Macerals microscopy

The contrast observed in electron microscopy depends largely on differences in atomic composition of the specimen. In this respect the coal macerals do not differ greatly from one another, and so very high contrast between them would not be expected and is not in fact obtained. Despite attempts to overcome this difficulty low contrast remains a serious limiting factor in observing structure. It is possible that staining techniques, as used in electron microscopy of biological specimens, may prove useful. 

With increasing rank, the differences between macerals become progressively smaller in electron as in light microscopy. Exinite in particular becomes difficult to distinguish in medium volatile coals, and could not be positively identified in the electron micrographs of low volatile bituminous coals so far examined. Similarly, the laminate structure of vitrinite described above could not be observed in low volatile bituminous coals. 

This volume covers a wide range of fundamental topics in coal maceral science that varies from the biological origin of macerals to their chemical reactivity. Several chapters report novel applications of instrumental techniques for maceral characterization. These new approaches include solid l3C NMR, electron spin resonance, IR spectroscopy, fluorescence microscopy, and mass spectrometry. A recently developed method for maceral separation is also presented many of the new instrumental approaches have been applied to macerals separated by this new method. The contributions in this volume present a sampling of the new directions being taken in the study of coal macerals to further our knowledge of coal petrology and coal chemistry. 

In the first half of this introductory chapter the maceral concept has been discussed and the main maceral groups and their important maceral types described. Emphasis has been placed on in situ characterization techniques which rely mostly on microscopy. The rest of this chapter will examine other techniques used for chemical characterization and examine the reactivity of coal macerals in thermal processes. The availability of separated maceral concentrates was a necessary component of the studies which will be described. 

The use of fluorescence microscopy to characterize coal macerals is a successful recent innovation. 

Compared to conventional white-light analysis, fluorescence microscopy reveals a greater number and variety of macerals, as well as, characteristic textures and structures. Although the spectra of fluorescent macerals are broad-peaked and, at this time, not suitable for chemical structure analysis, they are characteristic of maceral types and rank. 

Transmission electron microscopy has been used to determine the concentration of organic sulfur in coal. Because the electron beam can be focused to a fine spot on the coal specimen, the variation of organic sulfur in the macerals can be measured over distances as short as 1 pm or less. Thus, spatial variation can be determined within a particular maceral and across maceral boundaries in thinned coal foils. The excellent spatial resolution has also permitted us to measure the organic sulfur concentration in close proximity to sulfides we find that the organic sulfur concentration is constant over the maceral to within 1 pm of the pyrite. 

Transmission electron microscopy has been used to measure the spatial variation of organic sulfur within and between macerals. The spatial resolution of the measurements is superior to any other reported technique. 

Micropetrography evaluates the coal components ascertainable by microscopy. Figure 4 shows an extract of the results obtained from the combined maceral-microlithotype analysis after the International Handbook of Coal Petrography of the 15 brown coal lithotypes. 

The size and distribution of pores and the size, distribution, and identity of minerals in coal specimens from an eastern Kentucky splint coal and the Illinois No. 6 coal seam were determined by means of transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The observed porosity varies with the macerals such that the finest pores (<2-5 nm) are located in vitrinite, with a broad range of coarser porosity (40-500 nm) associated with the macerals exinite and inertinite. Elemental analyses, for elements of atomic number 11 or greater, in conjunction with selected area diffraction (SAD) experiments served to identify the source of the titanium observed in the granular material as the mineral rutile. Only sulfur could be de-tected in the other coal macerals. Dark-field microscopy is introduced as a means for determining the domain size of the coal macerals. This method should prove useful in the determination of the molecular structure of coal. 

A number of techniques have been used to determine mineralogies of coal As discussed in a recent review (J ) the most common techniques are x-ray diffraction infrared spectroscopy optical microscopyy and electron microscopy. X-ray diffraction and infrared spectroscopy can be considered "bulk methods because they are generally best performed on the mineral-matter concentrate obtained by removal of the macerals by low-ten5>era-ture ashing ( 2 ) The microscope methods can be considered "particulate methods because mineral grains in the coal are sized and classified individually These microscope methods are usually used without separation of minerals and macerals because the coal macerals can serve as a background matrix to separate mineral particles and to provide contrast for the dimensional measurement of the particle ... 

Coals are not homogeneous but are made up with macerals, these being recognizable components (by optical microscopy) within the coal derived from specific plant components. Coals may possess different quantities of macerals so accounting for small differences in rank. The exinite group of macerals exhibit maximum fluidity, the vitrinites have an intermediate position and the fusinites (like a wood charcoal) are non-fusible. 

Transmission electron microscopy (TEM) was utilized to examine the HNT dispersibility within fibrous networks. Because TEM technique generally deals with bulk film samples, fiber mats were embedded into Araldite epoxy resin to prepare fully cured epoxy composites. A 1 x 1 x 1 cm epoxy resin in volume was initially polymerized for 24 h. Subsequently, fiber mats were macerated in epoxy slurry and placed on prepolymerized solid epoxy before the repolymerization of composites. The composite films were sectioned into 100 nm layers using a diamond knife with a Leica EM UC6 microtome, which were further mounted on carbon grids. A JEOL 2011 transmission electron microscope was used at an accelerating voltage of 200 kV to explore the HNT dispersion level within liber mats reinforced epoxy composites. 

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