Duricrust

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Duricrust

From Wikipedia, the free encyclopedia (Magnesium and sodium don't seem to be involved ^-DC)

Key minerals Soluble minerals (Iron, Aluminum, Silica, Calcium, Carbon, Sulfur -DC), Climate Arid

Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters.

It is a general term (not to be confused with duripan) for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic and (or) non-pedogenic processes. It is typically formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward, or laterally by capillary action, commonly assisted in arid settings by evaporation.[1][2]

There are different types of duricrusts, each distinguished by a dominant mineralogy. For example, ferricrete (laterite) is dominated by sesquioxides of iron; alcrete (bauxite) is dominated by sesquioxides of aluminum; silcrete by silica; calcrete (caliche) by calcium carbonate, and gypcrete (gypcrust) by gypsum.[1]

ferricrete (laterite)
en.wikipedia.org/wiki/Laterite

Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolonged weathering of the underlying parent rock, usually when there are conditions of high temperatures and heavy rainfall with alternate wet and dry periods.[1] Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.

alcrete (bauxite)
en.wikipedia.org/wiki/Bauxite

Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), mixed with the two iron oxides goethite (FeO(OH)) and haematite (Fe2O3), the aluminium clay mineral kaolinite (Al2Si2O5(OH)4) and small amounts of anatase (TiO2) and ilmenite (FeTiO3 or FeO.TiO2).[1] Bauxite appears dull in luster and is reddish-brown, white, or tan in color.[2]

In 1821 the French geologist Pierre Berthier discovered bauxite near the village of Les Baux in Provence, southern France.[3][4]

silcrete by silica
en.wikipedia.org/wiki/Silcrete

Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferricrete, formed by iron oxide. It is a hard and resistant material, and though different in origin and nature, appears similar to quartzite. As a duricrust, there is potential for preservation of root structures as trace fossils.

Silcrete is common in the arid regions of Australia and Africa often forming the resistant cap rock on features such as the breakaways of the Stuart Range of South Australia. Silcrete can be found at a lesser extent throughout the world especially England (e.g. Hertfordshire puddingstone), and France.[1] In the Great Plains of the United States, polished silcrete cobbles are locally common on the surface and in river gravels east of the outcrops of the Ogallala Formation.[2][3]

In Australia, silcrete was widely used by Aboriginal people for stone tool manufacture, and as such, it was a tradeable commodity, and silcrete tools can be found in areas that have no silcrete groundmass at all, similar to the European use of flint.

Tools made out of silcrete which has not been heat treated are difficult to make with flintknapping techniques. It is widely believed by stone tool experts that the technology to treat silcrete by burying under a hot fire was known 25,000 years ago in Europe. Heating changes the stone structure making it more easily flaked.[4] This process may have been the first use of so-called pyrotechnology by early mankind.[5][6]

In South Africa at Pinnacle Point researchers have determined that two types of silcrete tools were developed between 60,000 and 80,000 years ago and used the heat treatment technique. There is evidence to suggest the technique may have been known as early as 164,000 years ago.[4][7]

The peoples of the African Middle Stone Age (MSA) showed a preference for silcrete tools, sourcing the material from up to 200 km to use in place of more accessible quartz and quartzite. MSA quarries have recently been found in Botswana south of the Okavango Delta. Evidence was found that raw silcrete blanks and blocks were transported prior to heat treating during the MSA. The geochemical signatures of the fragments can be used to identify where many of the individual pieces were quarried.[8]

In the Great Plains of the United States, silcrete cobbles and boulders up to 16 kilograms (35 lb) of Neogene/early-Quaternary age are found on uplands bordering the Ogallala outcrop and were used as chipped tool stone as early as the Early Ceramic (ca. 400–1100 CE) Keith phase of the Woodland culture.[3][2]

calcrete (caliche) by calcium carbonate
en.wikipedia.org/wiki/Caliche

Caliche (/kəˈliːtʃiː/) is a sedimentary rock, a hardened natural cement of calcium carbonate that binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or semiarid regions, including in central and western Australia, in the Kalahari Desert, in the High Plains of the western United States, in the Sonoran Desert, Chihuahuan Desert and Mojave Desert of North America, and in eastern Saudi Arabia at Al-Hasa. Caliche is also known as calcrete or kankar (in India). It belongs to the duricrusts. The term caliche is Spanish and is originally from the Latin calx, meaning lime.[1]

Caliche is generally light-colored, but can range from white to light pink to reddish-brown, depending on the impurities present. It generally occurs on or near the surface, but can be found in deeper subsoil deposits, as well. Layers vary from a few inches to feet thick, and multiple layers can exist in a single location. A caliche layer in a soil profile is sometimes called a K horizon.[2][3]

In northern Chile and Peru, caliche also refers to mineral deposits that include nitrate salts.[4][5] Caliche can also refer to various claylike deposits in Mexico and Colombia. In addition, it has been used to describe some forms of quartzite, bauxite, kaolinite, laterite, chalcedony, opal, and soda niter.

A similar material, composed of calcium sulfate rather than calcium carbonate, is called gypcrust.

gypcrete (gypcrust) by gypsum (sulfur)^-DC
en.wikipedia.org/wiki/Gypcrust

Gypcrete or gypcrust is a hardened layer of soil, consisting of around 95% gypsum (calcium sulfate). Gypcrust is an arid zone duricrust.[1] It can also occur in a semiarid climate in a basin with internal drainage, and is initially developed in a playa as an evaporate.[2] Gypcrete is the arid climate's equivalent to calcrete, which is a duricrust that is unable to generate in very arid climates.[3]

Composition

Gypcrust horizons can be up to 5 m (16 ft) thick with a 75-97% gypsum (CaSO4∙2H2O) content. The majority of gypsum-rich layers occur where the average annual rainfall is less than 250 mm because gypsum is moderately soluble (c. 2.6 g−1 at 25 °C) and is normally leached out under higher rainfall conditions. Gypsum cements are rarely, if ever, as strong as calcretes or silcretes.[1]
Formation

Gypcrust forms in a manner similar to that of caliche, which is composed of calcium carbonate. The development of gypcrust has 3 main stages. The first stage is primary crystallization of the surface brines or groundwater; the second stage is transportation and redeposition by wind or water; and the third stage is post-depositional alteration above or below the capillary fringe. Most gypcrust is formed either as a result of soil-forming processes or through the precipitation of cementing agents from groundwater.[1]
Influence of groundwater on the formation of duricrusts

There are two models that are used to illustrate the influence groundwater has on the formations of duricrusts like gypcrust: per ascensum and per descensum. The per ascensum model demonstrates a situation where the water table is relatively close to the surface, allowing solutions to be drawn upwards by evaporation and eventually cement near-surface sediments once they become concentrated enough to trigger precipitation. The per ascensum model is applicable to environments with high rates of surface evaporation like deserts. This type of system only produces thin duricrust layers since the process ultimately seals the surface horizons, which consequently decreases the potential for further evaporation. This model best depicts the formation of gypcrust. The per descensum model describes a system different from that of the formation of gypcrust in which precipitation of minerals occurs at a depth from downward-percolating solutions. This type of system explains the formation of thick duricrust horizons.[1]

Conditions for formation

Gypcretes form in four distinct conditions: in well-drained soils, as buried evaporates, in hydromorphic soils, or by the exposure of subsurface horizons by erosion.[1]

Profile

Gypcrete can be a loose and powdery deposit or a massive crystalline structure.[2] The profile of a gypcrust outcrop can have three layers. The bottom layer is the sand rose horizon at the water table where gypsum develops as aggregates of crystals. The middle layer is composed of massive gypcrete cemented sand, which forms above the water table during evaporation from the capillary fringe; newly formed gypcrete will be hard, and will soften with age. The uppermost layer is usually rich in gypsified roots and has a banded or nodular structure.[3]

Uses

Gypcrete has been used successfully for road construction in the Sahara.[1] Well-cemented gypcrusts may also provide adequate bearing capacity for structures, however it must be ensured that the underlying uncemented material is not overloaded to avoid collapse.[3]
See also

Gypsum flora of Nova Scotia – Group of plants in Nova Scotia, Canada

References

Walker, M.J. (2012). Hot Deserts: Engineering, Geology and Geomorphology: Engineering Group Working Day Report. Geological Society of London. ISBN 9781862393424. Retrieved 7 October 2013.
Britannica, Encyclopedia. "gypcrete". Encyclopædia Britannica, Inc. Retrieved 7 October 2013.
Bell, Fred G. (4 January 2002). Geological Hazards: Their Assessment, Avoidance and Mitigation. Taylor & Francis, 2002. ISBN 9780203014660. Retrieved 7 October 2013.

en.wikipedia.org/wiki/Gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.[4] It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite.

The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.

Duricrusts need to be formed in absolute accumulation, therefore they must have a source, transfer and precipitation.

Duricrust is often studied during missions to Mars because it may help prove the planet once had more water. Duricrust was found on Mars at the Viking 2 landing site, and a similar structure, nicknamed "Snow Queen", was found under the Phoenix landing site.[3] Phoenix's duricrust was later confirmed to be water-based.[4]

References

Dixon, J.C. and McLaren, S.J., 2009. Duricrusts. In A.J. Parsons and A.D. Abrahams, ed., pp. 123-151. Geomorphology of desert environments. Springer, Dordrecht . ISBN 978-1-4020-5718-2
Woolnough, W.G., 1930. The influence of climate and topography in the formation and distribution of products of weathering. Geological Magazine, 67(3), pp.123-132.
Rayl, A.J.S. (June 1, 2008). "Holy Cow, Snow Queen! Phoenix Landed on Ice, Team Thinks". The Planetary Society. Archived from the original on June 5, 2008. Retrieved November 12, 2008.

Nemiroff, R.; Bonnell, J., eds. (November 12, 2008). "Phoenix and the Holy Cow". Astronomy Picture of the Day. NASA. Retrieved November 12, 2008.

Further reading

DILL, H.G., WEBER, B. and BOTZ, R. (2013) Metalliferous duricrusts (“orecretes”) - markers of weathering: A mineralogical and climatic-geomorphological approach to supergene Pb-Zn-Cu-Sb-P mineralization on different parent materials.- Neues Jahrbuch für Mineralogie Abhandlungen, 190: 123-195

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Duripan

From Wikipedia, the free encyclopedia

A duripan is a diagnostic soil horizon of the USDA soil taxonomy that is cemented by illuvial silica into a subsurface hardpan. Similar to a fragipan, Petrocalcic Horizon and petrogypsic horizon, it is firmly cemented and restricts soil management. In soil descriptions, they are most often denoted by the symbol Bqm. The closest equivalent in the Canadian system of soil classification is called a duric horizon, although it does not mean exactly the same thing as a duripan in the United States. They form almost exclusively in arid or Mediterranean climates, and can be as hard as concrete, which makes plowing very difficult or impossible. Soils that include duripans are generally used for grazing or wildlife habitat, and are seldom cultivated.
Contents

1 Characteristics
2 Genesis
3 See also
4 References

Characteristics

The required characteristics for a duripan, according to the "Keys to USDA soil taxonomy, Tenth Edition, 2006" are:

The pan is cemented or indurated in more than 50 percent of the volume of some horizon; and
The pan shows evidence of accumulation of opal or other forms of silica, such as laminar caps, coatings, lenses, partly filled interstices, bridges between sand-sized grains, or coatings on rock or pararock fragments; and
Less than fifty percent of the volume of air-dry fragments slakes in 1 N HCl even during prolonged soaking, but more than 50 percent slakes in concentrated KOH or NaOH or in alternating acid and alkali; and
Because of lateral continuity, roots can penetrate the pan only along vertical fractures with a horizontal spacing of 10 cm or more.

What this means in plain English is:

More than half the horizon is made up of cemented materials.
There is evidence that the horizon was actually formed by movement of silica into the subsoil.
The purpose of the water, HCl, and KOH tests is to separate a duripan from a Petrocalcic Horizon or fragipan. These tests are seldom conducted in a field environment and usually are performed in a lab with prepared field samples.
The main purpose of describing a duripan is to denote limitations for land management. If plant roots are able to penetrate the duripan then there is no significant limitation and there would be little point in describing a duripan.

Genesis
Duripans occur mostly in arid or semiarid climates, where the soil is usually dry or seasonally dry. Soils with duripans are often geographically associated with areas of volcanic activity, and show evidence of ash or volcanic glass deposition. Volcanic glass weathers rapidly, providing an ample supply of soluble silica to cement the underlying soil. Other common sources of the silica cementing agent are iron-magnesium minerals and feldspars. The parent material of duripan soils usually does not contain a large amount of calcium carbonate, and if there is an abundance of calcium, a Petrocalcic Horizon can form in conjunction with the duripan. The weathered silica is mostly transported by water into the subsoil, where it precipitates and forms microaggregates that gradually grow into a full-fledged duripan. It cannot be considered a duripan until the cracks and gaps that allow roots to penetrate are 10 cm or more apart. A duripan can sometimes be broken up by earthquakes or periods of alternate wetting and drying that cause small volume changes.