Submicron-Scale Mineralogy of Lithotypes and the Implications for Trace Element Associations: Blue Gem Coal, Knox County, Kentucky

Coal and Related Carbon Resources / Lignite, Graphite, Graphene, & Carbon Uses

Hower, and four co-authors, reported in 2018 that transmission electron microscopy accompanied by energy-dispersive spectroscopy and selected area electron diffraction of density-gradient separates from two lithotypes of the low-ash, low-sulfur Blue Gem coal, eastern Kentucky, revealed an array of previously unrecognized (in this coal, and arguable in most others) sub-micron minerals, some <10 nm in size. The sample representing the 1.22–1.24 specific gravity fraction of the middle bench contains a mineral identified as a La-, Ce-, Nd-bearing monazite; other minerals with Cr-Fe, Cu-Fe-S, Fe-Zn-S, and Pb; and areas, probably comprising agglomerates of several grains, if not several minerals, with concentrations of Mg, Ca, Ti, Fe, Zn, Zr, and Mo.

The sample representing the 1.30–1.31 specific gravity fraction of the basal lithotype has aggregates of particles enriched in Mg, Ca, Ti, and Fe. Individual grains not specifically quantified include Cr-Ni-Mn-Cu-Fe-S, Ag-S, and Cu-S. Detailed investigation of one area (most of the variation within a <4 μm² region) demonstrates the presence of greenockite (CdS); minute phases containing Ni-Co-Ge and Ag-Cd-Bi, the latter with a more evident S association than the former; metallic Bi; nisnite (Ni₃Sn); silver cadmium; manganosite (MnO); and siderite. Some minerals, such as the monazite, are most likely of detrital or tuffaceous origin. Many of the other assemblages could be of hydrothermal origin, a hypothesis supported by known regional geochemical and coal rank trends, but not previously demonstrated in mineral assemblages at the 10's of nm scale in this region.

The TEM investigation of selected density-gradient separates of low-ash Blue Gem coal revealed that elements can have mineral associations. In bulk studies, such elements could have been noted, in general, as having an “organic association” simply because it was not routine, at least at the time of our 1989–1990 investigation (Hower et al., 1994a,b), to examine submicron to nanometer-scale particles. There is a fundamental difference between “organic association”, implying chemically associated (chelated) elements with organics, and an “association with organics”, implying elements incorporated in minerals that, basically, cannot be removed by float-sink or chemical methods.

The density-gradient separate from the basal bench, sample 5445, contains both detrital minerals, as indicated by the monazite studied by TEM, and sulfides, as suggested by the SEM-EDS examination. Sample 5447 from the vitrinite-rich portion of the middle bench contains a suite of possible hydrothermal Ag-, Cd-, Bi-, and other metal-bearing minerals but also (probable) low-temperature diagenetic minerals such as siderite. The overall picture emerging from this investigation shows that the incorporation of minerals within a low-ash coal such as the Blue Gem, or any coal for that matter, is complex and any attempt at a complete understanding of the story requires a wide range of tools.

Every coal could be a unique setting based on the original depositional setting, including both the maceral and mineral and non-mineral inorganic inputs and the diagenetic and metamorphic overprints. Broad, chemical-based and bulk-mineralogy-based inferences such as by those of Collins (1993) and Sakulpitakphon et al. (2004) cannot begin to unravel the  nature and origin of the fine element associations present in coals.