Monday, 6 October 2008
George R. Brown Convention Center, Exhibit Hall E
Dendrites are fern-like mineral coatings that normally occur on the fracture surfaces of rocks. Mn-oxide dendrites have historically been described as the mineral pyrolusite, but Potter and Rossman (1979) and McKeown and Post (2001) examined several dendrite samples and found a range of Mn-oxide minerals, including romanechite and hollandite. Unfortunately, a majority of their data were inconclusive due to the finely particulate and disordered nature of Mn oxides. We measured X-ray powder diffraction data (Cu K radiation) for a series of Mn dendrites occurring on rhyolite and dolostone collected from the area of Iron Mountain, Missouri. Diffraction data were measured on dendrite-coated rock slabs, and scraping was done only for large and thick dendrites. Analysis of the diffraction data revealed the presence of two Mn-oxide phases, the tunnel-structure mineral todorokite and the layer-structure mineral birnessite, along with substrate minerals. Scanning electron microscopy (SEM) showed the occurrence of well-defined todorokite crystals, but birnessite had poorly developed crystals. Although samples were dendritic on a macroscopic scale, dendritic morphology was not apparent using the SEM.
The formation mechanism of Mn dendrites is unknown, but it has been proposed by Chopard (1991) that they are deposited by a reaction-diffusion process when supersaturated Mn-rich solutions are exposed to air at rock surfaces. The mineralogy of the Iron Mountain dendrites, todorokite and birnessite, offers the possibility of a role for microorganisms in their formation. Several researchers have argued that rock varnish, a close relative to dendrites, is formed through bacterial precipitation (Dorn and Oberlander, 1981; Nagy et al., 1991). Mn-fixing bacteria precipitate Mn from solution by oxidizing Mn2+ and Mn3+ to Mn4+. Therefore, we are performing DNA extractions on dendrite samples in order to determine whether Mn-fixing bacteria are present. Their presence would be strong evidence for a biological contribution to dendrite formation.