Chemical data on desert varnish

Abstract

Desert varnish forms a dark coating up to 0.10 mm thick on the exposed surfaces of many stones and outcrops in southern California deserts. Wet chemical analyses were made of varnish, the underlying weathered rind, and fresh rock for a rhyolite and two andesites. The principal elements in varnish are O, H, Si, Al, Fe, and Mn, and the last two give the deposit its distinctive physical characteristics. H_2O, Fe_2O_3, and especially MnO show the greatest enrichment. Field observations and a number of partial analyses indicate that the best varnishes are on fine-grained rocks relatively rich in Fe and Mn. Spectrographic analyses were made of 22 varnishes, 14 rocks, 8 soils, and 5 samples of air-borne material. In the varnishes Ti, Ba, and Sr are by far the most abundant trace elements, followed by Cu, Ni, Zr, Pb, V, Co, La, Y, B, Cr, Sc, and Yb. Cd, W, Ag, Nb, Sn, Ga, Mo, Be, and Zn were recorded in some but not all varnishes. The trace-element content of all varnishes is similar, and the variations recorded are related to differences in the local geology. Most trace elements are considerably enriched in varnish—Cu and Co especially, and Ni, Pb, Ba, Cr, Yb, B, Y, Sr, and V. The chemical data suggest that (1) varnish on stones seated in soil or colluvium is derived largely from that material, (2) varnish on large bedrock exposures come from weathered parts of the rock, (3) air-borne material is probably a minor contributor. The formation of desert varnish is primarily a weathering process involving the solution, transportation, and deposition of Mn and Fe in particular and a host of trace elements. Most of these elements are derived from local sources, and the small amount of movement required can occur by transport in solution or possibly by ionic diffusion through moisture films. Dew may be as important a source of moisture as rain. Organic agents, such as bacteria, may cause deposition of varnish, but this has not yet been demonstrated. In the desert, evaporation and the catalytic action of MnO_2 should be capable of performing the task. The rate of varnish formation varies widely with local conditions. Hundreds and thousands of years may be required to form a dark coating in some situations, but at one locality in the Mojave Desert a good varnish formed on the surface stones of an alluvial deposit in 25 years. Although the widespread evidence of varnish deterioration may be due to climatological change, conditions in some parts of this desert area are currently favorable to varnish formation.

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