Initial Shape and Roundness of Sedimentary Rock Mineral Particles of Sand Size

Abstract

Measurements have been made of size, circularity (two dimensional sphericity) and roundness of crushed and screened fragments of nine minerals of sand size commonly found in sedimentary rocks. These are regarded as initial size, shape, and roundness before abrasion in ball mills. These are calcite, andalusite, quartz, garnet, zircon, tourmaline, microcline, sillimanite, and hornblende. While abrasion modifies shape of grains, the end shape ... appears to be determined by its original (See Pettijohn, 1949, p. 54.) It was found that each mineral possesses its own set of characteristics. These are related to type of cleavage and tenacity possessed by each mineral, but relationships are not simple. Sizing by screening introduces an element of shape. Similarly, microscopic measurements reflect a different aspect of shape due to preferred orientation of grains on slide. Such measurements are essentially two dimensional. The frequency distribution is given as per cent by number, so results may be more applicable to similar measurements in study of sedimentary rocks in form of thin section. A discussion of various methods is given Studies of abrasion of minerals in laboratory apparatus, such as ball mills, should recognize that each mineral has its own characteristics. Details of these differences are shown in table 9.