3.2.1 Macerals

The organic fragments in coal have been classified into three groups based on major characteristics pertaining to each group. These are the vitrinite maceral group, the inertinite maceral group and the liptinite maceral group. Each is subdivided into further macerals and sub-macerals.

Woody tissue (bark, cortex and xylem) falling into anaerobic water-logged conditions, such as prevailed over much of the hot equatorial swamps of Laurasia, over time becomes gelified, thereby trapping in the protoplasmic contents of the cells and softening the tough lignitic or cellulosic cell walls. Humic gels and humic acids are leached out and precipitated in layers alongside the gelifying woody materials (logs, branches, twigs and roots) or in the empty cells of woody tissues. In time, with the pressure of burden due to burial by overlying sediments or depth through mountain-building movements with faulting or graben structures, the gelified matter becomes compressed. In so doing, it loses moisture and volatiles, and ultimately hardens into the bright shiny bands seen in the bituminous and anthracitic coals of today.

The process of gelification or vitrinitisation (derived from an original French word meaning ‘to gelify’) gave rise to the term vitrinite in bituminous or anthracitic coals, i.e. in the hard coal range of rank. In lignites, between peat and the hard coal range, similar gelified materials are included in the huminite group of macerals.

The vitrinite maceral group is grey in colour with a reflectance generally between that of associated darker liptinites and lighter inertinites over the range of rank in which the three respective macerals can be readily recognised. The group includes three subgroups and six macerals derived from different humic materials and their varying pathways of transformation within the early peats – see Table 3.1. The three main subgroups include (i) relatively pure layers or lenses of homogeneous gelified material (Fig. 3.1a, telocollinite), some of which may also be found as amorphous cell fillings or as groundmass binding other coal components (Fig. 3.2, desmocollinite); (ii) well-preserved cell walls (Fig. 3.1b, telinite); and (iii) detrital fragments in an association of detrital organic and mineral materials (vitrodetrinite).

The chemical structure of vitrinite, in simple terms, is moderately aromatic with significant aliphatic chains in the bituminous range of rank. In comparison, inertinite is more structurally ordered and aromatic while liptinite is predominantly aliphatic. On heating vitrinite, volatile matter derived from the breakdown of the aliphatic chains is released relatively easily, and the remaining aromatic carbon-rich molecular structure softens, swells and becomes porous to varying degrees, subject to the level of rank of the coal.

Plant material which accumulates in drier environments with exposure to varying levels of aerobic conditions produces a variety of different organic fragments. Under these conditions, desiccation (drying out) and oxidation takes place, plant tissues are attacked by microbial fungi and bacteria to varying degrees, and some plant material may have been subjected to fire. If taken to extremes, such conditions lead to the complete destruction of content within the cell tissues, thereby leaving the most resistant, tough, fibrous outer cell wall material, namely lignin and cellulose, for burial in the accumulating peat swamp. In general, this maceral group is dense and structurally disordered, with tightly bonded aromatic carbon molecules and few if any aliphatic chains. As a consequence, this group produces limited or no volatile matter on heating, thereby rendering it relatively difficult to ignite and to burn out due to its dense molecular structure. For these reasons, this maceral group was considered to be inert during combustion and, as such, was termed ‘inertinite’. Typically the colour is white with a reflectance lighter than either vitrinite or liptinite in low to medium rank coals.

A wide range of forms are known in the inertinite maceral group. These range from fossil wood that had been burnt in ancient fires, resulting in dry, brittle, lightweight particles of porous strongly cellular fossil charcoal called ‘fusinite’. Under the petrographic microscope this maceral has a well-preserved cellular structure and appears hard (high relief), highly reflective, and very bright white to brassy yellow in colour. It occurs in stratified layers in a seam (possibly the remains of burnt logs, tree trunks and branches) or as broken fragments dispersed within the coally matrix – see Figs 3.3 and 3.4.

Fibrous organic fragments which display clear fibrous woody cell structure but lower levels of reflectance than fusinite with colours varying from white to very pale grey and varying degrees of gelification are termed ‘semifusinite’. Those semifusinites closer in structure, colour and reflectance to fusinite have been termed ‘inert semifusinite’, whereas those closer to vitrinite in terms of semi-gelified structure, greyer colour than fusinite but higher reflectance than vitrinite (indicating higher volatile matter content in this inertinite group) are termed ‘reactive semifusinite’. Such materials are particularly prevalent in Gondwana coals, but are relatively rare in the Laurasian coals – see Fig. 3.4.

Inertinite group macerals that have been broken into fragments between 2 and 30 μm in size are termed inertodetrinite – see Fig. 3.5. Subject to their colour and reflectance, these detrital fragments may be classified as inert or reactive in technological processes, in a manner similar to that applied to semifusinites. Such inertinite fragments are typically abundant in many Gondwana coal seams, often comprising the predominant maceral in thick seams or thick bands within seams, but they are not common in the Laurasian coalfields. These components are frequently found intimately associated with fine clay or quartz grains and, as such, are believed to have been removed from their original peat swamps and redeposited with silt and sand elsewhere as a result of the turbulent movement of water. Known as ‘drift coals’ they may have arisen, for example, through flooding by rivers bursting their banks following the seasonal melting of snow in the mountains surrounding the Gondwana coal-forming basins, the latter occurring after the cessation of the Carboniferous Ice Age in the southern hemisphere.

Also included in the inertinite group of macerals are secretinite (sclero- tinite), macrinite and micrinite. These components share the same white to pale grey colour of the inertinite group but vary in origin, structure and texture. Micrinite is composed of very small granules of inertinite (< 2 μm or less) found in concentrated lenses in vitrinite or inertinite, as fillings in the cellular cavities of fibrous woody tissue, or as the matrix in a collection of fragmented particles. This maceral is considered to be the solid residue of former oily, waxy or bitumen-like material that originated from liptinitic or hydrogen-rich vitrinitic substances. Macrinite occurs as an amorphous groundmass or as rounded fragments of discrete structureless bodies greater than 30 μm in size. It is analogous to collinite of the vitrinite group and is considered to have originated from flocculated humic substances which underwent dehydration and partial oxidation processes in an early stage during peatification due to temporarily lowered groundwater levels. Secretinite (previously known as sclerotinite) is composed of round or oval discrete bodies of fungal or cellular origin with well-preserved botanical structures in the centre (pores or shrinkage cracks). A summary of the characteristic forms in the inertinite group of macerals is presented in Table 3.2.

Apart from the fibrous, woody and fleshy tissues of trees and plants which gave rise to the macerals and maceral groups described above, another part of ancient plants needs to be considered. Known as the Liptinite group of macerals (previously the exinite group), these components originate from specialised hydrogen, waxy and oil-rich plant materials. Cutinite represents the waxy cuticles coating leaves and young shoots while sporinite represents the external coats (exines) of pollens and spores. This group also includes resin (Resinite) that exuded from the from the bark of ancient gymnospermous-like trees and algal bodies (Alginite), which occur either in concentrated layers or as discrete bodies within the coally matrix or in associated shales.

Liptinite macerals have characteristically low reflectances. They are brown to purple in colour in sub-bituminous and low rank bituminous coals (bituminous C and D) but range from grey to pale grey in the mid bituminous rank, after which they merge with vitrinite and disappear. The group is predominantly aliphatic in molecular structure and low in density. On heating, liptinite components in the bituminous C and D ranks of coal emit high proportions of H-enriched volatile matter followed by light and heavy hydrocarbons (bitumen, tars and waxes). Once emitted, little or no solid aromatic carbon remains. The macerals in the liptinite group are listed in Table 3.3.

3.2.2 Minerals

Minerals occur in various forms in coal. Minerals that were deposited at the same time as peat formation are termed syngenetic minerals, whereas those that formed after peat formation and at any time during coalification are epigenetic minerals, The latter minerals are usually derived from groundwater passing through cracks and fissures in the hardening or hardened coal seams. When saturated, the ions and elements in the ground water precipitate out and minerals form, taking the shape of the cracks or fissures in which they are deposited. Similarly, water passing through the xylem, phloem or cortical cells (supportive and ‘feeding’ cellular tissue) in ancient tree trunks also led to the precipitation of minerals in those cellular cavities, thereby ‘locking’ the minerals into those cell structures.

For practical purposes minerals may also be described as inherent, included or excluded. Inherent refers to minerals or elements bound to the organic cellular structure in coal (this includes trace elements which may have been taken up by the plant during growth and adsorbed into the plant’s tissues). Included mineral matter is that which forms a discrete mineral grain within the coally matrix, for example mineral grains of quartz and clay embedded in bands of vitrinite or inertinite in the manner of currants within a bun. Excluded mineral matter is that which is external to the coal or coal seam, namely particles of sandstone or shale which may have been derived from roof or floor rocks above or below the coal seam or from rock partings within the coal seam.

Within coal, the form in which mineral matter is found varies from extensive mineralised layers precipitated within the organic matrix to nodules, smaller grains, tiny crystals, fissure fillings, cell cavity fillings, or rock fragments which may be finely distributed throughout the coally matrix.

Within the syngenetic mineral category, those forms washed by water or wind blown into the peat swamp are predominantly clays and quartz and to a lesser extent feldspar, apatite, muscovite and related minerals. Those forms that precipitated in situ from colloidal, chemical and biochemical processes include pyrite, carbonate minerals and colloidal clays. Both groups are found intimately embedded within the organic matrix and are generally difficult to liberate during beneficiation. The epigenetic minerals that were precipitated during the later stages of coalification are generally found either in cleats and fissures or intruding into and disrupting the original layers of organic material. These include carbonate minerals (calcite, dolomite, ankerite, siderite), pyrite, marcasite and some clays. These may be coarsely crystalline or in the shape of nodules; in some cases minerals have been found to replace, transform or encapsulate older minerals. Carbonates that precipitate out in the cellular cavities of old tree trunks lying horizontally in peat have been found to be the hardest products known in coal. Called ‘hard fusite’ and comprising the fibrous cell walls (fusite) with calcite- or dolomite-filled cell cavities, such coally lithotypes have been found to wear mining picks away at extremely fast rates and be totally resistant to breakage in mills.

The most common minerals found in coal include the following:

Clay is overall the most abundant and widely spread mineral in coal. In the Gondwana coals, it often comprises up to 80% of the minerals present. It occurs as abundant small grains (from 1 to 2 μm up to 20 μm), is widely and finely distributed in the coally matrix, and is occasionally found as narrow lenses within the organic matrix or as thicker layers containing organic fragments. Some clay forms found filling cell cavities are considered to have formed by flocculation in situ and in many cases these fluoresce under ultraviolet light. Kaolinite is the most common of the clay minerals, although illite is also known to occur in many coalfields. When seen under the petrographic microscope, clays appear orange–brown to brown and black with an uneven granular structure.

Quartz is common in some seams, ranging in size from 5 to 50 μm. Often difficult to identify in oil immersion under the petrographic microscope, quartz grains have the appearance of glass. For these reasons, they are best viewed using other techniques (CCSEM and QEMSCAN). The form and shape of the grains range from rounded to angular, subject to their provenance and sedimentary history. Some grains bear rods of rutile within them.

Pyrite and marcasite, the most common iron sulphide minerals, are found to varying extents in all coals. Gondwana coals are generally low in iron sulphide minerals with minor exceptions, whereas these minerals are more common in Laurasian coals. Seen under the petrographic microscope they are bright yellow (up to 50 times brighter than the surrounding coal) and hard, generally exhibiting high relief. Pyritic minerals range from minute grains (1–2 μm) to large nodules (1 cm or more), as clusters of grains (framboids), and as lenses spread along strata within a seam. They are also frequently found as epigenetic minerals in cleats and fissures and as infillings in the cell cavities of massive woody fibrous bands of fusinite or semifusinites – see Fig. 3.6.

Carbonate minerals (calcite, dolomite, ankerite and siderite) are found both as syngenetic nodules within the organic matrix and in epigenetic forms as lenses intruded into early peats, as cell fillings, or as cleat deposits.

Other less frequent minerals include, but are not exclusive to, the following: feldspar, apatite, barite, gypsum, sphalerite and zircon. Radioactive forms of the latter mineral have been found in southern Africa.

Minerals in coal are important in the processes to which coal is exposed. Quartz is highly abrasive and is responsible for rapid wear in mills and ducts; clays are difficult to remove in beneficiation because of their very fine size, intimate intergrowth within the coally matrix, and often abundant distribution. Pyrite in cleat form is easier to remove due to ease of liberation during crushing relative to the forms of fine crystals embedded within organic matrices. Iron sulphide minerals emit SO₂, a greenhouse gas, on heating. Carbonates are brittle, emit CO₂ at low temperatures on heating, and play a role in reducing ash fusion temperatures when included in coal destined for a boiler. Collectively, minerals reduce heat when used for power generation and, subject to conditions in the boiler, certain forms can lead to slagging and fouling. They also reduce coking capacity if present in quantities higher than 14% ad.

3.3.1 Lithotypes and microlithotypes

In some cases, ancient peat-forming plant assemblages grew and died in situ, accumulating vegetation in the same swamp over time, fluctuating only in water level and geochemical environment over that time. Known as ‘autochthonous’ or in situ coals, these exhibit banded or stratified layers of vitrinite-rich, inertinite-rich or mixed organic matter contents. In hand specimen, these coals exhibit either bright (vitrinite-rich or vitrain) or dull (inertinite-rich or durain) bands or, in the case of multiple alternating narrow lenses of vitrinite and inertinite-rich layers, mixed banded coals. Known as lithotypes, the description ‘bright, dull or banded’ is a reflection of smaller microscopic bands of associated macerals and minerals. The latter are known as microlithotypes, and are defined in 50 μm bands of thickness.

Under conditions where developing peat swamps have been disrupted and broken up, either by flooding rivers, melting snows or incursions of the sea, the fragmented organic materials and accompanying minerals are carried in suspension to different locations and redeposited in the process of sedimentation in a manner similar to a river carrying sediments of sand and silt, which are deposited when the river meets a larger body of water. Suspended materials sink to the bottom, heaviest first, lightest last. In this manner, larger organic fragments and mineral grains are deposited before finer fragments in such a body of water. Coals formed in this manner are known as ‘allochthanous’ or drift coals, i.e. the fragmented materials have been removed, transported and deposited in a location different from their origin and have accumulated there to form the seam that is seen today. Under these conditions, a considerable quantity of sand or silt (fine clay) may accompany such organic accumulations. Subject to the type of sediment deposited, a further wide range of coals leading to sedimentary rock types may form. Such sediments are typically massive, exhibiting little or no banding in their lithotypes.

Subject to the quantity of organic matter relative to mineral matter in the mixture, a number of different ‘coally’ sediments may be classified. If the layer contains more maceral than mineral content, i.e. < 50% by mass of ash ad, the deposit would be classified as a coal and categorised into different grades subject to ash content i.e. from low-ash (< 10% ash) to high-ash (30–50% ash) categories of coal.

If, however, the inorganic mineral matter is more prolific in the mix than the organic matter, then, subject to the nature, size and content of minerals contained in the rocks, they may be termed dark grey to black carbonaceous shales, mudstones or siltstones, the degree of colour indicating the amount of organic matter associated within them. In some coal sequences, as for example the Coal Zones in some the Upper Permian Gondwana coalfields, rocks of this kind grade frequently from mineral-rich coally sediments such as siltstones and shales into true coal seams and vice versa over very short vertical distances (millimetres and centimetres). Known colloquially as ‘bar-code’ coals (due to their similarity to bar codes in retail stores), these seams are difficult to sample, analyse and evaluate for marketing purposes. Sampled too widely (incorporating multiple bands), the samples would be categorised as ‘rocks’, given average ash contents of well over 50% ad, but sampled in narrow bands, they are categorised as low to medium ash coals alternating with interbedded carbonaceous shales. It is these complex coally sediments with overall ash contents in excess of 50% that are included in the resources of some coal mines in a number of Gondwana countries. The run-of-mine coals from such seams are washed in various beneficiation processes to yield 12–15% of low-ash coals for high-value markets, a moderate yield of ‘middlings’ products with coals of 35–45% ash on average (including some 60–70% ash products) which are sent to local power stations, and a high yield of very high-ash material which is discarded. This is in stark contrast to the low-ash products produced from the typically low-ash, thick, vitrinite-rich bright seams encountered in many coalfields in the USA and EU (Germany, UK, and Russia).

In areas of relatively wide open water within a coal-swamp, other coally sediments containing macerals and minerals are laid down in the coal-to-sediment continuum. Pollens and spores and leaves that fall during autumn seasons are blown by wind and carried by water into that environment. Where stagnant conditions prevail, algal communities flourish, some of which die, sinking to the bottom to lie amongst the pollens, spores, leaves and fine clay or silt-rich sediments. When found in sufficient quantities, such liptinitic-rich sediments are known as sapropelic coals (deeper water coals). Due to the high ash content of such sediments, they are often overlooked in geological borehole core assessments but, if analysed, they may be found to yield high volatile matter content and valuable oily, waxy and bituminous- like substances, their primary drawback being the relatively high ash content. In some seams, bands of such sapropelic coals are sufficiently thick and organic-rich to warrant extraction for commercial coal-to-liquid purposes. The high volatile matter content and ease of ignition of such coals would explain the conundrum encountered in some mines where it is claimed that the ‘shales or mudstones’ exhibit faster rates of self-heating leading to spontaneous combustion than the associated coals themselves.

Where the spores, pollens and leaf cuticles concentrate in such sapropelic coals, these are specifically known as cannel coals, whereas those specifically enriched in algal matter are termed boghead coals. The inclusion of marine organisms leads to the development of oil shales in the natural sedimentary continuum.

The massive nature of the open water sapropelic coally sediments is in stark contrast to that found in coals accumulating on or close to land which supports terrestrial plants, namely, humic coals. Humic coals are generally stratified with bright bands (vitrinite-rich), often alternating with, or grading into, dull bands. Where bright and dull bands alternate rapidly in narrow layers they are known as banded coals. Sapropelic coals have a characteristic satin lustre, brown streak, and a specific form of breakage (conchoidal, in a circular or curved manner).

3.3.2 Abnormal conditions found in coal

A characteristic not often considered when evaluating the value and likely technological performance of a coal, and which is easily identified under a coal petrographic microscope, is the condition of a coal. This has become important in recent years especially in regions where

All factors listed above have deleterious impacts on the utilisation of coal.

No standards exist for such petrographic analyses, but a number of coal specialists have developed personal classification systems for the identification of such conditions and these are used on a routine basis for commercial application.

3.4.1 The concept of rank

During the process of biochemical degradation (early peat accumulation), plant material may either be submerged in water with restricted oxygen supply, which results in the production of humic acids and partial or full gelification of the woody plant tissue (peatification), or the plant material may undergo varying levels of oxidation and desiccation when exposed to air to varying degrees. In some cases, putrefaction (fermentation) occurs when plant material is subjected to reducing conditions. This results in anaerobic bacteria consuming the oxygen of organic substances, thereby transforming them into hydrogen-rich products. For example, the fermentation of cellulose leads to the production of hydrogen, methane, acetic acid, carbon dioxide and butyric acid. The coals which form from these processes are generally rich in extractable oils and nitrogen.

Once the accumulating plant matter is buried at depths below 1 m, biochemical degradation ceases to function and geochemical coalification takes place. During coalification, chemico-physical changes occur over time and under conditions of higher temperature and pressure, thereby giving rise to the process of maturation and metamorphosis (changes) in the various plant fragments (macerals). This results in the transformation of the original peat swamp plant materials through progressive stages of brown coal (lignite), sub-bituminous and bituminous coals to anthracite and meta-anthracite.The end-point of this continuum is graphite, which occurs only rarely in nature as a result of intense pressure and temperature in some coalfields of the world. The level to which a coal has reached in this coalification series is termed its rank or level of maturity. Chapter 2 describes this process in further detail.

The earlier process of peatification which resulted in the breakdown of plant matter into organic fragments in the original peat swamp (thereby controlling the proportions of maceral forms) is entirely independent of the maturing process or increase in rank.

The maceral groups, once formed, undergo different and varying levels of progressive physical, chemical and optical (microscopic) changes with increasing rank but their proportions and shape, with some exceptions, remain the same. In general, overall changes with rank are considered to take the form of loss of moisture and volatile matter with the consequent increase in carbon content and aromaticity. However, this is not necessarily the case in all coalfields.

As the inert forms of inertinite group macerals are highly aromatic from source in the peat stage, and therefore have little or no volatile matter to release, there is no discernible change with increasing rank. Their properties – aromaticity, carbon content, size and shape – remain the same through the entire rank process. Reactive inertinite macerals, on the other hand, change slightly with a limited loss of volatile matter and minor colour changes. Liptinite, in stark contrast, changes rapidly with increasing rank, first emitting high quantities of volatile matter from its abundant aliphatic chains in the early bituminous ranges of rank (bituminous D), followed by heavier hydrocarbons, waxes and tarry substances in early mid bituminous levels of rank (bituminous C). Thereafter, in the later mid bituminous range of rank (bituminous B to A), these macerals completely cease to exist, disappearing from view and merging into vitrinite. The use of liptinite is therefore as limited as inertinite in assessing the rank of coal when volatile matter or carbon content are used as rank-defining parameters.

In the case of the vitrinite maceral group, however, the loss of volatile matter and increase in carbon content has been shown to change in constant and linear manner with the increase of rank. Thus, as the coals of the Carboniferous period in Laurasia possess high vitrinite content (> 75%), volatile matter and carbon content have long been considered the definitive parameters indicative of the level of rank reached by a coal. In the Gondwana coalfields, however, where vitrinite varies from 0% to 75%, with an average of 25% for seams in that region, and as the inertinite group macerals with low and variable volatile matter contents dominate the coals, this is not necessarily the case.

In short, volatile matter and carbon content in Gondwana coals are not indicative of the true rank or maturity of a coal (unless containing high vitrinite content). In this coal-forming region, volatile matter and carbon content simply present values to which highly variable and unknown maceral proportions (some high and others low in volatile and carbon content), rank and condition all contribute. Factors such as heat effect by intrusive igneous sills and dykes, and weathering or oxidation of shallow seams superimpose further complications upon volatile matter and carbon contents, thereby once again disaffecting a true interpretation of those analytical parameters. It is possible, for example, to have a coal with a volatile matter content of 16% _ad which may appear to be a lean low volatile bituminous coal (or bituminous A), but in fact, based upon true determination of rank, is found to be a sub-bituminous coal with high proportions of oxidised and inert inertinite macerals. Such ‘mis’interpretations have serious practical and technological implications for such coals.

During the 1970s the need to refine a method for the true identification of rank on an international basis was initiated by the Coal Working Party of the UN ECE, given that the international trade in coal was expanding at that time. Results of investigations proved that increasing rank changed linearly in parallel with the proportion of light emanating from the surface of vitrinite particles. In the lignitic and sub-bituminous level of rank, vitrinite reflects medium to dark grey relative to normal white light. With increasing levels of rank, vitrinite passes through changing levels of grey, becoming increasingly lighter and whiter as it reaches its ultimate level of bright white in the meta-anthracite and graphite categories. Thus, based upon the regularity of this change, the reflectance of vitrinite values as defined in ISO 11760 (2005) was drawn up and designated as the official international method for the definition of the rank in the hard or black coal range (bituminous and anthracite), irrespective of the coal’s origin or maceral composition. Rank in the lignitic or sub-bituminous range of rank remains defined according to moisture level or heat content.

The reflectance of vitrinite is now reported as the conventional method of determination of rank, using the mean value of the quantity of light as statistically measured petrographically on the surface of specific number of vitrinite particles per sample. The value is reported as a percentage relative to white light, and either as a random value (Vr% random) or maximum value (Vm% maximum). The distribution of the values for a specific coal is portrayed as a histogram, indicating the range of values seen for a specific coal. Standard deviation values indicate the mode of distribution (single seam or blend with coals of several ranks). The internationally accepted categories of rank as defined by vitrinite reflectance (mean random) are presented in Table 2.6 in Chapter 2.

3.4.2 The impact of rank and maceral composition on coal utilisation

The rank and maceral composition of a coal have direct and relevant significance to the technological use of that coal. This includes five forms of reactivity in commercial operations, including reactivity to combustion, gasification, reduction in the metallurgical industry, hydrogenation/liquefaction and self-heating leading to spontaneous combustion. As these factors will be discussed in further chapters, only a summary of two areas of utilisation will be provided here.

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