Desert Varnish of the Sonoran Desert: Optical and Electron Probe Microanalysis

Optical and electron probe microanalysis of the dark patination coating rocks from the Sonoran Desert reveal a distinct, though sometimes discontinuous layer approximately 20 microns thick. This layer is characterized by concentrations of iron, manganese and magnesium considerably higher than their concentrations within the host rocks. The metal-rich layer is contained within a clay-like matrix and is, in some samples, separated from the host rock by a weathering cortex up to 1 mm thick. A large fraction of the metal in the outer layer appears to have been derived from sources external to the varnished rocks, probably wind-transported dust and clay.