Heterogeneity in Coal
The sum character of any coal may be considered the result of the collective operation of physical, chemical, and biological processes (Schopf, 1948a) and must include both contemporaneous and post-depositional changes. Differences in the history of formation of coal are reflected in its present lithology and properties. The coal inherits certain initial attributes due to plant morphology and environment. It acquires additional attributes resulting from diagenetic and metamorphic processes. Factors operating in the growth and depositional environment include depth of water, temperature, boundary conditions, chemical character of the depositional medium, and rate of burial. These factors largely determine the growth and distribution of the biological population, its death, degree, and manner of decomposition, and its accumulation and preservation. This combination of physical, chemical, and biological processes determines the initial attributes of the accumulating vegetable material.
Subsequent to deposition and prior to consolidation, certain diagenetic processes, which are largely biochemical, alter the vegetable accumulation to peat and superimpose additional attributes on the coalified material.
In the final stage of coalification, metamorphic processes become more and more intensive and may indeed proceed so far as to obliterate the early character of the coal.
The inherited and acquired attributes therefore produce a physical and chemical heterogeneity in coal which is called its constitution. Two general categories of variation in coal constitution are now recognized as the result of efforts to classify coals on the basis of their physical and chemical properties.
In the first category are rank variations which are established on the basis of carefully selected chemical criteria. The rank of coal is its stage of coalification in the series peat, lignite, bituminous coal, and anthracite coal. It depends largely on the extent to which metamorphic processes have affected the inherited attributes of a coal. An increase in rank is marked by the relative decrease of such constituents as moisture, oxygen, and volatile matter and the relative increase in carbon. Transitions in physical properties are also evident. The rank classification adopted by the American Society for Testing Materials (1938), as illustrated in Table 1, is in current usage. It is based on proximate analysis and calorific determinations calculated to the mineral-matter-free basis according to the Parr (1928) formulas. Physical criteria for differentiation also exist but are more difficult in application. McCabe (1937), for example, has shown that the angle of polarization and the index of refraction vary systematically with increase in rank.
Table 1--Modified A.S.T.M. classification by rank (A.S.T.M., 1938, p. 2)
|Class||Group||Limits of fixed carbon or B.t.u. on mineral-matter-free basis||Requisite
|1. Metanthracite||Dry F.C. 98 percent or more|
|2. Anthracite||Dry F.C. 92 percent or more and less than 98 percent||Nonagglomerating|
|3. Semianthracite||Dry F.C. 86 percent or more and less than 92 percent|
|1. Low-volatile bituminous||Dry F.C. 78 percent or more and less than 86 percent|
|2. Medium-volatile bituminous||Dry F. C. 69 percent or more and less than 78 percent|
|3. High-volatile A bituminous||Dry F. C. less than 69 percent* and moist B.t.u.** 14,000 or more|
|4. High-volatile B bituminous||Moist 13,000 B.t.u. or more but less than 14,000|
|5. High-volatile C bituminous||Moist B.t.u. 11,000 or more but less than 13,000||Either agglomerating or nonweathering|
|1. Subbituminous A coal||Moist B.t.u. 11,000 or more but less than 13,000||Both weathering and nonagglomerating|
|2. Subbituminous B coal||Moist B.t.u. 9,500 or more but less than 11,000|
|3. Subbituminous C coal||Moist B.t.u. 8,300 or more but less than 9,500|
|1. Lignite||Moist B.t.u. less than 8,300||Consolidated|
|2. Brown coal||Moist B.t.u. less than 8,300||Unconsolidated|
|* Coals having 69 percent or more fixed carbon (F.C.) on the dry, mineral-matter-free basis are classified according to fixed carbon regardless of B.t.u.|
|**Moist B.t.u. refers to coal having its natural bed moisture but not including visible water on the surface of the coal.|
Of a more fundamental nature are those variations caused by differences in the physical constitution of coal. These are called type variations and are most commonly determined by petrographic methods. Type variations are due to the properties acquired through the interplay of plant morphology, environment, and diagenetic processes. The importance of type variation declines with increasing rank since effects of progressive metamorphism tend to obliterate the original characteristics. Anthracites of a similar rank therefore display great similarity in appearance.
However, most low-rank coals are banded in appearance. Such coals consist of fine laminae and thicker bands of bright material alternating with duller material. The characterization of these banded ingredients has been the basis for classification according to type.
Dawson (1859) observed the banded materials megascopically and concluded that they were separate entities. Somewhat later, Muck (1881) recognized that there were at least three distinctly different comnonents which he called "elanzkohle," "mattkohle," and "faserkohle" (bright coal, dull coal, and mineral charcoal).
The application of the microscope to the investigation of coal is first recorded in the work of Witham (1833) and Hutton (1833), who demonstrated the vegetable origin of bituminous coals beyond question and supplied many data on the kinds of plants and plant structures preserved in coal. These early observations provided the background for modern research on the physical constitution of coal up to the time of the microscopic studies of White and Thiessen (1914).
Two different classes of petrograohic entities have been recognized in coal. Those based on recognition of plant parts and pieces as well as some kinds of decomposition products, have been called "phyterals" by Cady (1942, p. 347). The identity of a phyteral does not change throughout the metamorphic stages of coal formation although its chemical and physical composition vary in a pronounced manner. Phyteral content is therefore fixed at the beginning of coalification. However, recognition of a phyteral usually becomes more difficult with advanced metamorphism.
Entities based on the recognition of physical and chemical similarities were called "macerals" by Stopes (1935). They are identified on the basis of similarity in composition, as are minerals. A given phyteral may be represented by several kinds of macerals and a given maceral may be constituted from different types of phyterals.
Although the phyteral and maceral concepts were not expressed in the classifications of Stopes (1919) and Thiessen (1920), the implications were nevertheless present.
In her original classification, Stopes (1919) recognized four ingredients which she named vitrain, clarain, durain, and fusain. Definition and identification were based upon the properties of the hand specimen, supplemented by microscopic observations. The terminology was widely used in Europe although controversy was aroused by Stopes' observations concerning the nature and origin of vitrain. In 1935, Stopes expanded her original classification to reclassify vitrain, clarain, fusain, and durain as coal types and proposed an additional series of names to characterize the macerals or organic units of the coal types. This classification was adopted by the Second International Conference on Carboniferous Stratigraphy at Heerlen in 1935. All coals were regarded as aggregates of one or more of the primary types.
Vitrain--Vitrain occurs in thin horizontal bands up to 20 mm thick and has a brilliant glossy luster. It is microscopically structureless, homogeneous, and breaks with a conchoidal fracture. Vitrain was originally described as translucent in thin sections and microscopically structureless. In response to the controversy regarding microscopic structure, the term was expanded to include eu-vitrain (or structureless vitrain) and pro-vitrain (which shows structure). The maceral of vitrain is vitrinite. It is subdivided into collinite (vitrinite devoid of structure) and tellinite (vitrinite showing structure on polishing, etching, or in thin sectioning). Stopes also proposed that each recognizable plant tissue, organ, or secretion be given a distinct name within the general category vitrinite. Thus corky tissue in vitrain is called suberinite; material thought to be resin, resinite; exine material, exinite; cuticular material, cutinite.
Clarain--Clarain is a bright striated coal with a silky luster, not as brilliant or homogeneous as vitrain, lacking in conchoidal fracture, and consisting of thin bands stratified parallel to the bedding plane. It is predominantly translucent in thin section and may be composed of a variety of macerals of small size and concentrated to a varying degree. Vitrinite plus micronite (an opaque maceral) and fusinite (the maceral of fusain) are the usual constituents.
Durain--Durain is a hard, compact, dull coal which is megascopically structureless and gray to dull black in color. It is largely opaque in thin section since the predominant maceral is micronite. However, other macerals may be present in minor amount.
Fusain--Fusain consists of irregular wedges lying on bedding planes at various angles. It is fibrous and dull in appearance and consists of a porous, friable material resembling charcoal which breaks down to a fine dust. It is cellular and opaque in thin section and composed of the maceral fusinite.
Boghead and cannel coal--In addition to the banded coals, the nonbanded varieties, cannel and boghead, were recognized. Nonbanded coals contain essentially no vitrain but are composed of clarain and durain microdebris with large quantities of spore exines, pollen, and oil algae. If the algal content is low, the coal is a cannel; if high, it is a boghead. Megascopically, they are clean, compact blocks of massive structure and fine-grained texture. Usually they are dark gray to black, have a greasy luster, and a marked conchoidal fracture.
Thiessen (1920) considered coal to be composed of two visibly different major components present in various types of coal in varying proportions. He named these components anthraxylon and attritus. He accepted the well-established term fusain for designating the third (and minor) component. Thiessen's classification is held to be genetic in origin since the microscopically differentiated components (petrographic components) can be related to plant morphologic units now called phyterals.
Anthraxylon--Anthraxylon was described as relatively simple in structure and essentially homogeneous in appearance. It was recognized to be the fairly well-preserved cellular tissues of stems, branches, twigs, roots, sporangia, and leaves which survived plant decay in the early stages of coal formation. Anthraxylon bands are identified megascopically by their bright luster, black color, brittleness, and smooth fracture. In thin section anthraxylon appears in bright-orange, red, or brownish bands and often exhibits well-preserved cellular structure.
Attritus--Attritus was defined as a mixture of macerated plant debris, finely divided during the process of plant decay and subsequently coalified. Its composition is not simple and thin sections show that it is composed of many ingredients. Most attritus is translucent and exhibits the orange, red, and brown colors of anthraxylon although some of it is nearly or completely opaque at standard thicknesses of 10 microns. The opaque ingredients are called opaque attritus whereas the translucent material is translucent attritus. Translucent attritus includes humic degradation matter, a term applied to the cellulosic or lignocellulosic fragments of wood, phloem, cortex, and leaves; resin bodies, the dark-yellow to light-brown globules of resin which were once the cell contents of xylem and leaves; spore and pollen exines, the brilliant-orange to yellow cases of spores and pollen which have become flattened during coalification and appear as flattened rings in section; and cuticle, the bright yellow-golden bands with serrated edges which were the former coverings of leaves and stems. Megascopically, attrital coal is characterized by its dull color and striated appearance when interbanded with fine shreds of anthraxylon. It breaks irregularly into large fragments.
Fusain--The fusain of Thiessen's classification has the same meaning as it did in that of Stopes. It is a minor component of most coals and is characterized by its friability and softness. It resembles charred wood and is sometimes referred to as mineral charcoal. In thin section it is distinguished by its opaqueness and cellular structure.
Thiessen's system of classifying coal into types after thin section analysis is based on the aggregate character of the coal in terms of limiting quantities of the petrographic components or ingredients. It should be recognized that the term "coal type" as used by the Bureau of Mines differs from the "rock or coal type" of Stopes in which all coals are regarded as aggregates of one or more primary coal types. The Bureau of Mines (Table 2) now recognizes five types of coal and has established the critical limits for each type as recently stated by Parks and O'Donnell (1948, p. 537).
Table 2--Type classification of coals (U.S. Bureau of Mines) (Parks and O'Donnell, 1948, p. 537)
|Type||Critical amounts of components|
|Cannel coal||Less than 5 percent anthraxylon and predominantly transluscent attritus with little or no oil algae.|
|Boghead coal||Less than 5 percent anthraxylon and the transluscent attritus predominantly oil algae.|
|Bright coal||More than 5 percent anthraxylon and less than 20 percent opaque attritus.|
|Semisplint coal||More than 5 percent anthraxylon and 20 to 30 percent opaque attritus.|
|Splint coal||More than 5 percent anthraxylon and more than 30 percent opaque attritus.|
Despite claims to the contrary, it seems that only a few of the common petrographic terms used in either classification have either a precise botanical or compositional implication. Some of the maceral terms clearly denote parts of plants while others infer a chemical relation which can scarcely be determined petrographically. The terms opaque attritus and fusain as used by Thiessen are not botanical in origin. In addition, much uncertainty has existed concerning the significance of one set of names in terms of the other. Table 3 is an effort to show the correlation between the two different terminologies as suggested by Raistrick and Marshall (1939, p. 271). Several authors, in recent years, have attempted to resolve and clarify these differences and to develop the history of the nomenclature. Among these are Roos (1937), Cady (1945, pp. 86-102), and Raistrick and Marshall (1939, p. 178-205). Such efforts may be futile since there is still basic disagreement as to the meaning of the term "coal type."
Table 3--Nomenclature of coal petrology (Raistrick and Marshall, 1939, p. 271). British and German terminology as recommended by the International Committee at Heerlen, 1935. Corresponding American terms are those used by U.S. Bureau of Mines.
|Macroscopic character of the coal||British nomenclature||German nomenclature||American nomenclature|
|Rock types||Macerals (constituents)||Streifenarten||Gefuegebestandteile||Coal types||Constituents|
|Uniform brilliant black bands||Vitrain||Vitrinite: translucent in thin section; cellular structure may or may not be well preserved:
||Vitrit||Vitrinit||Anthraxylon. Term used to include the uniform brilliant bands (or their counterparts) in coals of all ages|
|Charcoal-like layers and fragments which readily soil the fingers||Fusain||Fusinite: cell structure well preserved. Cell walls opaque; cell cavities either empty or occupied by mineral matter||Fusit||Fusinit|
|Bright coal: clearly laminated; composed of innumerable brilliant fragments and bands with some duller material||Clarain||Containing (translucent orange or yellow in thin section):
Vitrinite; Resinite--resin bodies; Exinite--which includes
|Clarit||Bright coal||Containing: Anthraxylon; spores; cuticles; resins; etc., together with opaque and semitranslucent attritus; and fusain|
|Micrinite--granular opaque matter||Micrinit|
|Dull coal: dull and nonreflecting in the hand specimen; lamination poor or absent||Durain||Containing:
|Durit||Splint coal||Very largely opaque and semi-translucent attritus with spores, cuticles, resins, and a little anthraxylon|
In general, it may be said that both classifications depend to a certain extent on maceral and phyteral criteria. However, if the theoretical limitations of each are understood, there is no essential reason why either one cannot be used for certain types of petrographic analysis.
In the discussion and analyses of Kansas coals which follow, the phyteral classification of Thiessen and the Bureau of Mines will be used exclusively. The scheme is simple and, in addition, numerous petrographic analyses on a wide variety of coals have been made by the Bureau of Mines. These petrographic analyses provide a ready basis for comparison with this study.
Kansas Geological Survey, Geology
Placed on web November 2005; originally published May 1953.
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