Organic petrography:An approach for identification of maceral groups in Gheshlagh coal area, Eastern Alborz

Maceral is a term to introduce organic components visible under a microscope (Stopes, 1935). The physical and chemical characteristics of macerals such as elemental composition, moisture content, hardness, density and petrographic characteristics differ. The differences in the physical and chemical characteristics of macerals are reflected in their industrial behavior.(Parkash, 1985). Petrographic analysis provides information on the various physical components of coals (Suwarna and Hemanto, 2007) and determination of quality of coal, coalification rate, composition and characteristics of coke and paleoenvironmental deposition (Taylor et al., 1998).
Sampling and Coal samples were collected from freshly mined coal from 11 coal seams of 4 active coal mines (Cheshlagh, Zemestan Yourt, Narges Chal and Cheshmehsaran) for organic petrography in the Gheshlagh coal deposits. All samples were collected and stored in plastic bags to prevent contamination and weathering.
Samples were prepared for microscopic analysis by reflected light following ASTM Standard procedure D2797-04. For microscopic study, coal samples were crushed to1-mm size fraction (18 mesh size), mounted in epoxy resin and polished. Three polished samples were prepared for each coal seam. The petrographic composition was obtained by maceral analyses under standard conditions (ISO 7404/3, 2009, for maceral analysis). Maceral point counting (based on 400 points) analyses were performed using an Olympus BX51 reflected light microscope. The terminology used to identify and describe the organic matter particles is the one proposed by the International Committee for Coal and Organic Petrology (ICCP, 1998; ICCP, 2001; Scott and Glasspool, 2007; Taylor et al., 1998; Stach et al., 1982; Hower et al., 2009; Hower and Wagner, 2012).
Organic petrography of theGheshlagh coal seams: The vitrinite maceral group is dominant in all coal seams (66.2 to 87.2 vol.%) and includes collodetrinite, collotelinite, and corpogelinite macerals. Collodetrinite maceral (20 to 65.6 vol.%) is the most abundant maceral in the vitrinite group and is associated with inertodetrinite and microspornite macerals. Callotelinite (8.4 to 46.7 wt%) occurs as a structureless, homogeneous mass in the Gheshlagh coal seams. The inertinite group (4.9 to 23.3 vol%) includes fusinite, semifusinite, macrinite, secretinite, funginite and inertodetrinite macerals. Fusinite (1.7 to12.7 vol.%) is present in all coal seams in the Gheshlagh area. Cell cavities of fusinite are filled mostly by corpogelinite and clay minerals. Semifusinite occurs in appreciable concentrations (2.1to 14.3vol.%) and Cell lumens of this maceral are filled with mineral matter, pyrite and clay.
The liptinite group (nil to 3.5 vol%) includes sporinite, cutinite and resinite macerals. Sporinite is the dominant maceral in the liptinite group (nil to 2.3 vol.%) and occurs as elongated thread-like or spindle-shaped bodies and occurs as microspores and megaspores. Resinite (nil to 0.1 vol.%) occurs as round to oval bodies and as fillings of the cell cavities of fusinite, semifusinite and funginite.
The mineral matter content of most of the Gheshlagh coal seams varies between 3.1 and 24.9 vol.%. Mineral matter occurs in primary ground mass or secondary cavity filling form and includes clay minerals, carbonate and sulphide.
Based on organic petrographic studies carried out on four active coal mines in the Gheshlagh area, the presence of three maceral groups were determined. The vitrinite group (66.2 to 87.2 vol%) is the dominant maceral group, and callodetrinite maceral (20 to 65.6 vol%) is also abundant. The inertinite group contenthas a range of 4.9 to 23.3 vol% while the fusinite and semifusinite macerals are the most abundant of this group. The lowest volume percentage of macerals belongs to the liptinite group (0 to 3.5 vol%) with 2.3 vol% espornitebeing the most abundant Maceral of this group. The presence of espornite maceral at megaspore size in the S2 and K5 coal seams is very noticeable. The content of mineral matter of these coal seams varied from 5 to 24.9 vol%.