Anthracitic coals are high-rank coals. They are shiny (glassy) and break with a conchoidal (glass-like) fracture. Most coals do not reach anthracitic rank, which requires high heat from very deep burial, tectonic metamorphism, or contact metamorphism with igneous intrusions. The anthracitic rank is divided into three parts; semi-anthracite, anthracite, and meta-anthracite. In the U.S. rank classification system, anthracitic ranks are defined based on volatile matter and fixed carbon contents. Anthracitic coals have volatile matter less than 14% and fixed carbon contents greater than 86% on a dry mineral-matter free basis (ASTM).
Anthracite, which is a specific rank of anthracitic coals, has volatile matter from 2 to 8%, and fixed carbon contents of 92 to 98% on a dry mineral-matter free basis (ASTM; Jackson, 1977).
Physical and Chemical Changes (Anthracitic Rank)
Anthracitic rank marks the beginning of distinctive physical and chemical changes to coal termed anthracitization, graphitization, metadiagenesis, and telodiagenesis in different reports (see figure). The most distinctive physical change at anthracite rank, is the development of conchoidal fracture and the loss of cleats in coal. Conchoidal fractures are curved, reflective, glass-like surfaces. Chemically, anthracites show a sharp decrease in hydrogen content and the hydrogen to carbon (H/C) ratio relative to medium- and low-rank coals. At the molecular level, coalification at anthracitic ranks involves condensation of aromatic carbon-ring systems to larger ordered-carbon structures (Stach et al., 1982; Krevelen, 1993; Levine, 1993). This results in a very ordered, carbon-rich (98 to <100% C) coal at the meta-anthracite rank.
It was once thought that continued heating and burial of anthracite coal would result in the formation of the mineral graphite (a crystalline form of carbon), and then even diamond; the purest, most-ordered crystalline form of natural carbon. However, recent research suggests that the activation energy for the formation of graphite may be too high to be formed in nature just by normal geologic burial heating processes. Instead, it appears that considerable strain energy (as occurs in mountain-building areas of continental crust) are likely needed to transform anthracites into graphite (Wilkes et al., 1993; Bustin et al., 1995).