114
A. M. Sharaky
are classified as fine sands, well to moderately sorted, fine skewed, and mesokurtic.
Microenvironments of the dunes show much overlap in properties of the grain size
and small fairly consistent differences. The barchan crests and the eastern flanks
of the linear dunes are the coarsest, less sorted, more positive-skewed, and more
leptokurtic.
2.9 Sand Grain Morphology in Dakhla Oasis
The Dakhla dune sand grains are generally sub-rounded with an average roundness
of 3.3ρ. The microenvironments of the crescentic dunes show a slight variation in the
roundness of their quartz grains. The coarse grains are more rounded and spherical
than the fine grains. The sand grains did not show any regular change in roundness
with distance indicating that the sands are transported for a long distance. Quartz
grains forming linear dunes are slightly more rounded and spherical than those of the
crescentic dunes. The reason could be related to the greater surface area subjected
wind energy in linear dunes.
The examination of quartz grain surface using a scanning electron microscope
showed that the grains are mainly affected by mechanical features and less chemical
features. The mechanical features are represented by dish-shaped concavities, elongated depressions, upturned plates, parallel ridges, arcuate, circular, polygonal and
star features. The examined chemical features include solution etching pits, fracture
fillings, and silica precipitation to give the form of turtle-skin (Philip et al. 1992).
2.10 Mineral Composition of Dune Sands of Dakhla Oasis
Dakhla dune sands are generally uniform in mineral composition. They have mainly
composed quartz with a minor amount of carbonate grains (calcite and dolomite). A
minor amount of foraminifera, rock fragments of shale and phosphates are recorded
with quartz (Sharaky 1990). The carbonate content averages 5.8%, being slightly
higher in crescentic than in linear dunes. The source of carbonates is most probably
the Lower Eocene limestone capping the northern plateau.
Quartz is the most abundant light mineral in the mixture of fine to very fine sand
fractions, while feldspars are relatively rare. The quartz grains show frosting due to
pitted surfaces with grooves and cracks that filled by iron oxides.
The Dakhla dune sands varies in color from very pale brown (10 YR 7/4 in Munsell
Color Chart) to light yellowish brown (10 YR 6/4) in the dry state. The color of wet
sands is intense brown (7.5 YR 5/6). The reddening of sand grains in the Dakhla
depression slightly increases eastwards from the western belt and in the direction of
transportation southwards.
Heavy minerals of fine and very fine sand grains are mostly concentrated in the
outer belts, and locally on the crests of sand dunes. They average 0.9 and 0.7% in
A. M. Sharaky
are classified as fine sands, well to moderately sorted, fine skewed, and mesokurtic.
Microenvironments of the dunes show much overlap in properties of the grain size
and small fairly consistent differences. The barchan crests and the eastern flanks
of the linear dunes are the coarsest, less sorted, more positive-skewed, and more
leptokurtic.
2.9 Sand Grain Morphology in Dakhla Oasis
The Dakhla dune sand grains are generally sub-rounded with an average roundness
of 3.3ρ. The microenvironments of the crescentic dunes show a slight variation in the
roundness of their quartz grains. The coarse grains are more rounded and spherical
than the fine grains. The sand grains did not show any regular change in roundness
with distance indicating that the sands are transported for a long distance. Quartz
grains forming linear dunes are slightly more rounded and spherical than those of the
crescentic dunes. The reason could be related to the greater surface area subjected
wind energy in linear dunes.
The examination of quartz grain surface using a scanning electron microscope
showed that the grains are mainly affected by mechanical features and less chemical
features. The mechanical features are represented by dish-shaped concavities, elongated depressions, upturned plates, parallel ridges, arcuate, circular, polygonal and
star features. The examined chemical features include solution etching pits, fracture
fillings, and silica precipitation to give the form of turtle-skin (Philip et al. 1992).
2.10 Mineral Composition of Dune Sands of Dakhla Oasis
Dakhla dune sands are generally uniform in mineral composition. They have mainly
composed quartz with a minor amount of carbonate grains (calcite and dolomite). A
minor amount of foraminifera, rock fragments of shale and phosphates are recorded
with quartz (Sharaky 1990). The carbonate content averages 5.8%, being slightly
higher in crescentic than in linear dunes. The source of carbonates is most probably
the Lower Eocene limestone capping the northern plateau.
Quartz is the most abundant light mineral in the mixture of fine to very fine sand
fractions, while feldspars are relatively rare. The quartz grains show frosting due to
pitted surfaces with grooves and cracks that filled by iron oxides.
The Dakhla dune sands varies in color from very pale brown (10 YR 7/4 in Munsell
Color Chart) to light yellowish brown (10 YR 6/4) in the dry state. The color of wet
sands is intense brown (7.5 YR 5/6). The reddening of sand grains in the Dakhla
depression slightly increases eastwards from the western belt and in the direction of
transportation southwards.
Heavy minerals of fine and very fine sand grains are mostly concentrated in the
outer belts, and locally on the crests of sand dunes. They average 0.9 and 0.7% in