Chemical composition analysis of Mn/Fe ratio in California rock varnish

Rock varnish is a rock patina that occurs in arid and semi-arid regions and is composed of clay minerals, manganese and iron oxides, and trace amounts of more than 30 minor compounds. Previous work has determined that the top surfaces of desert pavement clasts have black, manganese-rich varnish and the underside of clasts have orange, iron-rich varnish. Some of the formation theories include bacteria on the surface of the rock oxidize and concentrate manganese, manganese and iron are leeched from the host rock and precipitated on the surface, and dust particles on the surface of the rock react with atmospheric fluids. The Mn/Fe ratio in manganeserich varnish from Death Valley, CA and the Mojave Desert, CA have been determined to be 0.3-1.2 and 0.9-2, respectively. The purpose of this study was to test the conclusion that the California Mn/Fe ratio in manganese-rich varnish is between 0.3-2 for multiple rock types using a Scanning Electron Microscope. Five samples from five locations were analyzed. The results suggest that not all California rock varnishes have a Mn/Fe ratio within the 0.3-2 range. Further studies may identify a correlation between rock types and the Mn/Fe ratio