Finely layered coatings, rich in manganese and iron and commonly called desert varnish, are common on rocks in desert environments worldwide. These coatings have been the subject of intense scientific debate and extensive research, owing to their potential for indicating past climates, for dating geological surfaces, and, via artwork carved in varnish, for providing information about ancient cultures.
The full scientific potential of desert varnish can only be realized through a rigorous probing of the physico-chemical variables and fundamental properties of varnish components, especially its mineralogical components.
Determining the mineralogy of the manganese- and iron-bearing materials is challenging because the minerals are extremely fine grained, generally down to nanometer-sized, and often poorly crystalline.
In addition, the thin film-like nature of varnish on rock makes separating and studying it difficult. Garvie et al. used novel sample preparation methods, high-resolution electron microscopy, and spectroscopic imaging to provide novel insights into desert varnish structure, mineralogy, and chemistry.
The spectroscopic imaging shows nanometer-scale separation of manganese- and iron-bearing phases, possibly reflecting differing degrees of chemical oxidation. A suite of late-grown manganese and iron phases commonly occur also, together with sparse barium and strontium sulfates, and rare, entrained, carbonaceous particles.
These data demonstrate that varnish remains a mineralogically and structurally active system. They furthermore suggest that there must be strict climate controls on varnish growth.
This research by Laurence Garvie et al., Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University was published in the March issue of Geology, Pages 215-218.
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