Abstract

Measurements have been made of the 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 the 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 the shape of grains, "the end shape ... appears to be determined by its original shape." (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 the 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 the slide. Such measurements are essentially two dimensional. The frequency distribution is given as per cent by number, so the results may be more applicable to similar measurements in the study of sedimentary rocks in the form of thin section. A discussion of the 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.

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