Chemical data on desert varnish [California]

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 (sub 2) O, Fe (sub 2) O (sub 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.