mineral matter from bituminous coal using hydrogen peroxide. Fuel, 53: 220-221. Ward, C.R., 1977. Mineral matter in the Harrisburg-Springfield (No. 5) Coal Member of the Carbondale Formation, Illinois basin. Ill. State Geol. Surv., Circ. 498, 35 pp. Ward, C.R., 1978. Mineral matter in Australian bituminous coals. Proc. Aust. Inst. Min. Metall., 267: 7-25. Ward, C.R., 1986. Review of mineral matter in coal. Aust. Coal Geol., 6: 87-110. Ward, C.R., in press. Distribution and origin of clay minerals in Australian bituminous coals. C. R. l l t h Int. Congr. Carboniferous Strat. Geol., Beijing (Sept. 1988). Ward, C.R., Bradley, G.M., Skilbeck, C.G. and Stutchbury, R., 1986. A Guide to Cored Rocks of the Sydney Basin. Earth Resources Foundation, University of Sydney, Australia, 150 pp. Williams, E.G. and Keith, M.L., 1963. Relationship between sulfur in coals and the occurrence of marine roof beds. Econ. Geol., 58: 720-729.