Scanning Electron Microscopy–Energy-Dispersive X-ray Spectrometry (SEM–EDS) Analysis of PM_1–2 Microspheres Located in Coal Char Particles with Different Morphologies

Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to analyze individual microspheres, 1–2 μm in size, located in coal char particles of Inertoid and Fusinoid/Solid morphological types. It was shown that PM1–2 (where PM = inorganic particulate matter) is formed in the porous structure of the carbon matrix, which controls the microsphere size, from authigenic minerals that determine their composition. Depending on the contents of SiO2, Al2O3, and FeO, the studied microspheres fall into various groups differing in mineral precursors. The precursor of the Group 1 microspheres with the specific composition of SiO2 + Al2O3 > 95 wt % and FeO ≤ 1.5 wt % is NH4–illite. Microspheres containing SiO2 + Al2O3 < 95 wt % and FeO in increasing content amounts up to 4, 6, and 10 wt %, included in Group 2, Group 3, and Group 4, respectively, are formed from mixed-layer K–illite–montmorillonite minerals subjected to cationic substitution with iron, followed by the entry of Fe3+ in interlayer sites. Calcite, dolomite, gypsum, magnesite, rutile, and siderite are involved in the formation of Group 5 microspheres with a high content of Ca, Mg, Ti, or Fe. The significant part of PM1–2 is represented by microspheres of Groups 2, 3, and 4 regardless of the type of coal char particles (62% for Inertoid ones and 75% for Fusinoid/Solid ones). About one-third of the microspheres for both char morphotypes refer to Group 5. Microspheres of Group 1 (8%) are located only in the Inertoid char particles, which results from the characteristic effect of the maceral–mineral composition of original coal. It has been suggested that Inertoid and Fusinoid/Solid char particles are formed from various macerals, semifusinite and fusinite, respectively. Due to the closed-cell structure, semifusinite contains noncation-exchanged NH4–illite, the mineral precursor of microspheres with low contents of Fe, K, Na, and Mg. The fusinite structure allows cationic substitution in NH4–illite with the formation of mixed-layer K–illite and montmorillonite, the mineral precursors of a significant part of PM1–2.