Rock varnish formed on top of granodiorite at Kitt Peak, Arizona. The varnish is bright in this backscattered electron microscope image because it has lots of manganese and iron.  The varnish is about 40 microns thick (or about 4 widths of a human hair).

Rock Varnish: Basic Information

Approximately one-sixth of the earth's land surface consists of 'bare' rock surfaces.  Yet the true mineral faces of the rock are rarely seen.  Instead, a variety of paper-thin accretions coat rocks in all terrestrial environments. Since weathering includes the formation of new compounds that are more in equilibrium with the environment at and near the earth's surface, rock coatings fall under the interdisciplinary field of weathering. Intellectual curiosity about the physical and chemical characteristics, origin, geography, and utility of these encrustations has spawned over 3000 scientific papers.

A type of a rock coating that is dark in color and is characterized by clay minerals (~40-60%) cemented to rock surfaces by oxides and hydroxides of manganese (birnessite) and iron (goethite and hematite) that typically comprise 20-40%. The oxides nanometer is size and they are a by product of the weathering of bacterial casts.  Clay minerals are ubiquitous, but they will not form rock varnish by themselves.  Rock varnish grows only where and when the nanometer-scale remnants of bacteria maneuver in between the broken and decayed fragments of clay minerals weathered at the nanometer scale. Most varnishes seen at the surface today actually start in the subsurface in fissures. It forms in all terrestrial environments, but the term "desert varnish" is common because varnishes are most geochemically stable in the subaerial environment in deserts. Typical thicknesses are less than 100µm.   Usually dull in luster, its occasional sheen comes from a smooth surface micromorphology in combination with manganese enrichment at the very surface of the varnish. The constituents in varnish accrete on the host rock. The most volumetrically significant post-depositional modification is the leaching of cations from rock varnish.  Varnishes are also eroded by lithobionts.  But in places where varnishes are intact black (manganese-rich) and orange (iron-rich) microlaminae may indicate past changes in climate.  Rock varnishes sometimes act as case-hardening agents for the underlying rock. Many varnishes also encapsulate organic matter; unfortunately, radiocarbon dating of these organics is not useful because the entombed organics form an open system of older and younger organics.  The most useful application of rock varnish rests in the layering pattern of black (manganese-rich) layers and orange (manganese-poor) layers, which reveal the age of the varnish and past environmental changes experienced by the rock surface.

[Refereed publication to cite for this information:
Dorn, R. I. 1998. Rock coatings. Amsterdam: Elsevier. 429 pp.]