Coal Petrography

the description of the components of coal as studied macroscopically and in thin and polished sections under the microscope. The founders of coal petrography in the USSR were Iu. A. Zhemchuzhnikov and M. D. Zalesskii. Outside the USSR, R. Thiessen of the USA and M. Stopes of Great Britain made seminal contributions to the development of the field.

As its methods were refined and its range of problems grew, coal petrography became concerned with the composition, structure, and origin of coals. In other words, it developed into the field of coal petrology. The principal task of coal petrology is the study of the elementary components of the organic matter of coal beds. These components are the remains of plants, which, in the process of biochemical decomposition into the peat stage of coal formation, lost or preserved to some extent their form and structure. A distinction is made here between the tissue elements and the matrix, which, taken together, constitute the components of coal. Also, macrocomponents, such as vitrain and fusain, are distinguished from microcomponents, such as spores and cuticles.

Coal petrography employs various methods of investigation. In methods of optical microscopy, for example, the specimen may be viewed in air or in immersion; both transmitted-light and reflected-light methods are used, and the light may be ordinary, polarized, or ultraviolet. Specimens may be separated into groups of components of similar density in heavy liquids (mixtures, for example, of C₆H₆, CC1₄, or CHBr₃). Maceration methods are used to identify and then study stable components. Etching with strong oxidizers is employed to reveal the cryptographic structure of coal. Thermal and chemical analyses may be conducted of components or groups of components of coal samples. Quantitative methods are of great importance. They include the determination of the reflectance, refractive index, and hardness of vitrinite; the establishment of the color and brightness of luminescence of liptinite; and the calculation of the content of components in average bed samples and in chunks of coal.

From 14 to 40 petrographic components have been identified in coal by different investigators. Depending on the source material and the conditions of its transformation in the peat stage of coal formation, the micropetrographic components are classified into three basic groups (vitrinite, or gelinite; fusinite; and liptinite) and two intermediate groups (weakly gelified and weakly fusinized).

The composition of the source plants and the quantitative relations between micropetrographic components determine the genetic types of coal, which are characterized by definite chemical and technological properties within each stage of coalification.