Title: An Occurrence of Natural Pitch Coke, Raton Formation, Purgatoire River Valley, Colorado
Authors: John C. Crelling and Susan M. Rimmer, Department of Geology, Southern Illinois University Carbondale, Carbondale, IL 62901
Publication: The Outcrop, June 2011, p. 5-9
Article Type: Lead Story
Occurrences of natural coke produced by the igneous intrusion of coal have been reported in the Spanish Peaks region of south-central Colorado (Eby 1925, McFarland 1929, Johnson 1961, Dutcher et al. 1966, Crelling and Dutcher 1971, Podwysocki and Dutcher 1971, Cooper et al. 2007, and Pollock et al. 2010). However, in a recent study of an intruded section of the Raton Formation (Upper Cretaceous-Paleocene) along the Purgatoire River near Medina Plaza, CO, coke that was possibly derived from pitch has been observed. This material (herein referred to as pitch coke) occurs in a carbonaceous Type III shale directly above and below a lamprophyre sill. This pitch coke is characterized by a remarkable flow mosaic texture, high reflectance (Ro > 7%) and anisotropy, abundant devolitilization vesicles, and an absence of inertinite inclusions. Only rare shale inclusions were observed. Along boundaries of the coke “fingers” there is evidence for multiple stages of accumulation, including coarse or ribbon coke edged by vapor-deposited carbon (pyrolytic carbon), secondary layers of highly porous coke (not unlike sponge coke), and small areas of mesophase spheres. Under the microscope, this pitch coke looks identical to commercial needle coke manufactured from petroleum residue.
This pitch coke is quite different from natural coke derived from coal at the same locality. The coked coal has a medium-grained circular mosaic texture that is consistent with the high volatile bituminous rank of the unaltered coal in the area. When bituminous coal is heated in the absence of air it softens at about 350°C and then melts,devolatilizes, vesiculates, and finally hardens into coke at about 550°C forming anisotropic domains that give coke its characteristic mosaic cell-wall texture. The nature of this optical texture is rank dependent, thus coal always develops a texture true to its rank. Whereas the coal-derived coke has devolitilization vesicles similar to the pitch coke, it also has numerous inclusions of inertinite macerals such as fusinite and secretinite that are not seen in the pitch coke. Geochemically, raw samples of the pitch coke “fingers” have low HI and OI values (<10), high %C (~70%), low N contents (~0.6%), C/N ratios of ~120, and d13C of -26‰.
These observations suggest that the coke found in this study was not formed by the direct coking of coal, but from a mobile phase (pitch) that was subsequently coked by the intrusion. This pitch could have been derived from the coal that lies several feet above the shale that is in contact with the sill or from the organic matter contained within the carbonaceous shales (in which case it could be considered a natural petroleum coke), or both. However, coal tar pitch associated with the coked coal investigated by Podysocki and Dutcher (1971) showed only a fine to course circular mosaic texture, quite different from the ribbon mosaic texture of the pitch coke.
In summary, the lack of coal maceral inclusions, its occurrence throughout the shale, and the ribbon mosaic texture that is inconsistent with the unaltered coal rank all suggest that the pitch coke was derived from the carbonaceous shale. The thermal history of the site is interpreted as a rapid heating by the intrusion causing the release and coking of pitch from the shale and the melting and coking of the coal above the shale. This was followed by cooling that caused shrinkage fractures that were filled with vapor-deposited (pyrolitic) carbon and frozen residual mesophase.
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