Silva and 4 co-authors reported in late 2020 that they conducted petrographic, geochemical, and electron-microbeam investigations of selected anthracites and anthracite-rank coaly shales. The Pennsylvania, Rhode Island, and KwaZu-Natal anthracites show an enrichment in the middle to heavy rare earths (REE) while the Virginia semi-anthracite exhibited a decreasing trend through the light REE, a peak in Gd, and a depletion in the heavy REE. Substitution of As, Hg, and Se in pyrite was most apparent in the Virginia coal. The Pennsylvania, Rhode Island, and KwaZu-Natal anthracites contained spherical carbon nanotube (CNT) structures. The CNTs contained several elements, including Hg, Cd, F, Cl, and Br. While CNTs are known to be produced from coals of varying ranks, this seems to be the first report of naturally occurring CNTs.
The authors concluded that their study consisted of a survey of a few Paleozoic-age anthracites; it is not a comprehensive overview of all anthracites. However, it does provide a basis for the identification of naturally occurring C-nanoparticles in anthracites and for studies of their impact on the properties of the coals.
Among the chemical properties of the anthracites, the enrichment of Pb and As in the Rhode Island and Virginia anthracites corresponds to expectations with respect to metamorphism by hydrothermal fluids. The Rhode Island anthracite is in a region with notable metamorphic facies gradients, from low-grade metamorphic rocks in the north to staurolite zone metamorphism in the south [Refs:19–20].
The Valley Fields, Virginia, semi-anthracites were influenced by hydrothermal fluid flow [17]. The Pennsylvania anthracite, while in a region subject to intense thrust faulting and known fluid flow, does not have as high of a concentration of chalcophile elements as the Rhode Island and Virginia coals. Instead, the metamorphism of the Pennsylvania anthracites is reflected in the mineralogy of clays and other phyllosilicates in the coal, cleat, and surrounding rocks [24–28,31]. Some of the variability in the concentration of chalcophile elements lies in their varied concentrations in forms of pyrite and marcasite and in the occurrence of metalloid forms with As, Co, Cu, Cd, Ga, Hg, Ni, Pb, and Se.
The Pennsylvania anthracite had the highest P2O5, Sr, Ba, and REE content of the coals analyzed. The ash-basis REE exceeds 1700 ppm in a Pennsylvania Northern Anthracite Field coal [49]. In general, the Bragg et al. [49] analyses fall into the mid-100’s-1000-ppm-REE range, similar to Pennsylvania anthracites analyzed by Hower and Dai [48] and Hower et al. [47].
The Rhode Island, Pennsylvania anthracites, and KwaZu-Natal samples, the three highest-reflectance coals in the sample set contained carbon nanotube (CNT) structures. Several inorganic elements, including Hg and the halogens, are associated with the CNTs. To the best of our knowledge, this is the first report of naturally occurring CNTs in anthracites.
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