The influence of mineral detritus on rock varnish formation

•Different high resolution electron microscope methods examine the influence of aeolian dust on varnish formation.•Sampling sites include 19 locations on five different continents.•Aeolian mineral dust is enveloped by rock varnish in different ways.•Very dusty locations can result in alternating layers of dust-rich and varnish-rich layers.

A mix of high resolution electron microscope methods imaged the textures and chemistry of rock varnish samples from 19 field sites on five continents. The vast majority of aeolian mineral is not incorporated into manganiferous rock varnish. Of those dust particles that are enveloped, submicron sized oval-shaped quartz minerals are the most common type of detritus seen, as they rest conformably between laminated layers. The dominance of quartz as the most common detrital mineral, combined with the relative rarity of feldspars – is consistent with the hypothesis that feldspars experience in situ decay into clay minerals. After the detritus is buried in varnish, mineral boundaries often develop enhanced porosity. Some porous zones around dust particles develop submicron skins of redeposited Mn–Fe. In other cases, the porous zones aid in the transport of capillary water that mobilizes and redeposits Mn–Fe as stringers in fissures. Larger dust particles ∼10 μm in diameter are deposited in microtopographic depressions, such as tubes created by acid-producing lithobionts. Varnishes growing in particularly dusty regions form alternating dust-rich and varnish-rich layers that potentially correlate to alternating dusty and less dusty periods. The very foundation of varnish, the underlying rock, is often less stable in the surficial environment than varnish – leading to enhanced porosity and mineral decay in the substrate. Sometimes, physical collapse of varnish into the underlying void space mixes varnish and rock; more commonly, however, remobilization of varnish constituents into these pore spaces creates case hardening of the weathering rind in the underlying rock.