This widespread desert phenomena has been considered by some to be a biogeochemical process that occurs in arid regions throughout the world (e.g., Grote and Krumbein, 1992). Desert varnish coatings on rocks may only be 0.01 mm thick, but they can turn entire desert mountain ranges black or reddish brown. Desert varnish is the result of a disequilibrium process incorporating oxides of manganese and iron, along with clays that form together on the surface of rocks exposed to the Sun for thousands of years. Desert varnish is considered by some to have formed by the actions of microscopic bacterial colonies living on the rock surface, but evidence for this process has not been documented. Trace amounts of manganese and iron are absorbed from the atmosphere and precipitated as a black layer of manganese oxide or reddish iron oxide on the rock surface. This thin layer also includes cemented clay particles. It has been estimated that up to 10,000 years are required for a complete varnish coating to form in extreme arid desert regions. A diagramatic representation of a possible biogeochemical process is shown below:
Perry et al. (2006) present their analysis of desert varnish in "Baking black opal in the desert sun: The importance of silica in desert varnish". They revealed the presence of amorphous hydrated silica (opal) and the silica mineral moganite, similar to findings from siliceous hot-spring deposits. They conclude "... the slow dissolution of silica from anhydrous and hydrous minerals, and its subsequent gelling, condensation, and hardening, provides a simple explanation of a formation mechanism for desert varnish and silica glazes and the incorporation of organic material from local environments."
In their paper "Nanometer-scale complexity, growth, and diagenesis in desert varnish", Garvie et al. (2008) report on a high-resolution and spectroscopic study of desert varnish on a variety of rock samples from the Sonoran Desert of Arizona. They found that clay is the primary component of desert varnish incorporating nm-scale layers rich in Mn-bearing phases and Fe oxide (hematite), along with some secondary and trace elements proposed to be derived from atmospheric aerosols. Although C grains within the varnish possibly represent fungal and/or bacterial remnants, the formation of desert varnish is considered to be the result of mainly abiotic processes of diagenesis including hydration, evaporation, dissolution, freezing, baking, redox, and other forms of chemical and structural modification. They contend that opal may serve as a cementing material in some rocks. It is calculated that desert varnish is very slowly accumulated at rates of <1µm to perhaps tens of µm/t.y.