II. DESERT SOILS
57
C. SILICATES
L Characteristics
Silica, that presumably has been solubilized by the alkaline reaction of
desert soils, may be found associated with carbonates to form a cemented
layer or with iron in alternating thin layers to form a duripan. Certain
other desert duripans are cemented by an agent, suspected as being silica
and/or aluminum silicate, that is soluble in concentrated alkali. Abu Fakhr
(1961) describes a "gatch" hardpan in soils of Kuwait which is predominantly sand with a high carbonate content. The cementing agents are presumed to be, in order of concentration, silica, calcium, and magnesium
carbonates, magnesium hydroxide, gypsum, alumina, and iron oxide. The
last three are believed not to play as important a role in gatch pan formation as the other constituents but are mostly incidental inclusions. The
saturation extract of gatch is higher in sodium than calcium, which is noted
also by the pH of 8.5 and above.
2. Origin
Siliceous layers are found in some desert soils of the Kalahari Desert.
These appear to be similar to those found in Australia where soils have
developed on geologically old relics. Siliceous duripans (silcrete) are located below the indurated carbonate layer (calcrete) in Kalahari (Debenham, 1952).
Siliceous crusts have been described as appearing on soils of the Southern Sahara Desert. Silicates also have been claimed to be an important
cementing agent of desert pavements. The stony Australian deserts are covered with broken silcrete which is described by Jackson (1957) as having
a hard, intense varnish. The stony tablelands of Australia as well as North
Africa and Southwest Asia are noted for their flat-topped mounds capped
with silcrete. These tableland soils are usually calcareous, gypsiferous, or
saline at some depth, as a probable result of earlier salt accumulations
between the silcrete mounds.
3. Distribution
Chemical analyses of world desert materials indicate the widespread occurrence of precipitated silica in indurated "caliche" layers. The chemistry
of weathering and soil formation of desert materials and accumulations
of carbonate and sulfate under semiarid and arid alkaline conditions will
allow for solubilization and precipitation of silica. The precipitation of
silica or silicates in indurated lime layers under desert conditions may be
responsible for imparting the hardness and impenetrable nature to these
57
C. SILICATES
L Characteristics
Silica, that presumably has been solubilized by the alkaline reaction of
desert soils, may be found associated with carbonates to form a cemented
layer or with iron in alternating thin layers to form a duripan. Certain
other desert duripans are cemented by an agent, suspected as being silica
and/or aluminum silicate, that is soluble in concentrated alkali. Abu Fakhr
(1961) describes a "gatch" hardpan in soils of Kuwait which is predominantly sand with a high carbonate content. The cementing agents are presumed to be, in order of concentration, silica, calcium, and magnesium
carbonates, magnesium hydroxide, gypsum, alumina, and iron oxide. The
last three are believed not to play as important a role in gatch pan formation as the other constituents but are mostly incidental inclusions. The
saturation extract of gatch is higher in sodium than calcium, which is noted
also by the pH of 8.5 and above.
2. Origin
Siliceous layers are found in some desert soils of the Kalahari Desert.
These appear to be similar to those found in Australia where soils have
developed on geologically old relics. Siliceous duripans (silcrete) are located below the indurated carbonate layer (calcrete) in Kalahari (Debenham, 1952).
Siliceous crusts have been described as appearing on soils of the Southern Sahara Desert. Silicates also have been claimed to be an important
cementing agent of desert pavements. The stony Australian deserts are covered with broken silcrete which is described by Jackson (1957) as having
a hard, intense varnish. The stony tablelands of Australia as well as North
Africa and Southwest Asia are noted for their flat-topped mounds capped
with silcrete. These tableland soils are usually calcareous, gypsiferous, or
saline at some depth, as a probable result of earlier salt accumulations
between the silcrete mounds.
3. Distribution
Chemical analyses of world desert materials indicate the widespread occurrence of precipitated silica in indurated "caliche" layers. The chemistry
of weathering and soil formation of desert materials and accumulations
of carbonate and sulfate under semiarid and arid alkaline conditions will
allow for solubilization and precipitation of silica. The precipitation of
silica or silicates in indurated lime layers under desert conditions may be
responsible for imparting the hardness and impenetrable nature to these