Observations and experiments on the origin and formation of inertinite group macerals

The inertinite group macerals include fusinite, semifusinite, inertodetrinite, macrinite, micrinite and, funginite and secretinite, which together replace sclerotinite. The macrolithotype fusain comprises fusinite and semifusinite, and is now widely accepted as charcoal formed by wildfire activity. However, alternative origins for fusinite and semifusinite are still claimed. This paper considers the use of the terms pyrofusinite, degradofusinite, rank fusinite and primary fusinite misleading, while the definitions of the terms funginite and secretinite are also considered problematic. Observations made on modern, wildfire derived and volcanogenic, charcoal assemblages using reflectance and scanning electron microscopy, together with experimental charcoalification studies, demonstrate that most, if not all, inertinite macerals have acquired their optical and physical characteristics through the action of elevated temperatures. The criteria for the identification of charcoals produced by wildfire and volcanic activity are outlined and the role of the petrographic nomenclature system is discussed.