Striated conical structures and related fractures in bituminous coal of the southern Canadian Rocky Mountains

Striated structures, many of which are conical in form, are common mesoscopic features in bituminous and semi-anthracite coals of the southern Canadian Rocky Mountains. The structures are planar, conical to pyramidal in outline and are characterized by striae which radiate from a common apex and bifurcate or “horsetail” to form secondary structures on the master surface. They are up to 20 cm in length, 4 cm in width and have apical angles varying from 10° to 50°. The striated structures occur at numerous localities in both highly sheared and relatively unsheared coal seams.The conical striated structures closely resemble some shatter cones developed in fine-grained rocks, whereas the planar striated structures are somewhat similar to chevron-structured, hackle-marked joints. The pyramidal structures are apparently unique to coal. All three types of structures are considered to be the result of brittle shear fracture based on their occurrence, orientation, and morphology. The apparently restricted occurrence of the structures to coal is likely a function of the homogeneity, and low tensile and compressive strength of bituminous and semi-anthracite coals, possibly augmented by high inter- and/or intra-particle gas pressure resulting from devolatization of the coal during progressive coalification.