Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy

X-ray absorption data were collected for a series of varnish and dendrite M n oxide coatings on rock substrates containing a wide variety of mineralogi es exposed to a variety of environments. Near-edge spectra of the coatings indicate that the Mn-oxide phases present have Mn valences between 3+ and 4 +, with average Mn valences for the varnishes closer to 4+ than those for t he dendrites. Mn EXAFS data and analyses indicate that Mn-oxide structure t ypes for the varnishes range, perhaps continuously, from large tunnel phase s, similar to todorokite and romanechite, to layer phases, i.e., birnessite -family. Similar results were found for the dendrite samples, except that t he variety of Mn-oxide phases is somewhat larger than those found for the v arnishes. No correlations were found between Mn-oxide structure-type within these coatings and the corresponding substrate petrology.