Rock varnish

Garza-Valdes, L. A. and B. Stross (1992), Rock varnish analysis, in Vandiver, P. J., D. Druzik, G. S. Wheeler, and I. C. Freestone (eds.), Materials Issues in Art and Archaeology III, a Symposium in San Francisco, California, April 27-May 1, Materials Research Society Symposium Proceedings, Pittsburgh, PA, Vol. 267, pp. 891-900. 

O Grady, C. (2005), The occurrence of rock varnish on stone and ceramic artifacts, Rev. Conservation 6, 31-38. 

Rock salt semiconductors, 22 141 dating, 21 317-318 selenium occurrence in, 22 78 Rock surface chemistry, in volumetric sweep efficiency, 18 621 Rock varnish, photocatalytic origin of, 19 100-101... 

Grote, G. Krumbein, W. E. (1992). Microbial precipitation of manganese by bacteria and fungi from desert rock and rock varnish. Geomicrobiology Journal, 10, 49-57. 

Dorn, R. I. (1984). Cause and implications of rock varnish microchemical laminations. 

Krumbein, W. E. Jens, K. (1981) Biogenic rock varnish of the Negev desert (Israel) an ecological study of iron and manganese transfer by cyanobacteria and fungi. Oecologia, 50, 25-38. 

Liu, T. (2003) Blind testing of rock varnish microstratigraphy as a chronometric indicator results on late Quaternary lava flows in the Mojave Desert, California. 

Liu, T., Broecker, W.S., Bell, J.W. Mandeville, C. (2000) Terminal Pleistocene wet event recorded in rock varnish from the Las Vegas Valley, southern Nevada. Palaeogeography, Palaeoclimatology, Palaeoecology 161, 423 433. 

Rock varnish (often called desert varnish when seen in drylands) is a paper-thin mixture of about two-thirds clay minerals cemented to the host rock by typically one-fifth manganese and iron oxyhydroxides. Upon examination with secondary and backscattered electron microscopy, the accretionary nature of rock varnish becomes obvious, as does its basic layered texture imposed by clay minerals (Dorn and Oberlander, 1982). Manganese enhancement, two orders of magnitude above crustal values, remains the geochemical anomaly of rock varnish and a key to understanding its genesis. 

Rock varnish Clay minerals, Mn and Fe oxides, and minor and trace elements colour ranges from orange to black due to variable concentrations of different... 

Figure 8.1 This road cut between Death Valley, California and Las Vegas, Nevada illustrates the ability of 10-100 jLim-thick rock varnish (darkens debris slope surface on upper half of image) to mask the appearance of the host igneous and metamorphic rocks (light-appearing road cut on lower half of image).

The problem for any researcher new to the topic of manganese-rich varnish is that the vast majority of papers are written by investigators who collect only a few samples from a few locales and simply assume that their samples, somehow, are equivalent to varnishes collected in completely different biogeochemical settings. The only formal classification of rock varnish thus far presented (Dorn, 1998, pp. 214-224) maintains the goal of forcing varnish researchers to ensure that they have sampled the same variant in any comparisons. For the sake of clarity and brevity, this chapter presents illustrations only for varnishes formed in subaerial settings. 

Analytical chemist Celeste Engel completed the first Masters thesis research on rock varnish and teamed up with geomorphologist Robert Sharp to write a seminal paper on rock varnish in the Mojave Desert (Engel and Sharp, 1958). The major elements are O, H, Si, Al, and Fe in approximately equal abundance with Mn. The key mystery of varnish formation (von Humboldt, 1812 Lucas, 1905 Engel and Sharp, 1958 Jones, 1991) is... 

Table 8.2 A few of the misunderstandings in the literature surrounding rock varnish and its environmental relations. See chapter 10 in Dorn (1998) for a more detailed account of the history of thought on varnish... 

Rock varnish is too thin to be an effective case hardening agent... 

Rock varnish grows best on certain lithologies such as fine-grained extrusive rocks... 

Figure 8.5 Varnish micromorphology form ranges from botryoidal to lamellate (A and C). Two types of imagery show botryoidal varnishes from Kitt Peak, Arizona (A) the topography by secondary electrons (B) the same structures from the bottom upwards with back-scattered electrons - showing the layering structures inside each nudeation centre. (C and D) Scanning electron microscopy images of lamellate clay minerals accreting on rock varnish in Death Valley, California. (C) Individual clay platelets overlap as they cement onto the surface. (D) The clays impose a lamellate structure in cross-section, as first noticed by Potter and Rossman (1977).

Silica and aluminum, taken together, comprise the bulk of rock varnish - consistent with clays being the dominant mineralogy. 

Manganese and iron oxides typically comprise one-quarter to one-third of rock varnish with high point-to-point variability at scales from nanometres to kilometres. 

Varnish minerals were originally reported to be amorphous (Engel and Sharp, 1958), with goethite (Scheffer et al., 1963) and ferric chamosite (Washburn, 1969) as important components. Seminal research conducted with infrared spectroscopy, X-ray diffraction and electron microscopy at the California Institute of Technology revealed that the bulk of rock varnish... 

Table 8.3 Examples of elemental variation exhibited in bulk chemical analyses of rock varnishes found in desert regions. Results are normalised to 100% with measurements by particle induced X-ray emission (Cahill, 1986), with NA indicating not available and BLD below the limit of detection... 

Detritus, both organic and inorganic, often settles in morphological depressions on varnish. These pieces are sometimes trapped by accreting rock varnish, leading to a wide variety of other minerals that are sometimes found in varnish samples such as quartz, feldspars and magnetite (Potter and Rossman, 1979a Dorn, 1998, p. 198 Mancinelli et al., 2002). 

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