Silica glaze coats rocks in a wide variety of settings. After only a few centuries, the lava flows of Hualalai Volcano, Hawai'i develop a brownish-red coloration — shown here through backscattered electron microscopy.
Rock Coatings
[Cite the following publication for this information:
Dorn, R. I. 1998. Rock coatings. Amsterdam: Elsevier. 429
pp.]
The earliest humans studied rock coatings. Prehistoric artists used rock coatings as a backdrop for engravings. After rock coatings were mentioned in the Old Testament (Levithicus 13,14), Nabateans used visual differences between rock coatings and the underlying sandstone to highlight monumental architecture in Petra at about the same time as Nasca cultures juxtaposed dark and light rock coatings to create giant ground figures in the Peruvian Desert. The phrases "bare rock" or "naked rock" are almost always misnomers, for you almost never see true exposures of mineral rock material in natural situations.
Some 14 different classes of rock coatings have been identified thus
far:
| Term |
|
Related Terms |
| Carbonate Skin | Coating composed primarily of carbonate, usually calcium carbonate, but could be combined with magnesium or other cations. | Caliche, calcrete, patina, travertine, carbonate skin, dolocrete, dolomite |
| Case Hardening
Agents |
Addition of cementing agent to rock matrix material; the agent may be manganese, sulfate, carbonate, silica, iron, oxalate, organisms, or anthropogenic. | Sometimes called a particular type of rock coating |
| Dust Film | Light powder of clay- and silt-sized particles attached to rough surfaces and in rock fractures. | Gesetz der Wüstenbildung; clay skins; clay films; soiling |
| Heavy Metal Skins | Coatings of iron, manganese, copper, zinc, nickel, mercury, lead and other heavy metals on rocks in natural and human-altered settings | Described by chemical composition of the film |
| Iron Film | Composed primarily of iron oxides or oxyhydroxides; unlike orange rock varnish because it does not have clay as a major constituent | Ground patina, ferric oxide coating, red staining, ferric hydroxides, iron staining, iron-rich rock varnish, red-brown coating |
| Lithobiontic Coatings | Organic remains form the rock coating, for example lichens, moss, fungi, cyanobacteria, algae. | Organic mat, biofilms, |
| Nitrate Crust | Potassium and calcium nitrate coatings on rocks, often in caves and rock shelters in limestone areas | saltpeter; niter; icing |
| Oxalate Crust | Mostly calcium oxalate and silica with variable concentrations of magnesium, aluminum, potassium, phosphorus, sulfur, barium, and manganese. Often found forming near or with lichens. Usually dark in color, but can be as light as ivory | Oxalate patina, lichen-produced
crusts, patina,
scialbatura |
| Phosphate Skin | Various phosphate minerals (e.g. iron phosphates or apatite) that are mixed with clays and sometimes manganese | Organophosphate film; epilithic biofilm |
| Pigment | Human-manufactured material placed on rock surfaces by people. | Pictograph, paint, some-times described by the nature of the material |
| Rock Varnish | Clay minerals, Mn and Fe oxides, and minor and trace elements; color ranges from orange to black in color produced by variable concentrations of different manganese and iron oxides. | Desert varnish, desert lacquer, patina, manteau protecteaur, Wüstenlack, Schutzrinden, cataract films, |
| Salt Crust | The precipitation of sodium chloride on rock surfaces | Halite crust, efflorescence, salcrete |
| Silica Glaze | Usually clear white to orange shiny luster, but can be darker in appearance, composed primarily of amorphous silica and aluminum, but often with iron. | Desert glaze, turtle-skin patina, siliceous crusts, silica-alumina coating, silica skins |
| Sulfate Crust | Composed of the superposition of sulfates (e.g., barite, gypsum) on rocks; not gypsum crusts that are sedimentary deposits | Gypsum crusts; sulfate skin |
Rock Varnish is the most studied rock coating. Although it is found in all terrestrial settings, it is most easily recognized on the plentiful bare rocks seen in deserts (hence another common name is desert varnish). Also, the physical and chemical stability of rock surfaces in deserts allows sufficient time for this slow-growing accretion to form. It is structured much like a brick wall, but its thickness (ranging from <5µm to almost 500µm) is typically less than 30µm The clay minerals that comprise the bulk of the structure (~50-70%) are cemented to the rock by hydroxides of manganese and iron (15-50%). The enigma of varnish is its great enrichment in manganese, typically over 50 times compared with the adjacent environment (soils, underlying rock, dust).
The debate over the origin of rock varnish revolves around the source of the manganese and mechanisms of enrichment. Until the late 1970's and the more common use of scanning electron microscopy, the vast majority of researchers thought that the manganese and the varnish derived from the underlying rock. Now, investigators see a clear morphologic varnish/rock boundary—and other evidence that varnish is an accretion. Although most investigators now agree on an external origin, there are still two hypotheses on the mechanism of manganese concentration. The abiotic hypothesis holds that small changes in pH can concentration manganese by geochemical processes. The biotic hypothesis holds that bacteria, and perhaps other microorganisms, concentrate manganese—supported by culturing experiments and direct observations of bacterial enhancement of manganese.
Silica Glaze is a broad category of rock coatings that are dominated by amorphous silica with variable amounts of aluminum and iron. They are usually less than 200µm thick, with a clear white to orange shiny luster, but they can be darker in appearance. Silica glazes have been noted in warm deserts, cold deserts like Antarctica, on dry tropical islands, along tropical rivers, mid-latitude humid temperate settings, and various archaeological contexts. Silica glazes probably precipitate from soluble Al-Si complexes [Al(OSi(OH)3)2+] that are released from the weathering of phyllosilicate minerals.
Iron Films. Rusty-colored rocks are readily recognized as 'iron oxides' and are often readily dismissed once such a label has been given. Of course, once iron films are examined in detail, they display a wide variety characteristics and different types of iron films occur in very different environmental circumstances. For example, subaerial rock surfaces in hyper-arid deserts host iron films characterized by clay minerals cemented with ~10% iron. Subaerial dolerite rock surfaces in the Dry Valleys of Antarctica are rimmed by iron oxyhydroxides that both form an accretion and a weathering rind over a millimetre thick. Rocks in acid streams in Arctic and alpine settings are often impregnated with iron hydroxides that can physically separate pieces of the rock, much like salt or frost weathering.
Other rock coatings. Biofilms are organic coatings, for example lichens, moss, fungi, cyanobacteria, algae. Carbonate skins are composed primarily of carbonate, usually calcium carbonate, but the carbonate is sometimes combined with magnesium. They can occur as tufas, caliche exposed by rock spalling, or even as coatings on urban buildings. Case hardening agents represent the addition of cementing agents to rock matrix material; the agent may be manganese, sulfate, carbonate, silica, iron, oxalate, organisms, or anthropogenic. Dust films are a light powder of clay- and silt-sized particles that adhere to rough surfaces and in rock fractures. Heavy metal skins form on rocks down stream from mine tailings; they can have high concentrations of copper, lead, zinc, manganese, iron, cadmium, mercury and other heavy metals. Nitrate crusts of potassium and calcium nitrate are most often found in caves and rock shelters in limestone areas. Oxalate crusts are typically composed of calcium oxalate and silica with variable concentrations of magnesium, aluminum, potassium, phosphorus, sulfur, barium, and manganese. They are often less than a millimetre thick and can be found forming near or with lichens. Phosphate skins are comprised of various phosphate minerals (e.g. iron phosphates or apatite) that are mixed with clays and sometimes manganese. Polish films form where aeolian and glacial abrasion produce micrometre-scale films that reflect the composition of the underlying rock. Sulphate skins are the superposition of sulfates (e.g., barite, gypsum) on rocks; these differ from gypsum crusts in the soils literature.
[Cite the following for this information:
Dorn, R. I. 1998. Rock coatings. Amsterdam:
Elsevier. 429 pp.]