Revisiting Alexander von Humboldt's initiation of rock coating research

Secondary, backscattered, high-resolution transmission, and energy-dispersive spectroscopy tools of electron microscopy reexamined Alexander von Humboldt's field site of "brownish black crust[s]" covering rocks along cataracts of the Orinoco River. Modern tools confirm eighteenth-century analysis that the basic composition includes an abundance of manganese, iron, and carbon. Additional major constituents include clay minerals, calcium, and sometimes barium and cobalt. Backscattered and secondary electron imaging confirms the 2-century-old hypothesis of an accretionary nature of the Orinoco coating. The remarkable Orinoco sheen is produced by a smooth lamellate micromorphology and high concentrations of manganese-the same conditions required to produce lustrous varnishes formed in warm deserts. Although von Humboldt deduced, and we agree, that coating constituents must derive from the Orinoco, electron microscope observations of Mn-enriched diatom fragments, Mn-enriched cocci-bacterial forms, and microstromatolitic textures suggests a role for microorganisms in the 60- to 70-times enhancement of Mn over Fe in these varnishes. With the retrospect of 2 centuries of scholarship, Alexander von Humboldt rightfully deserves to be considered the father of rock coating research.