Farouk El-Baz

Document Type

Article

Publication Date

1979

Abstract

The color of desert surfaces is indicative of the cou- position of the exposed rock, rock rubble, soil, and sand. Deposits of particulate material in the desert are often light-toned with predominantly yellow and yellowish-red colors. The reddish component in desert sands has been attri- buted to the presence of hematite on individual sand grains. This hematite coating was previously thought to have origi- nated in lateritic soils of humid climates. Today, however, many investigators agree that the red coating is acquired in the aeolian environment itself. Observations from space have confirmed that the red color of dune sand increases with age and as the distance fron the sand source increases. Visual observations by astronauts and color photographs taken from orbit show transitions from less red to redder sands in the same field in numerous deserts. In general the color of dune sand lepends on the com- position of the source rock. For example, white dunes in Bermuda are made of calcite. resulting fcon the erosion of limestone and sea shells, and dunes of the White Sands in New Mexico are made up of nearly pure gypsum grains. Some dunes near the Red Sea coast are made up of pink coral grains, and others are covered with black magnetite particles. Furthermore, dune sand that is composed mostly of quartz may display color variations depending on the amount of lighter or darker components. However, in most cases the color of quart sand depends largely on the red coating on individual grains. Study of the amount and nacare of reddish coatings on sand grains can be perforaed lby siaple microscopic xamiaation. The aan stain on the quartz grains appears as a frost of yellowish-red color. Darker and thicker coatings occur within grooves or indentations on grains. The mlaeralogy, texture, and chemical composition of both types of coatings can be determined by a combination of X-ray diffractlon techniques and scanning electron picroscopy. Sanples so studied fron the Western Desert of Egypc that the coating is basically made of platelets of kaolinite, which are covered by submicroscopic particles oI henatite. importance of |the clay component in the coatings suggests method of formation similar to chat respousible for desert varnish on rocks in aeolian environments. Eurthermore, coatings on quartz grains in sanples froa three locations Great Sand Sea increase in thickness from about 0.5 alcro- ueters in the aorth to about 1.5 micrometers in the middle, between and micrometers in the south. These results confirn the observation of reddening of sands as the transport distance increeses. They also suggest show The a the in che the that the quattz grains acguire the coat ings during aeolian Cransport This property can ibe used ol |determine the relative ages of color zones in the sane field, and thus can be indica- tive of the transport direction of the sand.

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