Origin of Desert Varnish

Caltech News Release via California Geology (3/78)

Lithosphere (April 1993); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA

Desert varnish, a smooth black coating that accumulates slowly on rock formations ranging from small boulders to cliffs hundreds of feet high, has puzzled naturalists for many years. On the underside, where it is not exposed to the air, a varnished rock often acquires a glossy red-orange finish through interaction with water and minerals in the soil. Because of its striking appearance and widespread occurrence, the varnish frequently attracts attention, especially in national parks such as Grand Canyon and Zion where visitors query rangers about its cause.

For years scientists have assumed that desert varnish was composed primarily of manganese and iron oxides, precipitated out of the rock through weathering processes. However, the varnish structure eluded precise analysis because it is composed of particles too fine to be characterized by X-rays, the main diagnostic tool of mineralogical investigation.

Infrared spectroscopy, a technique which illuminates mineral samples with infrared light and records the pattern of absorbed wavelengths, was applied to the study of desert varnish by two Caltech scientists. George R. Rossman, associate professor of mineralogy, and Russel M. Potter, Caltech graduate student, used rock samples from 20 locations in California, New Mexico, and Arizona in the analysis of desert varnish. The samples included quartz, granite, basalt, rhyolite, quartzite, feldspar, and sandstone.

The results of the analysis revealed that the main constituent in desert varnish, totaling about 70%, is clay, not manganese and iron oxides. The oxides form the remaining 30%. The red coating on the underside of the varnished rocks, previously believed to be iron oxide, turned out to be 90% clay incorporating an iron oxide stain, similar to the iron in the black finish on the rocks' exposed portions. In addition it was found that all desert varnish, whether it formed on the side of a cliff or on a 10-inch boulder, shares a similar composition.

It was concluded that most of the coating collects from sources outside the rock rather than from material leached out of it, as many geologists had believed. One reason for this conclusion is that varnish is found covering non-manganese or iron bearing quartz crystals. Although some rocks may contribute oxides through weathering, the primary source seems to be wind deposited particles.

Fine, windblown clay particles are a critical ingredient in forming the varnish which first forms on rough, porous surfaces. These surfaces allow dew and other moisture to collect, depositing a thin film of clay when the water evaporates. This film of sediment on the rock's surface encourages water to migrate through tiny pores inside the film, depositing traces of manganese and iron as the water evaporates.

The formation of desert varnish is interdependent upon the clay and oxides. The dry, fluffy clay particles depend on the oxides to form a resistant cementing agent. The oxides, in turn, require clay particles for transportation and deposition. This is the underlying reason why all desert varnish that was examined contained both clay and manganese and iron oxides -- never one without the other.

The preceding article was published in the April 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

The material is in the public domain, and may be republished freely.

Last updated: 18 September 2002