Gypcrete or gypcrust is a hardened layer of

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

, consisting of around 95%

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

(

calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...

). Gypcrust is an arid zone

duricrust 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 ...

. It can also occur in a semiarid climate in a basin with internal drainage, and is initially developed in a playa as an evaporate. Gypcrete is the arid climate's equivalent to calcrete, which is a duricrust that is unable to generate in very arid climates.

Composition

Gypcrust horizons can be up to 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.

Formation

Gypcrust forms in a manner similar to that of caliche, which is composed of

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

. The development of gypcrust has three 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.

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.

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.

Profile

Gypcrete can be a loose and powdery deposit or a massive crystalline structure. 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.

Uses

Gypcrete has been used successfully for road construction in the Sahara. 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.

References

{{Soil type Soil Sedimentary rocks