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

Rock varnish is a dark-colored Mnand Fe-rich coating that forms on exposed rock surfaces in almost every terrestrial weathering environment, particularly in arid and semiarid regions. Varnishes average about 100 μm in thickness but can reach thicknesses of several hundred micrometers, with accumulation rates of 1–15 μm/1000 years (Liu and Dorn 1996). Rock varnishes have attracted considerable research interest as a potential Quaternary dating tool for rock surfaces, archeological features and artifacts (e.g., Dorn 1989); as indicators of paleoclimatic change (Dorn 1994; Liu and Dorn 1996); to assist in interpretations of remote sensing studies of varnished rock surfaces on Earth and on Mars (Israel et al. 1997); because of the likely role of microorganisms in their formation (e.g., Perry and Adams 1978; Nagy et al. 1991); and simply because they are so ubiquitous and obvious. Rock varnishes are also being studied as possible environmental monitors because of the great scavenging abilities of Mn oxides for certain heavy metals (Dorn 1991; Fleisher et al. 1999). During the past thirty years, an impressive volume of literature has been published addressing the detailed morphology, chemistry, formation, and potential uses of rock varnishes. Nevertheless, the formation mechanism and detailed mineralogy of rock varnish is incompletely understood. A growing number of researchers, however, concur that it forms as an external accretion with its constituents probably derived from windblown material, and does not have any chemical or mineralogical relationship to the underlying rock (Potter and Rossman 1977, 1979; Allen 1978; Dorn and Oberlander 1981). Studies have shown a distinct morphological boundary between ABSTRACT