ABSTRACT

The degree of development of undulatory extinction and fractures as measures of the deformed condition of quartz grains in clastic sands was in terms of grain-size. Four samples from the Oligocene clastic deposits and separate sand-dunes from four different localities around Cairo were used in this work. On the basis of close similarity in their physical constitution and textural composition, the microscopic studies were carried out on quartz of three size fractions from each sand, representing the coarse, moderate, and fine fractions. The frequency of occurrence of secondary enlargements on detrital sand grains was also studied in terms of grain-size. It was found that the degree of development of the undulatory extinction phenomenon, the frequency of occurrence of fractures and seco dary enlargements, and roundness all decrease with the decrease in grain-size. The frequency of occurrence of fractures as well as secondary enlargements was also found to decrease with the decrease in the degree of development of undulatory extinction. The mechanical means of size reduction, as fracturing processes, are discussed in terms of the deformed condition of the structure of clastic quartz.