ABSTRACT: Petrographic Methods for the Comparison of Modern Peats and a Miocene-Age Lignite, Kalimantan, Indonesia

Two modern peat deposits and a Tertiary lignite from Kalimantan, Indonesia have been characterized on the basis of megascopic appearance and microscopic composition. In an effort to understand the relationship between the organic components of the two types of deposits, they were microscopically analyzed using the same methodology. Although both peat and coal are composed essentially of the same material (plant remains in various stages of decomposition) they are not usually analyzed using similar techniques. This paper proposes that a single technique be used for microscopic analysis of peat and coal. Peat and coal are composed of particles of plant material that vary in size from several microns to tens of centimeters. The plant material is set in a matrix of fine-grained components that vary in size from a few microns to a millimeter. At the megascopic level, the description of peat and coal is similar. The amount and the size of the larger plant fragments (in peat) or vitrain bands (in coal) are compared to the amount and the character of the matrix (or attriius); these properties are used to differentiate between peat and coal types. Additional megascopic descriptors include degree of humification for peat and brightness (luster) and color of attrital layers for coal. Unfortunately, the techniques commonly employed in the microscopic characterization of peat and coal and the terminology used in their description are dissimilar. Although the commonly used methods for peat and coal petrography have provided much useful information, the differing techniques and terminology often produce different kind3 of data and make direct comparisons difficult. Therefore, a methodology is proposed for both peat and coal that identifies botanical characteristics (commonly used in peat petrography) by using reflected-light, oil-immersion microscopy (commonly used in coal petrography). The techniques used in preparing and petrographically characterizing peat and coal samples are very different. Peat petrography is commonly performed on intact microtomed sections (15 pm thick) in transmitted-light, and botanical characteristics (such as plant-part type and species) of the organic components are identified (Figure 1; Cohen, 1974). In contrast, coal is generally examined by using reflected-light, oil-immersion microscopy on either oriented blocks or, more commonly, pellets made of coal that have been crushed to <1 mm. Discrete organic components (macerals) in coal are identified using a wide range of terminology that depends, in part, on the rank of coal; but these organic descriptors do not differentiate between the origins of the large plant-part particles. Identification of botanical components in peat and coal is desirable because it allows for compositional and genetic comparisons. The proposed new method uses reflected-light, oilimmersion microscopy on large (5 x 9 cm), oriented blocks of both peat and coal, which allows for direct comparisons of the organic components. Similar terminology can then be used that identifies parlicles of plant parts (such as leaves, roots, and bark) and other fine-grained matrix