Diversity of Desert Sand

Abstract

The stability of minerals in igneous rocks when subject to weathering is the reverse of the crystallization order following Bowen's reaction series. The last-formed minerals (e.g., quartz, muscovite, and potash feldspar) in igneous rocks are more stable at ordinary temperatures than are the minerals (e.g., olivine, pyroxene, and calcic plagioclase) formed at the early stage of crystallization. Quartz is very resistant to chemical weathering under surface temperature conditions. On the contrary, olivine and calcic plagioclase are readily decomposed by weathering.
The Taklimakan desert sand is composed of various kinds of mineral grains, showing that it is immature. On the contrary, the Libyan desert sand is composed mainly of quartz grains signifying that the sand is mature.
In this paper, desert sands from the Kalahari and Namib Deserts were examined, and the character was discussed together with the description of grain size, mineral variety, rock grain and roundness.
Most desert sands from the Kalahari Desert are composed of quartz grains (94.7%-99.9%), with a small amount of feldspar grains (0%-0.5%), mafic mineral grains (0%-4.0%), and rock grains (0.3%-2.0%), indicating the sand is mature.
One of the desert sands from the Kalahari Desert is mainly composed of quartz grains (72.7%) and rock grains (23.7%), with some feldspar grains (1.3%) and mafic mineral grains (2.3%), indicating immaturity. Another type of desert sand is of pure quartz sand, but the quartz grains are ill-rounded in roundness class, indicating the sand is also immature.
Two types of desert sands from the Namib Desert are examined. The main composition of one type is quartz grains (98.5%), with some feldspar grains (1.0%), mafic mineral grains (0%), and rock grains (0.5%), indicating maturity. The other type is composed of quartz grains (58.9%), feldspar grains (16.5%), mafic mineral grains (13.1%) and rock grains (11.5%), indicating immaturity.