Geoscience Reference
In-Depth Information
Table 8.7
Major categories of rock coating (Dorn, 2007).
Coating
Description
Carbonate skin
Coating composed primarily of carbonate, usually CaCO 3 , but sometimes combined with magnesium
Case hardening
agents
Addition of cementing agent to rock matrix material; the agent may be manganese, sulfate, carbonate,
silica, iron, oxalate, organisms or anthropogenic
Dust film
Light powder of clay- and silt-sized particles attached to rough surfaces and in rock fractures
Heavy metal
skins
Coatings of iron, manganese, copper, zinc, nickel, mercury, lead and other heavy metals on rocks in
natural and human-altered settings
Iron film
Composed primarily of iron oxides or oxyhydroxides
Lithobiontic
coatings
Organic remains form the rock coating, e.g. lichens, moss, fungi, cyanobacteria, algae.
Nitrate crust
Potassium and calcium nitrate coatings on rocks, often in caves and rock shelters in limestone areas
Oxalate crust
Mostly calcium oxalate and silica with variable concentrations of magnesium, aluminium, potassium,
phosphorous, sulfur, barium and manganese. Often found forming near or with lichens. Usually dark in
colour, but can be as light as ivory
Phosphate skin
Various phosphate minerals (e.g. iron phosphates or apatite) that are mixed with clays and sometimes
manganese; can be derived from decomposition of bird excrement
Pigment
Human-manufactured material placed on rock surfaces by people
Rock varnish
Clay minerals, Mn and Fe oxides, and minor and trace elements; colour ranges from orange to black in
colour produced by variable concentrations of different manganese and iron oxides
Salt crust
The precipitation of chlorides on rock surfaces
Silica glaze
Usually clear white to orange shiny luster, but can be darker in appearance, composed primarily of
amorphous silica and aluminium, but often with iron
Sulfate crust
Composed of the superposition of sulfates (e.g. barite, gypsum) on rocks; not gypsum crusts, which are
sedimentary deposits
Figure 8.16 A 10 to 100 ยต m-thick rock varnish (darker debris slope surface on upper half of image) masks the appearance of
the host igneous and metamorphic rocks in a road cut between Death Valley, California, and Las Vegas, Nevada, USA. Photograph
courtesy of Ronald Dorn.
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